Origin of Petrified Wood Color

Abstract

Fossil forests have world-wide distribution, commonly preserving mineralized wood that displays vivid hues and complex color patterns. However, the origin of petrified color has received little scientific attention. Color of silicified wood may be influenced by the presence of relict organic matter, but the most significant contribution comes from trace metals. This study reports quantitative analysis of trace metals in 35 silicified wood samples, determined using LA-ICP-MS spectrometry. The most important of these metals is Fe, which can produce a rainbow of hues depending on its abundance and oxidation state. Cr is the dominant colorant for bright green fossil wood from Arizona, USA and Zimbabwe, Africa. Complex color patterns result from the progressive nature of the fossilization process, which causes wood to have varying degrees of permeability during successive episodes of permineralization. These processes include simple diffusion, chromatographic separation, infiltration of groundwater along fractures and void spaces, and oxidation/reduction.

Full text

geosciences
Article
Origin of Petrified Wood Color
George Mustoe
1,
* and Marisa Acosta
2
1
Geology Department, Western Washington University, Bellingham, WA 98225, USA
2
Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USA;
macosta@uoregon.edu
* Correspondence: mustoeg@wwu.edu; Tel.: +1-360-650-3582
Academic Editor: James Schmitt
Received: 16 March 2016; Accepted: 3 May 2016; Published: 9 May 2016
Abstract:
Fossil forests have world-wide distribution, commonly preserving mineralized wood that
displays vivid hues and complex color patterns. However, the origin of petrified color has received
little scientific attention. Color of silicified wood may be influenced by the presence of relict organic
matter, but the most significant contribution comes from trace metals. This study reports quantitative
analysis of trace metals in 35 silicified wood samples, determined using LA-ICP-MS spectrometry.
The most important of these metals is Fe, which can produce a rainbow of hues depending on
its abundance and oxidation state. Cr is the dominant colorant for bright green fossil wood from
Arizona, USA and Zimbabwe, Africa. Complex color patterns result from the progressive nature
of the fossilization process, which causes wood to have varying degrees of permeability during
successive episodes of permineralization. These processes include simple diffusion, chromatographic
separation, infiltration of groundwater along fractures and void spaces, and oxidation/reduction.
Keywords: petrified wood; transition elements; trace elements; ICP analysis
1. Introduction
In a modern tree, the color is due to the presence of lignin and other organic constituents. When
wood is mineralized, cellular features may be preserved in great detail, but the original wood color is
lost. For woods that are mineralized with iron pyrite, iron oxide, or copper minerals, the color of the
fossil wood is determined by the mineral color. More often, wood is mineralized with silica. For these
specimens, the origin of color involves two phenomena: trace metals that play a role in controlling
color of silicified wood, and physical and chemical factors that cause these hues to sometimes be
distributed in complex patterns.
This study reports on samples of silicified wood that span a broad color spectrum, ranging from
vivid primary colors to shades of white, brown, and black. Samples include specimens from two
localities that are well-known for producing bright colored specimens: multicolored “rainbow wood”
from the Triassic Chinle Formation from two localities in Arizona, USA, and red and green wood from
the Eocene Clarno Formation at Hampton Butte, Oregon, USA. Other specimens came from Nevada
and Oregon, USA, and Zimbabwe and Madagascar, Africa.
2. Metallic Elements as a Cause of Mineral Color
Although wood can be mineralized with calcium carbonate, calcium phosphate, iron oxide, and
other non-silicate minerals, silica is by far the most common agent for wood petrifaction, Silicified
wood can be mineralized with opal, chalcedony, or microcrystalline quartz. In pure form, these SiO
2
polymorphs are colorless, but in nature they may occur in a wide range of colors. Crystal lattice
defects produce transparent gray “smoky” quartz, and opaque “milky” quartz may be caused by an
abundance of microscopic fluid inclusions. Optical effects produce the iridescent “fire” in precious
Geosciences 2016, 6, 25; doi:10.3390/geosciences6020025 www.mdpi.com/journal/geosciences

Geosciences 2016, 6, 25 2 of 24
opal, where the orderly arrangement of silica microspheres causes reflected visible light to be divided
into spectral colors.
Trace amounts of transition metals are the most common source of color in quartz family
minerals [
1
], and colored gems [
2
–
8
]. Elements with atomic no. 21–29 (Sc, Ti, V, Cr, Mn, Fe, Co,
Ni, Co) have partially filled d-orbitals. Electrons transitioning between d-orbital positions absorb
energy in the visible spectrum, and minerals containing even trace amounts of these elements may
have bright color. These colors are related to concentration and oxidation state. In the absence of trace
element pigments, quartz may appear white because of microscopic fluid inclusions; silicified wood
may be opaque white because of light scattering caused by the relict fibrous wood structure. When
trace elements are present, intensity of the color is controlled by the amount of colorant that is added
to the white base color. In the case of red-hued hematite, colors may range from pale pink to vivid red,
depending on the hematite concentration. Similarly, goethite may produce tints that range from pale
yellow to dark yellowish brown. If two or more pigment compounds are presented, a complex range
of colors may result. This phenomenon may explain the subtle color gradations that may occur within
fossil wood samples from a single locality and variations that occur within a single specimen.
3. Previous Work
In the absence of analytical studies, speculations regarding silicified wood color have commonly
involved unsupported and often conflicting claims [
9
,
10
]. A detailed study reported elemental data
from black, red, and beige zones in a single specimen from Petrified Forest National Park, Arizona,
USA [
11
]. Daniels and Dayvault [
12
] provide a lengthy but rather speculative discussion of trace
elements as a cause of silicified wood color. The only data cited for their interpretations are unpublished
results from qualitative trace element studies performed on 10 samples at Argonne National Laboratory,
Chicago, IL, USA. Thus, the present study is an important contribution because it is the first time
precise quantitative analyses of a broad spectrum of trace elements have been performed on a wide
variety of silicified wood specimens.
4. Methods
Fossil wood samples were mounted with epoxy cement on glass microscope slides, and reduced
to approximately 200 micrometer thickness using Wards Ingram petrographic machines. Specimen
surfaces were polished on lapidary wheels using silicon carbide abrasives. Trace element concentrations
were determined for 35 specimens (Appendix). Specimens contained chalcedony/microcrystalline
quartz as the only discernable mineral, except for specimens from Nye County, Nevada, USA, which
are permineralized with common opal (opal-CT). Mineralogy was determined by X-ray diffraction
patterns of packed powders with a Rigaku Geigerflex diffractometer using Ni-filtered Cu K
α
radiation.
The following elements ( mass numbers) were analyzed for each sample using an Agilent 7500ce
ICP-MS spectrometer with New Wave UP213 Laser ablation system at the Western Washington
University Advanced Materials Science and Engineering Center: Ti(49), V(51), Cr(53), Mn(55), Fe(56),
Co(59), Ni(60), Cu(63), U(238). Beam diameter was 55 micrometers, with laser output set at 70%
of full power. These operating conditions were selected to yield precise laser tracks, as confirmed
by microscopy using a Tescan Vega scanning electron microscope and a Wilde model 420 optical
microscope (Figure 1). Six 2 mm long parallel lines were laser-etched on the sample surface. For each
laser line, data were collected as 15 replicate measurements for each element. Data were averaged,
applying background corrections based on data from blank samples. PPM concentrations were
calculated using U.S. National Institute of Standards and Technology glass reference samples NIST
610, NIST 612, and NIST 614 for calibration. Results were plotted using Microsoft Excel. Specimens are
archived at the Western Washington University Geology Department.

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describe bright-colored petrified wood. For this study, colors of polished specimens are described in
the appendix using the Pantone color matching system (www.Pantone.com). Reference colors can be
viewed online at http://rgb.to/pantone.
Figure 1. Samples for elemental analyses were prepared as microscope slides that contain several
polished specimens. Laser tracks are visible in reflected light (upper right) and SEM
photomicrographs (lower right).
5. Results
LA-ICP-MS analyses reveal that a broad spectrum of colors can be produced by only a few trace
elements. Iron, in various oxidation states and levels of abundance, accounts for most of these hues.
Chromium is the dominant trace element in some bright green specimens.
5.1. Green Wood
Green silicified wood has commonly been assumed to be tinted with chromium, but elemental
analyses of samples from five locations (Figure 2) show that this trace metal is a dominant colorant
only in some bright green specimens. Dark green fossil woods are colored by iron, which may be
present in amounts up to several weight %. These results are consistent with previous qualitative
evidence [12]. Pale green opalized wood from Nye County, Nevada, USA, contains Fe as the major
trace metal, but at concentrations much lower than dark green specimens (Figure 3).
Figure 2. Green silicified wood. (A) Zimbabwe, Africa; (B) Hampton Butte, Deschutes County,
Oregon, USA; (C) Nazlini Canyon, Apache County, Arizona, USA; (D) Winslow, Navajo County,
Arizona, USA; (E) Nye County, Nevada, USA.
Figure 1.
Samples for elemental analyses were prepared as microscope slides that contain several
polished specimens. Laser tracks are visible in reflected light (
upper right
) and SEM photomicrographs
(lower right).
Colors of geological materials are commonly described using the Munsell color space system,
using Munsell Soil Color Charts M50215B (Munsell Color, Baltimore, MD, USA) and Geological
Society of America Rock-Color Chart (Boulder, CO, USA). These charts lack sufficient range to describe
bright-colored petrified wood. For this study, colors of polished specimens are described in the
appendix using the Pantone color matching system (www.Pantone.com). Reference colors can be
viewed online at http://rgb.to/pantone.
5. Results
LA-ICP-MS analyses reveal that a broad spectrum of colors can be produced by only a few trace
elements. Iron, in various oxidation states and levels of abundance, accounts for most of these hues.
Chromium is the dominant trace element in some bright green specimens.
5.1. Green Wood
Green silicified wood has commonly been assumed to be tinted with chromium, but elemental
analyses of samples from five locations (Figure 2) show that this trace metal is a dominant colorant
only in some bright green specimens. Dark green fossil woods are colored by iron, which may be
present in amounts up to several weight %. These results are consistent with previous qualitative
evidence [
12
]. Pale green opalized wood from Nye County, Nevada, USA, contains Fe as the major
trace metal, but at concentrations much lower than dark green specimens (Figure 3).
Geosciences 2016, 6, 25 3 of 23
describe bright-colored petrified wood. For this study, colors of polished specimens are described in
the appendix using the Pantone color matching system (www.Pantone.com). Reference colors can be
viewed online at http://rgb.to/pantone.
Figure 1. Samples for elemental analyses were prepared as microscope slides that contain several
polished specimens. Laser tracks are visible in reflected light (upper right) and SEM
photomicrographs (lower right).
5. Results
LA-ICP-MS analyses reveal that a broad spectrum of colors can be produced by only a few trace
elements. Iron, in various oxidation states and levels of abundance, accounts for most of these hues.
Chromium is the dominant trace element in some bright green specimens.
5.1. Green Wood
Green silicified wood has commonly been assumed to be tinted with chromium, but elemental
analyses of samples from five locations (Figure 2) show that this trace metal is a dominant colorant
only in some bright green specimens. Dark green fossil woods are colored by iron, which may be
present in amounts up to several weight %. These results are consistent with previous qualitative
evidence [12]. Pale green opalized wood from Nye County, Nevada, USA, contains Fe as the major
trace metal, but at concentrations much lower than dark green specimens (Figure 3).
Figure 2. Green silicified wood. (A) Zimbabwe, Africa; (B) Hampton Butte, Deschutes County,
Oregon, USA; (C) Nazlini Canyon, Apache County, Arizona, USA; (D) Winslow, Navajo County,
Arizona, USA; (E) Nye County, Nevada, USA.
Figure 2.
Green silicified wood. (
A
) Zimbabwe, Africa; (
B
) Hampton Butte, Deschutes County, Oregon,
USA; (
C
) Nazlini Canyon, Apache County, Arizona, USA; (
D
) Winslow, Navajo County, Arizona, USA;
(E) Nye County, Nevada, USA.

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Figure 3. Trace element levels in green silicified wood.
5.2. Red Wood
Oxidized iron is the important colorant in red silicified wood (Figures 4 and 5). Dark red color
is associated with high Fe levels; much lower Fe occurs in a light red sample from Texas Spring,
Nevada, USA. The specimen from Nazlini Canyon, Arizona, USA, contains 15 ppm uranium, but this
element is probably not significant as a colorant; relatively high U levels occur in specimens that have
a variety of colors (Appendix).
Figure 4. (A) Triassic Chinle Formation, Nazlini Canyon, Apache County, Arizona, USA; (B) Triassic,
Madagascar, Africa; (C) Triassic Chinle Formation, Holbrook, Navajo County, Arizona, USA.
5.3. Rainbow Colors
Petrified Forest National Park, Arizona, USA, is famous for the abundance of “rainbow wood”
[13], where even a small specimen may contain a broad range of colors. This study determined trace
element compositions from two locations in the Triassic Chinle Formation. Holbrook specimens are
correlative to the fossil forests within the nearby park; Nazlini Canyon specimens are from a more
distant location, located approximately 100 km northeast of Petrified Forest National Park. The
stratigraphic relationship is uncertain; Nazlini Canyon specimens are characterized by pastel colors,
compared to the bright hues typical of specimens from the park area (Figure 6).
Figure 3. Trace element levels in green silicified wood.
5.2. Red Wood
Oxidized iron is the important colorant in red silicified wood (Figures 4 and 5). Dark red color is
associated with high Fe levels; much lower Fe occurs in a light red sample from Texas Spring, Nevada,
USA. The specimen from Nazlini Canyon, Arizona, USA, contains 15 ppm uranium, but this element
is probably not significant as a colorant; relatively high U levels occur in specimens that have a variety
of colors (Appendix).
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Figure 3. Trace element levels in green silicified wood.
5.2. Red Wood
Oxidized iron is the important colorant in red silicified wood (Figures 4 and 5). Dark red color
is associated with high Fe levels; much lower Fe occurs in a light red sample from Texas Spring,
Nevada, USA. The specimen from Nazlini Canyon, Arizona, USA, contains 15 ppm uranium, but this
element is probably not significant as a colorant; relatively high U levels occur in specimens that have
a variety of colors (Appendix).
Figure 4. (A) Triassic Chinle Formation, Nazlini Canyon, Apache County, Arizona, USA; (B) Triassic,
Madagascar, Africa; (C) Triassic Chinle Formation, Holbrook, Navajo County, Arizona, USA.
5.3. Rainbow Colors
Petrified Forest National Park, Arizona, USA, is famous for the abundance of “rainbow wood”
[13], where even a small specimen may contain a broad range of colors. This study determined trace
element compositions from two locations in the Triassic Chinle Formation. Holbrook specimens are
correlative to the fossil forests within the nearby park; Nazlini Canyon specimens are from a more
distant location, located approximately 100 km northeast of Petrified Forest National Park. The
stratigraphic relationship is uncertain; Nazlini Canyon specimens are characterized by pastel colors,
compared to the bright hues typical of specimens from the park area (Figure 6).
Figure 4.
(
A
) Triassic Chinle Formation, Nazlini Canyon, Apache County, Arizona, USA; (
B
) Triassic,
Madagascar, Africa; (C) Triassic Chinle Formation, Holbrook, Navajo County, Arizona, USA.

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Figure 5. Trace element levels in red silicified wood.
Figure 6. Silicified wood from Triassic Chinle Formation: (A) Holbrook, Navajo County, Arizona,
USA. Squares show color zones analyzed for trace elements; (B,C) Nazlini Canyon, Apache County,
Arizona, USA.
In this study, trace metals were measured for six color zones within a single specimen,
guaranteeing that the various colors are not related to differing age or conditions of deposition or
diagenesis. Several additional specimens were analyzed. Analyses of Nazlini Canyon wood were
made on individual samples, each having a particular color (Appendix).
For Holbrook and Nazlini Canyon specimens, the various hues are all associated with the
presence of iron. At both locations, bright red samples contain slight Mn elevations (120 ppm for
Nazlini wood and 67 ppm for Holbrook wood). Bright red wood colors also occur in samples that
have low Mn values (Figure 5, Appendix) suggesting this element is not an essential colorant. For
both Arizona locations, “rainbow” colors are related to Fe concentrations, which progressively
decrease in intensity from bright to pale (Figure 7). These results are consistent with data from Sigleo
11], who measured trace elements in black, red, and beige zones in a single specimen, finding Fe to
be the primary colorant, with the intensity of color proportional to the abundance of that element.
Figure 5. Trace element levels in red silicified wood.
5.3. Rainbow Colors
Petrified Forest National Park, Arizona, USA, is famous for the abundance of “rainbow wood” [
13
],
where even a small specimen may contain a broad range of colors. This study determined trace element
compositions from two locations in the Triassic Chinle Formation. Holbrook specimens are correlative
to the fossil forests within the nearby park; Nazlini Canyon specimens are from a more distant
location, located approximately 100 km northeast of Petrified Forest National Park. The stratigraphic
relationship is uncertain; Nazlini Canyon specimens are characterized by pastel colors, compared to
the bright hues typical of specimens from the park area (Figure 6).
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Figure 5. Trace element levels in red silicified wood.
Figure 6. Silicified wood from Triassic Chinle Formation: (A) Holbrook, Navajo County, Arizona,
USA. Squares show color zones analyzed for trace elements; (B,C) Nazlini Canyon, Apache County,
Arizona, USA.
In this study, trace metals were measured for six color zones within a single specimen,
guaranteeing that the various colors are not related to differing age or conditions of deposition or
diagenesis. Several additional specimens were analyzed. Analyses of Nazlini Canyon wood were
made on individual samples, each having a particular color (Appendix).
For Holbrook and Nazlini Canyon specimens, the various hues are all associated with the
presence of iron. At both locations, bright red samples contain slight Mn elevations (120 ppm for
Nazlini wood and 67 ppm for Holbrook wood). Bright red wood colors also occur in samples that
have low Mn values (Figure 5, Appendix) suggesting this element is not an essential colorant. For
both Arizona locations, “rainbow” colors are related to Fe concentrations, which progressively
decrease in intensity from bright to pale (Figure 7). These results are consistent with data from Sigleo
11], who measured trace elements in black, red, and beige zones in a single specimen, finding Fe to
be the primary colorant, with the intensity of color proportional to the abundance of that element.
Figure 6.
Silicified wood from Triassic Chinle Formation: (
A
) Holbrook, Navajo County, Arizona,
USA. Squares show color zones analyzed for trace elements; (
B
,
C
) Nazlini Canyon, Apache County,
Arizona, USA.

Geosciences 2016, 6, 25 6 of 24
In this study, trace metals were measured for six color zones within a single specimen,
guaranteeing that the various colors are not related to differing age or conditions of deposition
or diagenesis. Several additional specimens were analyzed. Analyses of Nazlini Canyon wood were
made on individual samples, each having a particular color (Appendix).
For Holbrook and Nazlini Canyon specimens, the various hues are all associated with the presence
of iron. At both locations, bright red samples contain slight Mn elevations (120 ppm for Nazlini wood
and 67 ppm for Holbrook wood). Bright red wood colors also occur in samples that have low Mn
values (Figure 5, Appendix) suggesting this element is not an essential colorant. For both Arizona
locations, “rainbow” colors are related to Fe concentrations, which progressively decrease in intensity
from bright to pale (Figure 7). These results are consistent with data from Sigleo [
11
], who measured
trace elements in black, red, and beige zones in a single specimen, finding Fe to be the primary colorant,
with the intensity of color proportional to the abundance of that element.
Geosciences 2016, 6, 25 6 of 23
Figure 7. Trace elements in “rainbow wood” from Triassic Chinle Formation, Holbrook, Arizona,
USA. Note that compared to other histograms in this paper, X and Y axis labels are reversed to more
clearly show the relation of wood color to Fe concentration.
5.4. Black, White, Clear
It is not surprising that nearly colorless silicified wood specimens contain very low concentrations
of trace elements (Figure 8). Two samples of black silicified wood, from Holbrook, Arizona, USA, and
Nye County, Nevada, USA, were likewise found to contain low trace element levels. Opaque white
specimens showed variations in composition, ranging from 1683 ppm Fe to 200 ppm or less in six
other specimens. Black wood results from a variety of causes. Some Chinle formation specimens,
particularly those from Nazlini Canyon, Arizona, USA, have thin external coatings of “desert varnish,”
Mn/Fe-enriched coating formed on rock surfaces that have had prolonged surface exposure. The black
zone may extend shallow distances along fractures in silicified wood (Figure 9). The common
assumption that the black color is caused by elevated levels of Mn or Fe is not valid for samples in
this study. For mineralized wood, black color is rarely related to prehistoric burning, in contrast to
the widespread abundance of unmineralized charcoal in the fossil record, which provides clear
evidence of ancient forest fires [14]. One cause of black color in silicified wood may simply be the low
reflectance of translucent chalcedony.
Figure 8. Trace elements in black, white, and clear silicified wood.
Figure 7.
Trace elements in “rainbow wood” from Triassic Chinle Formation, Holbrook, Arizona, USA.
Note that compared to other histograms in this paper, X and Y axis labels are reversed to more clearly
show the relation of wood color to Fe concentration.
5.4. Black, White, Clear
It is not surprising that nearly colorless silicified wood specimens contain very low concentrations
of trace elements (Figure 8). Two samples of black silicified wood, from Holbrook, Arizona, USA,
and Nye County, Nevada, USA, were likewise found to contain low trace element levels. Opaque
white specimens showed variations in composition, ranging from 1683 ppm Fe to 200 ppm or less in
six other specimens. Black wood results from a variety of causes. Some Chinle formation specimens,
particularly those from Nazlini Canyon, Arizona, USA, have thin external coatings of “desert varnish,”
Mn/Fe-enriched coating formed on rock surfaces that have had prolonged surface exposure. The
black zone may extend shallow distances along fractures in silicified wood (Figure 9). The common
assumption that the black color is caused by elevated levels of Mn or Fe is not valid for samples in
this study. For mineralized wood, black color is rarely related to prehistoric burning, in contrast to the
widespread abundance of unmineralized charcoal in the fossil record, which provides clear evidence of
ancient forest fires [
14
]. One cause of black color in silicified wood may simply be the low reflectance
of translucent chalcedony.

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Figure 7. Trace elements in “rainbow wood” from Triassic Chinle Formation, Holbrook, Arizona,
USA. Note that compared to other histograms in this paper, X and Y axis labels are reversed to more
clearly show the relation of wood color to Fe concentration.
5.4. Black, White, Clear
It is not surprising that nearly colorless silicified wood specimens contain very low concentrations
of trace elements (Figure 8). Two samples of black silicified wood, from Holbrook, Arizona, USA, and
Nye County, Nevada, USA, were likewise found to contain low trace element levels. Opaque white
specimens showed variations in composition, ranging from 1683 ppm Fe to 200 ppm or less in six
other specimens. Black wood results from a variety of causes. Some Chinle formation specimens,
particularly those from Nazlini Canyon, Arizona, USA, have thin external coatings of “desert varnish,”
Mn/Fe-enriched coating formed on rock surfaces that have had prolonged surface exposure. The black
zone may extend shallow distances along fractures in silicified wood (Figure 9). The common
assumption that the black color is caused by elevated levels of Mn or Fe is not valid for samples in
this study. For mineralized wood, black color is rarely related to prehistoric burning, in contrast to
the widespread abundance of unmineralized charcoal in the fossil record, which provides clear
evidence of ancient forest fires [14]. One cause of black color in silicified wood may simply be the low
reflectance of translucent chalcedony.
Figure 8. Trace elements in black, white, and clear silicified wood.
Figure 8. Trace elements in black, white, and clear silicified wood.
Geosciences 2016, 6, 25 7 of 23
Figure 9. Black “desert varnish” penetrating fractures, Holbrook, Arizona, USA.
5.5. Brown
Large variations in trace element abundances were observed for brown-hued silicified wood
(Figure 10). These fossil wood samples contain Fe as a dominant trace element, but at levels that
seldom exceed a few hundred ppm. The intensity of brown color does not necessarily correspond to
the Fe concentration. For example, dark brown wood from Hampton Butte, OR, USA, and medium
brown woods from Texas Spring, NV, USA, and Madagascar, Africa all contain low abundances for
all measured trace elements. A possible explanation is that brown colors may sometimes be caused
by relict organic matter rather than trace element colorants. More research is needed to explain the
apparently diverse origin of tan and brown colors.
Figure 10. Trace element levels in brown silicified wood. Light brown bars represent tan wood, other
bars represent darker shades of brown.
6. Discussion
An important result of this study is the discovery that a variety of colors in silicified wood can
be produced by Fe in varying abundances and oxidation states. Cr is important only for a few bright
Figure 9. Black “desert varnish” penetrating fractures, Holbrook, Arizona, USA.
5.5. Brown
Large variations in trace element abundances were observed for brown-hued silicified wood
(Figure 10). These fossil wood samples contain Fe as a dominant trace element, but at levels that
seldom exceed a few hundred ppm. The intensity of brown color does not necessarily correspond to
the Fe concentration. For example, dark brown wood from Hampton Butte, OR, USA, and medium
brown woods from Texas Spring, NV, USA, and Madagascar, Africa all contain low abundances for
all measured trace elements. A possible explanation is that brown colors may sometimes be caused
by relict organic matter rather than trace element colorants. More research is needed to explain the
apparently diverse origin of tan and brown colors.

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Figure 9. Black “desert varnish” penetrating fractures, Holbrook, Arizona, USA.
5.5. Brown
Large variations in trace element abundances were observed for brown-hued silicified wood
(Figure 10). These fossil wood samples contain Fe as a dominant trace element, but at levels that
seldom exceed a few hundred ppm. The intensity of brown color does not necessarily correspond to
the Fe concentration. For example, dark brown wood from Hampton Butte, OR, USA, and medium
brown woods from Texas Spring, NV, USA, and Madagascar, Africa all contain low abundances for
all measured trace elements. A possible explanation is that brown colors may sometimes be caused
by relict organic matter rather than trace element colorants. More research is needed to explain the
apparently diverse origin of tan and brown colors.
Figure 10. Trace element levels in brown silicified wood. Light brown bars represent tan wood, other
bars represent darker shades of brown.
6. Discussion
An important result of this study is the discovery that a variety of colors in silicified wood can
be produced by Fe in varying abundances and oxidation states. Cr is important only for a few bright
Figure 10.
Trace element levels in brown silicified wood. Light brown bars represent tan wood, other
bars represent darker shades of brown.
6. Discussion
An important result of this study is the discovery that a variety of colors in silicified wood can
be produced by Fe in varying abundances and oxidation states. Cr is important only for a few bright
green wood colors. This observation will hopefully reduce the misconceptions that have resulted from
using trace element colors in gemstones as an analog for wood petrifaction. The differences in the
origin of color in silicified wood and non-silicate minerals probably stems from the differing lattice
structures. The possibility of incorporation of a trace element within a lattice depends not only on the
presence of the element during paragenesis, but on its compatibility with the molecular architecture of
the host mineral. For example, Ti may be readily incorporated into iron-bearing minerals because of
the similarity in atomic radii and valence between Fe and Ti. Also, mineral color may be related to
the proximity of neighboring atoms, which affects possibilities for electron charge transfer. In copper
minerals, Cu bonded to oxygen in malachite (Cu
2
CO
3
(OH)
2
) and azurite (Cu
3
(CO
3
)
2
(OH)
2
) produces
green and blue colors, respectively. These colors are very different from those of minerals where
Cu is associated with sulfur, as in opaque metallic lusters typical of chalcocite (Cu
2
S), and bornite
(Cu
5
FeS
4
). Similar color variations occur at trace levels, as evidenced by beryl (Be
3
Al
2
Si
16
O
8
), where
color changes from red (ruby) to green (emerald) as the concentration of Cr increases as a substitute
for Al in the Al
2
O
3
lattice framework [15].
Possibilities for ionic substitutions as a source of color are very limited in silicified wood
because the lattice structure of quartz is well-defined, compared to silicate minerals families that are
characterized by variable structure (e.g., clays, zeolites, garnet, amphibole, pyroxenes, etc.). Because of
the differences in ionic radii between atoms of given transition metal and silicon, metallic substitutions
distort the quartz crystal lattice; the concentration of trace elements necessary to produce vivid colors
in microcrystalline SiO
2
grains would produce unacceptably large kinetic stresses. When an element
is present at a concentration sufficient to generate pigmentation, it is likely present as a separate
cryptocrystalline solid phase intermixed with the SiO
2
. Colored areas in specimens used for this study
seldom contain pigment grains that are detectable by optical microscopy, but there are exceptions
(Figures 11 and 12).

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green wood colors. This observation will hopefully reduce the misconceptions that have resulted
from using trace element colors in gemstones as an analog for wood petrifaction. The differences in
the origin of color in silicified wood and non-silicate minerals probably stems from the differing
lattice structures. The possibility of incorporation of a trace element within a lattice depends not only
on the presence of the element during paragenesis, but on its compatibility with the molecular
architecture of the host mineral. For example, Ti may be readily incorporated into iron-bearing
minerals because of the similarity in atomic radii and valence between Fe and Ti. Also, mineral color
may be related to the proximity of neighboring atoms, which affects possibilities for electron charge
transfer. In copper minerals, Cu bonded to oxygen in malachite (Cu
2CO3(OH)2) and azurite
(Cu
3(CO3)2(OH)2) produces green and blue colors, respectively. These colors are very different from
those of minerals where Cu is associated with sulfur, as in opaque metallic lusters typical of chalcocite
(Cu
2S), and bornite (Cu5FeS4). Similar color variations occur at trace levels, as evidenced by beryl
(Be
3Al2Si16O8), where color changes from red (ruby) to green (emerald) as the concentration of Cr
increases as a substitute for Al in the Al
2O3 lattice framework [15].
Possibilities for ionic substitutions as a source of color are very limited in silicified wood because
the lattice structure of quartz is well-defined, compared to silicate minerals families that are
characterized by variable structure (e.g., clays, zeolites, garnet, amphibole, pyroxenes, etc.). Because
of the differences in ionic radii between atoms of given transition metal and silicon, metallic
substitutions distort the quartz crystal lattice; the concentration of trace elements necessary to
produce vivid colors in microcrystalline SiO
2 grains would produce unacceptably large kinetic
stresses. When an element is present at a concentration sufficient to generate pigmentation, it is likely
present as a separate cryptocrystalline solid phase intermixed with the SiO
2. Colored areas in
specimens used for this study seldom contain pigment grains that are detectable by optical
microscopy, but there are exceptions (Figures 11 and 12).
Figure 11. (A) Silicified Triassic wood from Madagascar owes its red color to discrete Fe-rich zones
present within the permineralized tissue; (B) Uniform red color in Triassic “Araucarioxylon” wood
from Holbrook, Arizona. USA, showing two adjacent tracheids, with well-preserved bordered pits.
Figure 12. Miocene wood from Grassy Mountain, Lane County, Oregon, USA. The light-colored
chalcedony has orange zones caused when iron minerals were later deposited within open pores and
Figure 11.
(
A
) Silicified Triassic wood from Madagascar owes its red color to discrete Fe-rich zones
present within the permineralized tissue; (
B
) Uniform red color in Triassic “Araucarioxylon” wood from
Holbrook, Arizona. USA, showing two adjacent tracheids, with well-preserved bordered pits.
Geosciences 2016, 6, 25 8 of 23
green wood colors. This observation will hopefully reduce the misconceptions that have resulted
from using trace element colors in gemstones as an analog for wood petrifaction. The differences in
the origin of color in silicified wood and non-silicate minerals probably stems from the differing
lattice structures. The possibility of incorporation of a trace element within a lattice depends not only
on the presence of the element during paragenesis, but on its compatibility with the molecular
architecture of the host mineral. For example, Ti may be readily incorporated into iron-bearing
minerals because of the similarity in atomic radii and valence between Fe and Ti. Also, mineral color
may be related to the proximity of neighboring atoms, which affects possibilities for electron charge
transfer. In copper minerals, Cu bonded to oxygen in malachite (Cu
2CO3(OH)2) and azurite
(Cu
3(CO3)2(OH)2) produces green and blue colors, respectively. These colors are very different from
those of minerals where Cu is associated with sulfur, as in opaque metallic lusters typical of chalcocite
(Cu
2S), and bornite (Cu5FeS4). Similar color variations occur at trace levels, as evidenced by beryl
(Be
3Al2Si16O8), where color changes from red (ruby) to green (emerald) as the concentration of Cr
increases as a substitute for Al in the Al
2O3 lattice framework [15].
Possibilities for ionic substitutions as a source of color are very limited in silicified wood because
the lattice structure of quartz is well-defined, compared to silicate minerals families that are
characterized by variable structure (e.g., clays, zeolites, garnet, amphibole, pyroxenes, etc.). Because
of the differences in ionic radii between atoms of given transition metal and silicon, metallic
substitutions distort the quartz crystal lattice; the concentration of trace elements necessary to
produce vivid colors in microcrystalline SiO
2 grains would produce unacceptably large kinetic
stresses. When an element is present at a concentration sufficient to generate pigmentation, it is likely
present as a separate cryptocrystalline solid phase intermixed with the SiO
2. Colored areas in
specimens used for this study seldom contain pigment grains that are detectable by optical
microscopy, but there are exceptions (Figures 11 and 12).
Figure 11. (A) Silicified Triassic wood from Madagascar owes its red color to discrete Fe-rich zones
present within the permineralized tissue; (B) Uniform red color in Triassic “Araucarioxylon” wood
from Holbrook, Arizona. USA, showing two adjacent tracheids, with well-preserved bordered pits.
Figure 12. Miocene wood from Grassy Mountain, Lane County, Oregon, USA. The light-colored
chalcedony has orange zones caused when iron minerals were later deposited within open pores and
Figure 12.
Miocene wood from Grassy Mountain, Lane County, Oregon, USA. The light-colored
chalcedony has orange zones caused when iron minerals were later deposited within open pores
and fractures. (
A
), Transverse vew showing annual rings, with ferrugionous zones concentrated near
exterior surface and along radial fractues; (
B
), Close-up view showing vessels containing iron oxides.
Iron Oxides
LA-ICPMS analysis revealed iron to be the dominant element responsible for silicified wood color,
with the exception of the bright green specimens that contain high levels of chromium. The wide
range of colors that can be produced by small amounts of iron can be explained by the diversity of
naturally-occurring iron oxides and hydroxides (Table 1 [16,17]).
The ability of iron oxides to determine silicified wood color is related to tinting strength and
hiding power (opacity). Tinting strength refers to the ability of the pigment to impart color to material;
all iron oxides have high tinting strength, but some minerals have higher tinting strength than others.
For example, the pigmenting power of hematite is far greater than that of goethite [
18
]. For iron oxides,
substitution for some Fe by another element may affect the color. For goethite, substitution of small
amounts of Mn produces an olive brown to blackish color, while V substitution causes a greenish hue.
Mn-substituted hematite is blackish [18].
As noted previously, particle size can influence color. Goethite and akaganéite crystals of
0.1–1 µm
diameter are yellow, but larger particles are more brown [
15
]. Aqueous suspensions of hematite
particles with diameters <0.1
µ
m were orange, red in the range of 0.1–0.5
µ
m, and purple for particles
>1.5
µ
m [
18
]. Particle shape also influences color, e.g., acicular hematite has a more yellowish hue than
more symmetric particles [
19
]. Violet color may result when red hematite nanoparticles are present in
bluish translucent chalcedony [12].
Colors observed in silicified wood containing Fe as the major colorant element are consistent with
evidence for soil color. Secondary iron oxides are the most important pigment for low-organic content
soil [
20
]. Soils containing only goethite are yellowish brown. Presence of hematite may mask color

Geosciences 2016, 6, 25 10 of 24
contribution from goethite, yielding reddish color. For 33 Brazilian soils, intensity of red color was
linear in relation to hematite content [20].
Table 1. Iron oxide minerals. Data from Cornell & Schwertmann [16], Barrón & Torrent [17].
Mineral Formula Xl System Color
Iron oxides:
Magnetite Fe
2
+Fe
3
+2O
4
Isometric Black
Wüstite FeO Isometric
Grayish white to yellow
or brown
Hematite α-Fe
2
O
3
Hexagonal
Earthy variety: dull red
to bright red
Maghemite γ-Fe
2
O
3
Isometric brown
Akageneite
Fe
3+
O(OH,Cl)
Monoclinic
Yellow brown, reddish
brown
Iron hydroxides:
Bernalite Fe(OH)
3
Isometric Green
Oxide/hydroxides:
Goethite α-FeO(OH) Orthorhombic
Yellowish to reddish to
dark brown
Limonite FeO(OH)¨ nH
2
O Amorphous Yellow to brown
Lepidocrocite γ-FeO(OH)
3
Orthorhombic Red to reddish brown
Feroxyhyte
Geosciences 2016, 6, 25 9 of 23
fractures. (A), Transverse vew showing annual rings, with ferrugionous zones concentrated near
exterior surface and along radial fractues; (B), Close-up view showing vessels containing iron oxides.
Iron Oxides
LA-ICPMS analysis revealed iron to be the dominant element responsible for silicified wood
color, with the exception of the bright green specimens that contain high levels of chromium. The
wide range of colors that can be produced by small amounts of iron can be explained by the diversity
of naturally-occurring iron oxides and hydroxides (Table 1 [16,17]).
The ability of iron oxides to determine silicified wood color is related to tinting strength and
hiding power (opacity). Tinting strength refers to the ability of the pigment to impart color to
material; all iron oxides have high tinting strength, but some minerals have higher tinting strength
than others. For example, the pigmenting power of hematite is far greater than that of goethite [18].
For iron oxides, substitution for some Fe by another element may affect the color. For goethite,
substitution of small amounts of Mn produces an olive brown to blackish color, while V substitution
causes a greenish hue. Mn-substituted hematite is blackish [18].
Table 1. Iron oxide minerals. Data from Cornell & Schwertmann [16], Barrón & Torrent [17].
Mineral
Formula
Xl System
Color
Iron oxides:
Magnetite
Fe2+Fe3+2O4
Isometric
Black
Wüstite
FeO
Isometric
Grayish white to yellow or brown
Hematite
α-Fe2O3
Hexagonal
Earthy variety: dull red to bright red
Maghemite
γ-Fe2O3
Isometric
brown
Akageneite
Fe
3+
O(OH,Cl)
Monoclinic
Yellow brown, reddish brown
Iron hydroxides:
Bernalite
Fe(OH)3
Isometric
Green
Oxide/hydroxides:
Goethite
α-FeO(OH)
Orthorhombic
Yellowish to reddish to dark brown
Limonite
FeO(OH)·nH2O
Amorphous
Yellow to brown
Lepidocrocite
γ-FeO(OH)3
Orthorhombic
Red to reddish brown
Feroxyhyte
ɗ-FeO(OH)
Hexagonal
Brown, yellowish brown
Ferrihydrite
(Fe
3+
)2O3·0.5H2O
Hexagonal
Dark brown, yellowish brown
Fougèrite
Fe4
2+
Fe2
3+
(OH)32(CO3)·3H2O
Hexagonal
Gray-green
Mӧssbauerite
Fe
3+
6O4(OH)8(CO3)·3H2O
Hexagonal
Blue-green
Trébeurdenite
Fe2
2+
Fe4
3+
(OH)10CO3·3H2O
Hexagonal
Gray-green
Schwertmannite
Fe8O8(OH)6(SO)·nH2O
Tetragonal
Orange
As noted previously, particle size can influence color. Goethite and akaganéite crystals of 0.1–1 μm
diameter are yellow, but larger particles are more brown [15]. Aqueous suspensions of hematite
particles with diameters <0.1 μm were orange, red in the range of 0.1–0.5 μm, and purple for particles
>1.5 μm [18]. Particle shape also influences color, e.g., acicular hematite has a more yellowish hue
than more symmetric particles [19]. Violet color may result when red hematite nanoparticles are
present in bluish translucent chalcedony [12].
Colors observed in silicified wood containing Fe as the major colorant element are consistent
with evidence for soil color. Secondary iron oxides are the most important pigment for low-organic
content soil [20]. Soils containing only goethite are yellowish brown. Presence of hematite may mask
color contribution from goethite, yielding reddish color. For 33 Brazilian soils, intensity of red color
was linear in relation to hematite content [20].
As noted in Table 1, colors of iron oxides and hydroxides are typically shades of yellow, brown,
red, or black, but green iron compounds are known. These compounds are commonly referred to as
“green rust”, following the 1935 discovery of a green corrosion product of metallic iron [21].
Subsequently, green rust minerals were identified in natural occurrences, beginning with the
discovery of fougèrite in forest soil [22], mӧssbauerite [23], and trébeurdenite [24], both from
intertidal mud. Investigations of these green rusts are proceeding rapidly, as evidenced by a 2015
FeO(OH) Hexagonal Brown, yellowish brown
Ferrihydrite
(Fe
3+
)
2
O
3
¨ 0.5H
2
O
Hexagonal
Dark brown, yellowish
brown
Fougèrite
Fe
4
2+
Fe
2
3+
(OH)
32
(CO
3
)
¨
3H
2
O
Hexagonal Gray-green
Mössbauerite
Fe
3+
6
O
4
(OH)
8
(CO
3
)
¨
3H
2
O
Hexagonal Blue-green
Trébeurdenite
Fe
2
2+
Fe
4
3+
(OH)
10
CO
3
¨
3H
2
O
Hexagonal Gray-green
Schwertmannite Fe
8
O
8
(OH)
6
(SO)¨ nH
2
O Tetragonal Orange
As noted in Table 1, colors of iron oxides and hydroxides are typically shades of yellow, brown, red,
or black, but green iron compounds are known. These compounds are commonly referred to as “green
rust”, following the 1935 discovery of a green corrosion product of metallic iron [
21
]. Subsequently,
green rust minerals were identified in natural occurrences, beginning with the discovery of fougèrite
in forest soil [
22
], mössbauerite [
23
], and trébeurdenite [
24
], both from intertidal mud. Investigations
of these green rusts are proceeding rapidly, as evidenced by a 2015 special volume [
21
], but their range
of occurrences in natural environments remains enigmatic. High iron concentrations in some green
silicified samples suggest the possible presence of these minerals.
Although Fe is likely present in fossil wood in the form of ferruginous minerals, the amounts are
too low to be detected by X-ray diffraction for the samples used in this study. However, the range of
colors in the minerals listed in Table 1 is consistent with the observation that multi-colored silicified
wood commonly contains Fe as the predominant trace element.
7. Geologic Source of Colorant Elements
What is the origin of colorant phases found in silicified wood? Colorants present in fossil wood
have mostly been introduced during fossilization, based on the very low trace element concentrations
typical of the original tissue [
25
]. Two sources are likely: elements derived from matrix enclosing the
buried wood, and elements transported from a more distant source by groundwater. Some, perhaps
many, deposits involve both phenomena. It is probably not a coincidence that the brightest-colored
wood specimens in this study came from two locations where wood was preserved in weathered
volcaniclastic formations noted for vivid strata. In the case of the Triassic Chinle Formation, the
elemental composition of sediment inclosing silicified wood contains several percent iron, and
elevated Mn levels [
11
]. The color of the fossil wood is not always correlative with the color of

Geosciences 2016, 6, 25 11 of 24
the host strata (Figure 13), suggesting that groundwater was sometimes important for the transport of
dissolved elements.
Geosciences 2016, 6, 25 10 of 23
special volume [21], but their range of occurrences in natural environments remains enigmatic. High
iron concentrations in some green silicified samples suggest the possible presence of these minerals.
Although Fe is likely present in fossil wood in the form of ferruginous minerals, the amounts
are too low to be detected by X-ray diffraction for the samples used in this study. However, the range
of colors in the minerals listed in Table 1 is consistent with the observation that multi-colored silicified
wood commonly contains Fe as the predominant trace element.
7. Geologic Source of Colorant Elements
What is the origin of colorant phases found in silicified wood? Colorants present in fossil wood
have mostly been introduced during fossilization, based on the very low trace element concentrations
typical of the original tissue [25]. Two sources are likely: elements derived from matrix enclosing the
buried wood, and elements transported from a more distant source by groundwater. Some, perhaps
many, deposits involve both phenomena. It is probably not a coincidence that the brightest-colored
wood specimens in this study came from two locations where wood was preserved in weathered
volcaniclastic formations noted for vivid strata. In the case of the Triassic Chinle Formation, the
elemental composition of sediment inclosing silicified wood contains several percent iron, and
elevated Mn levels [11]. The color of the fossil wood is not always correlative with the color of the
host strata (Figure 13), suggesting that groundwater was sometimes important for the transport of
dissolved elements.
Figure 13. Triassic Chinle Formation, Petrified Forest National Park, Arizona, USA. (A) “Painted
Desert” shows beds of weathered volcaniclastic sediment that range from bland to very colorful. Fe
is the predominate source of color [9]; (B) Bright-colored slicified logs weathered from pale-colored
strata; (C) Logs weathered from reddish beds; (D) Wood containing variegated yellow and red colors
preserved in volcaniclastic matrix that contains yellow and red color patches.
Silicified wood is common in Eocene Clarno Formation beds at Hampton Butte, Deschutes
County, OR, USA. At the Painted Hills type locality at John Day Fossil Beds National Monument,
Clarno strata are composed of weathered volcaniclastic strata similar to the Arizona Chinle
Formation (Figure 14). At Hampton Butte, specimens were collected from excavations in pale-colored
soil; the original color of the host sediment is unknown.
Figure 13.
Triassic Chinle Formation, Petrified Forest National Park, Arizona, USA. (
A
) “Painted
Desert” shows beds of weathered volcaniclastic sediment that range from bland to very colorful. Fe
is the predominate source of color [
9
]; (
B
) Bright-colored slicified logs weathered from pale-colored
strata; (C) Logs weathered from reddish beds; (D) Wood containing variegated yellow and red colors
preserved in volcaniclastic matrix that contains yellow and red color patches.
Silicified wood is common in Eocene Clarno Formation beds at Hampton Butte, Deschutes County,
OR, USA. At the Painted Hills type locality at John Day Fossil Beds National Monument, Clarno strata
are composed of weathered volcaniclastic strata similar to the Arizona Chinle Formation (Figure 14).
At Hampton Butte, specimens were collected from excavations in pale-colored soil; the original color
of the host sediment is unknown.
Geosciences 2016, 6, 25 11 of 23
Figure 14. (A) Eocene Clarno Formation Painted Hills locality; (B) Collecting wood from soil at
Hampton Butte, Oregon, USA; (C–E) Freshly-excavated specimens. Dark green is the predominate
color, while red zones appear to result from oxidation.
8. Origin of Color Patterns
The chemical composition of fossil wood can be determined with a high degree of accuracy, but
the interpretation of the way in which elemental variations produce color patterns is more subjective.
If wood had a homogeneous structure and permineralization occurred in a single step, and if no
remobilization of elemental consituents occurred during diagenesis, silicified wood would be
expected to have a uniform color. Instead, fossil wood commonly has color patterns that are highly
variable in hue and complex in geometry. The development of complex color patterns begins with
infiltration of mineral-laden groundwater, followed by some or all of the following effects.
8.1. Groundwater Penetration along Natural Cellular Pathways
Fluid transport in plants stems is highly directional, a phenomenon that persists long after death.
Groundwater percolating through porous tissue follows elongated trachieds that are connected
laterally by circular apertures (pits), and along larger conductive vessels in angiosperm woods. This
permeability commonly causes petrifaction colors to be aligned with the wood grain (Figure 15).
Figure 15. Scanning electron microscope image of modern conifer wood cut in transverse section,
showing rectangular tracheids connected to one another along their radial walls by circular pits.
An important aspect of stepwise mineralization is that differential permeability may persist in
partially mineralized wood. One example is during incipient stages of silicification, when cell walls
incorporate silica, but lumen remain open. In this situation, the permeability of groundwater
Figure 14.
(
A
) Eocene Clarno Formation Painted Hills locality; (
B
) Collecting wood from soil at
Hampton Butte, Oregon, USA; (
C
–
E
) Freshly-excavated specimens. Dark green is the predominate
color, while red zones appear to result from oxidation.
8. Origin of Color Patterns
The chemical composition of fossil wood can be determined with a high degree of accuracy, but
the interpretation of the way in which elemental variations produce color patterns is more subjective.
If wood had a homogeneous structure and permineralization occurred in a single step, and if no
remobilization of elemental consituents occurred during diagenesis, silicified wood would be expected

Geosciences 2016, 6, 25 12 of 24
to have a uniform color. Instead, fossil wood commonly has color patterns that are highly variable in
hue and complex in geometry. The development of complex color patterns begins with infiltration of
mineral-laden groundwater, followed by some or all of the following effects.
8.1. Groundwater Penetration along Natural Cellular Pathways
Fluid transport in plants stems is highly directional, a phenomenon that persists long after
death. Groundwater percolating through porous tissue follows elongated trachieds that are connected
laterally by circular apertures (pits), and along larger conductive vessels in angiosperm woods. This
permeability commonly causes petrifaction colors to be aligned with the wood grain (Figure 15).
Geosciences 2016, 6, 25 11 of 23
Figure 14. (A) Eocene Clarno Formation Painted Hills locality; (B) Collecting wood from soil at
Hampton Butte, Oregon, USA; (C–E) Freshly-excavated specimens. Dark green is the predominate
color, while red zones appear to result from oxidation.
8. Origin of Color Patterns
The chemical composition of fossil wood can be determined with a high degree of accuracy, but
the interpretation of the way in which elemental variations produce color patterns is more subjective.
If wood had a homogeneous structure and permineralization occurred in a single step, and if no
remobilization of elemental consituents occurred during diagenesis, silicified wood would be
expected to have a uniform color. Instead, fossil wood commonly has color patterns that are highly
variable in hue and complex in geometry. The development of complex color patterns begins with
infiltration of mineral-laden groundwater, followed by some or all of the following effects.
8.1. Groundwater Penetration along Natural Cellular Pathways
Fluid transport in plants stems is highly directional, a phenomenon that persists long after death.
Groundwater percolating through porous tissue follows elongated trachieds that are connected
laterally by circular apertures (pits), and along larger conductive vessels in angiosperm woods. This
permeability commonly causes petrifaction colors to be aligned with the wood grain (Figure 15).
Figure 15. Scanning electron microscope image of modern conifer wood cut in transverse section,
showing rectangular tracheids connected to one another along their radial walls by circular pits.
An important aspect of stepwise mineralization is that differential permeability may persist in
partially mineralized wood. One example is during incipient stages of silicification, when cell walls
incorporate silica, but lumen remain open. In this situation, the permeability of groundwater
Figure 15.
Scanning electron microscope image of modern conifer wood cut in transverse section,
showing rectangular tracheids connected to one another along their radial walls by circular pits.
An important aspect of stepwise mineralization is that differential permeability may persist in
partially mineralized wood. One example is during incipient stages of silicification, when cell walls
incorporate silica, but lumen remain open. In this situation, the permeability of groundwater resembles
the direction of fluid transport in the orignal wood (Figure 16). The primary permeability is lengthwise,
but because adjacent tracheids are connected by intervessel pits, fluids may also diffuse radially. If
these pits remain open during early petrifaction, or if tracheid walls become fractured, mineral-bearing
groundwater may migrate laterally. Because of their larger diameter, conductive vessels in angiosperm
wood may remain open after tracheids have become mineralized; subsequent precipitation episodes
may cause these vessels to be mineralized with silica of a different color.
Geosciences 2016, 6, 25 12 of 23
resembles the direction of fluid transport in the orignal wood (Figure 16). The primary permeability
is lengthwise, but because adjacent tracheids are connected by intervessel pits, fluids may also diffuse
radially. If these pits remain open during early petrifaction, or if tracheid walls become fractured,
mineral-bearing groundwater may migrate laterally. Because of their larger diameter, conductive
vessels in angiosperm wood may remain open after tracheids have become mineralized; subsequent
precipitation episodes may cause these vessels to be mineralized with silica of a different color.
Figure 16. Permineralization commonly proceeds according to the permeability variations, causing
color patterns to reflect the tree’s original anatomy. (A) Miocene wood from Saddle Mountain, Grant
County, Washington, USA; (B) Miocene wood from Hubbard Basin, Elko County, NV, USA.
8.2. Multiple Episodes of Permineralization
Complex color patterns commonly result when wood becomes mineralized in a series of discrete
episodes. Silicification does not always follow a single pathway [26]. The first step typically involves
precipitation of amorphous silica on cell walls, a process of “organic templating” caused by the
molecular affinity of cellulose and lignin for silica [27]. Later steps include filling of intracellular
spaces, cell lumina, and conductive vessels (Figure 17). If these steps in the permineralization process
occur in discrete episodes, the concentration of elements in groundwater may change, along with pH,
eH, and temperature. These variations may cause various anatomical features to be permineralized
in different colors.
Figure 17. Wood may retain directional permeability during early stages of permineralization, as
evidenced by Miocene opalized wood from Nevada, USA. (A,B) Transverse views of wood from
Virgin Valley, Humbold County, showing tracheids with open lumina; (C) Conifer wood from Lyon
County, oblique view of a tracheid showing mineralized cell walls with well-preserved pits and
empty lume; (D) Transverese view of Virgin Valley wood showing mineralized tracheids, with non-
mineralized intercellular spaces.
Figure 16.
Permineralization commonly proceeds according to the permeability variations, causing
color patterns to reflect the tree’s original anatomy. (
A
) Miocene wood from Saddle Mountain, Grant
County, Washington, USA; (B) Miocene wood from Hubbard Basin, Elko County, NV, USA.

Geosciences 2016, 6, 25 13 of 24
8.2. Multiple Episodes of Permineralization
Complex color patterns commonly result when wood becomes mineralized in a series of discrete
episodes. Silicification does not always follow a single pathway [26]. The first step typically involves
precipitation of amorphous silica on cell walls, a process of “organic templating” caused by the
molecular affinity of cellulose and lignin for silica [
27
]. Later steps include filling of intracellular spaces,
cell lumina, and conductive vessels (Figure 17). If these steps in the permineralization process occur
in discrete episodes, the concentration of elements in groundwater may change, along with pH, eH,
and temperature. These variations may cause various anatomical features to be permineralized in
different colors.
Geosciences 2016, 6, 25 12 of 23
resembles the direction of fluid transport in the orignal wood (Figure 16). The primary permeability
is lengthwise, but because adjacent tracheids are connected by intervessel pits, fluids may also diffuse
radially. If these pits remain open during early petrifaction, or if tracheid walls become fractured,
mineral-bearing groundwater may migrate laterally. Because of their larger diameter, conductive
vessels in angiosperm wood may remain open after tracheids have become mineralized; subsequent
precipitation episodes may cause these vessels to be mineralized with silica of a different color.
Figure 16. Permineralization commonly proceeds according to the permeability variations, causing
color patterns to reflect the tree’s original anatomy. (A) Miocene wood from Saddle Mountain, Grant
County, Washington, USA; (B) Miocene wood from Hubbard Basin, Elko County, NV, USA.
8.2. Multiple Episodes of Permineralization
Complex color patterns commonly result when wood becomes mineralized in a series of discrete
episodes. Silicification does not always follow a single pathway [26]. The first step typically involves
precipitation of amorphous silica on cell walls, a process of “organic templating” caused by the
molecular affinity of cellulose and lignin for silica [27]. Later steps include filling of intracellular
spaces, cell lumina, and conductive vessels (Figure 17). If these steps in the permineralization process
occur in discrete episodes, the concentration of elements in groundwater may change, along with pH,
eH, and temperature. These variations may cause various anatomical features to be permineralized
in different colors.
Figure 17. Wood may retain directional permeability during early stages of permineralization, as
evidenced by Miocene opalized wood from Nevada, USA. (A,B) Transverse views of wood from
Virgin Valley, Humbold County, showing tracheids with open lumina; (C) Conifer wood from Lyon
County, oblique view of a tracheid showing mineralized cell walls with well-preserved pits and
empty lume; (D) Transverese view of Virgin Valley wood showing mineralized tracheids, with non-
mineralized intercellular spaces.
Figure 17.
Wood may retain directional permeability during early stages of permineralization, as
evidenced by Miocene opalized wood from Nevada, USA. (
A
,
B
) Transverse views of wood from Virgin
Valley, Humbold County, showing tracheids with open lumina; (
C
) Conifer wood from Lyon County,
oblique view of a tracheid showing mineralized cell walls with well-preserved pits and empty lume;
(
D
) Transverese view of Virgin Valley wood showing mineralized tracheids, with non-mineralized
intercellular spaces.
8.3. Diffusion
Percolation of groundwater in ancient wood affects color in several ways. In wood that retains
permeability, transport of colorants may involve longtitudinal and radial diffusion. Unlike the initial
directional infiltration described above, these diffusion zones may cause more generalized color
patterns (Figure 18).
Geosciences 2016, 6, 25 13 of 23
8.3. Diffusion
Percolation of groundwater in ancient wood affects color in several ways. In wood that retains
permeability, transport of colorants may involve longtitudinal and radial diffusion. Unlike the initial
directional infiltration described above, these diffusion zones may cause more generalized color
patterns (Figure 18).
Figure 18. Diffusion of mineral-bearing groundwater along wood grain may cause color patterns.
(A,B) Miocene conifer wood, Saddle Mountain, Grant County, Washington, USA; (C) Miocene Prunus
wood, McDermitt, Malhuer County, OR, USA.
8.4. Bleaching
Previously-precipitated compounds may be dissolved during subsequent episodes of
groundwater infiltration, producing a local lightening of color. External surfaces may simply be
bleached as a result of removal of colorants, but in interior regions dissolved colorants may be
reprecipitated to create a darkened halo bordering the bleached area (Figure 19).
Figure 19. Miocene wood from McDermitt, Malheur County, Oregon, USA, showing bleached zones
following fractures and voids caused by wood rot. (A), Longitudinal view; (B), Transverse view.
8.5. Chromatography
Color variations created by remobilization of colorants suggests the possibility that trace
elements can be segregated by a process of natural chromatography, where soluble components
Figure 18.
Diffusion of mineral-bearing groundwater along wood grain may cause color patterns.
(
A
,
B
) Miocene conifer wood, Saddle Mountain, Grant County, Washington, USA; (
C
) Miocene Prunus
wood, McDermitt, Malhuer County, OR, USA.

Geosciences 2016, 6, 25 14 of 24
8.4. Bleaching
Previously-precipitated compounds may be dissolved during subsequent episodes of
groundwater infiltration, producing a local lightening of color. External surfaces may simply be
bleached as a result of removal of colorants, but in interior regions dissolved colorants may be
reprecipitated to create a darkened halo bordering the bleached area (Figure 19).
Geosciences 2016, 6, 25 13 of 23
8.3. Diffusion
Percolation of groundwater in ancient wood affects color in several ways. In wood that retains
permeability, transport of colorants may involve longtitudinal and radial diffusion. Unlike the initial
directional infiltration described above, these diffusion zones may cause more generalized color
patterns (Figure 18).
Figure 18. Diffusion of mineral-bearing groundwater along wood grain may cause color patterns.
(A,B) Miocene conifer wood, Saddle Mountain, Grant County, Washington, USA; (C) Miocene Prunus
wood, McDermitt, Malhuer County, OR, USA.
8.4. Bleaching
Previously-precipitated compounds may be dissolved during subsequent episodes of
groundwater infiltration, producing a local lightening of color. External surfaces may simply be
bleached as a result of removal of colorants, but in interior regions dissolved colorants may be
reprecipitated to create a darkened halo bordering the bleached area (Figure 19).
Figure 19. Miocene wood from McDermitt, Malheur County, Oregon, USA, showing bleached zones
following fractures and voids caused by wood rot. (A), Longitudinal view; (B), Transverse view.
8.5. Chromatography
Color variations created by remobilization of colorants suggests the possibility that trace
elements can be segregated by a process of natural chromatography, where soluble components
Figure 19.
Miocene wood from McDermitt, Malheur County, Oregon, USA, showing bleached zones
following fractures and voids caused by wood rot. (A), Longitudinal view; (B), Transverse view.
8.5. Chromatography
Color variations created by remobilization of colorants suggests the possibility that trace elements
can be segregated by a process of natural chromatography, where soluble components separate into
discrete zones as they travel through porous substrate. Two laboratory applications of chromatography
may be relevant. Paper chromatography uses a sheet of filter paper as the stationary phase, akin to the
porous cellulose/lignin structure of unmineralized wood. Thin-layer chromatography employs a glass
plate coated with inert powder (commonly silica gel), similar to the porous nature of wood that is in
early stages of silicification. In both types of chromatography, the separation of constituent compounds
is controlled by differences in polarity between the individual compounds, the solvent that transports
them, and the porous solid. In natural environments, the solvent is water, a strongly polar material.
In paper chromatography, polar molecules bind to the polar cellulose fibers of the absorbant paper.
In thin-layer chromatography, silica gel is likewise a polar substance, producing similar separation
patterns. Chromatographic separation has not previously been suggested as a cause of silicified wood
color patterns, but the processes appears to be common (Figure 20).

Geosciences 2016, 6, 25 15 of 24
Geosciences 2016, 6, 25 14 of 23
separate into discrete zones as they travel through porous substrate. Two laboratory applications of
chromatography may be relevant. Paper chromatography uses a sheet of filter paper as the stationary
phase, akin to the porous cellulose/lignin structure of unmineralized wood. Thin-layer
chromatography employs a glass plate coated with inert powder (commonly silica gel), similar to the
porous nature of wood that is in early stages of silicification. In both types of chromatography, the
separation of constituent compounds is controlled by differences in polarity between the individual
compounds, the solvent that transports them, and the porous solid. In natural environments, the
solvent is water, a strongly polar material. In paper chromatography, polar molecules bind to the
polar cellulose fibers of the absorbant paper. In thin-layer chromatography, silica gel is likewise a
polar substance, producing similar separation patterns. Chromatographic separation has not
previously been suggested as a cause of silicified wood color patterns, but the processes appears to
be common (Figure 20).
Figure 20. When trace elements are mobilized and infiltration of groundwater, development of bands
may result from a process of chromatographic separation. Samples of Miocene wood from Cherry
Creek, White Pine County, NV, USA. (A,B), Longitudinal views; (C) Transverse view , with close-up
image (D).
8.6. Internal Voids and Open Fractures
Petrifaction commonly begins with precipitation of silica in small spaces: cell wall interstices,
followed by tracheid lumina and intercellular spaces [26]. Larger openings (e.g., vessels), and voids
left by decay or fracturing may remain open long after the wood has reached a stage of initial
silicification. These openings may allow later episodes of groundwater infiltration, when changes in
trace element abundance or changes in pH/eH/temperature influence coloration (Figure 21).
Figure 20.
When trace elements are mobilized and infiltration of groundwater, development of bands
may result from a process of chromatographic separation. Samples of Miocene wood from Cherry
Creek, White Pine County, NV, USA. (
A
,
B
), Longitudinal views; (
C
) Transverse view , with close-up
image (D).
8.6. Internal Voids and Open Fractures
Petrifaction commonly begins with precipitation of silica in small spaces: cell wall interstices,
followed by tracheid lumina and intercellular spaces [
26
]. Larger openings (e.g., vessels), and voids left
by decay or fracturing may remain open long after the wood has reached a stage of initial silicification.
These openings may allow later episodes of groundwater infiltration, when changes in trace element
abundance or changes in pH/eH/temperature influence coloration (Figure 21).
Geosciences 2016, 6, 25 15 of 23
Figure 21. Red patterns in Miocene wood from Hubbard Basin, Elko County, Nevada, USA, follow
open voids created by decay and along fractures; (A) Transverse view, with close-up image (B).
In later stages of petrifaction, widespead silicification may greatly limit permeability. This
permineralization increases the brittleness of the fossilized wood; stresses may result in fracture
zones that may provide conduits for fluid. Groundwater penetrating along these fractures can
produced localized bleaching, precipitation, or chromatographic redistribution. These processes may
produce a heterogeneous distribution within a single specimen (Figure 22).
Figure 22. Red zones follow fractures in Triassic wood from the Chinle Formation, Holbrook, Navajo
County, Arizona, USA. (A, B), Transverse views showing coloration following radial fractures; (C),
Coloration following radial fractures and extending along interfaces between adjacent annual rings..
If fractures become filled with silica, wood color may become compartmentalized, where certain
regions are no longer subject to infiltration by groundwater, even though the material within these
zones retains permeability (Figure 23).
Figure 21.
Red patterns in Miocene wood from Hubbard Basin, Elko County, Nevada, USA, follow
open voids created by decay and along fractures; (A) Transverse view, with close-up image (B).
In later stages of petrifaction, widespead silicification may greatly limit permeability. This
permineralization increases the brittleness of the fossilized wood; stresses may result in fracture zones

Geosciences 2016, 6, 25 16 of 24
that may provide conduits for fluid. Groundwater penetrating along these fractures can produced
localized bleaching, precipitation, or chromatographic redistribution. These processes may produce a
heterogeneous distribution within a single specimen (Figure 22).
Geosciences 2016, 6, 25 15 of 23
Figure 21. Red patterns in Miocene wood from Hubbard Basin, Elko County, Nevada, USA, follow
open voids created by decay and along fractures; (A) Transverse view, with close-up image (B).
In later stages of petrifaction, widespead silicification may greatly limit permeability. This
permineralization increases the brittleness of the fossilized wood; stresses may result in fracture
zones that may provide conduits for fluid. Groundwater penetrating along these fractures can
produced localized bleaching, precipitation, or chromatographic redistribution. These processes may
produce a heterogeneous distribution within a single specimen (Figure 22).
Figure 22. Red zones follow fractures in Triassic wood from the Chinle Formation, Holbrook, Navajo
County, Arizona, USA. (A, B), Transverse views showing coloration following radial fractures; (C),
Coloration following radial fractures and extending along interfaces between adjacent annual rings..
If fractures become filled with silica, wood color may become compartmentalized, where certain
regions are no longer subject to infiltration by groundwater, even though the material within these
zones retains permeability (Figure 23).
Figure 22.
Red zones follow fractures in Triassic wood from the Chinle Formation, Holbrook,
Navajo County, Arizona, USA. (
A, B
), Transverse views showing coloration following radial fractures;
(
C
), Coloration following radial fractures and extending along interfaces between adjacent annual rings.
If fractures become filled with silica, wood color may become compartmentalized, where certain
regions are no longer subject to infiltration by groundwater, even though the material within these
zones retains permeability (Figure 23).
Geosciences 2016, 6, 25 16 of 23
Figure 23. (A), Geometric color patterns caused when areas of Miocene wood have been isolated by
chalcedony-filled fractures); (B) Close-up view of distinct color boundaries controlled by silicified
fractures, marked with arrows. Specimen from Rogers Mountain, Lynn County, Oregon, USA. (A),
8.7. Oxidation/Reduction
In the above examples, percolation of groundwater through permeable wood produced color
changes by local precipitation or dissolution of mineral pigments. However, color changes may be
caused by changes in redox conditions. In this case, the concentration of mineral pigments may
remain constant, but changes in their oxidation states may cause color change. This color change is
evident in specimens from Hampton Butte, Oregon, USA. Despite a long history of avocational
collecting, the locality has not been studied in detail. Silicified logs show evidence of decay prior to
fossilization; permineralized tissues commonly are highly brecciated, showing multiple episodes of
silica deposition. Specimens may be light brown, dark green, or bright red, with void spaces filled
with white quartz or chalcedony. Variations in structure and coloration are evidence of a complex
diagenetic history, but green is the most common color for Hampton Butte speciemns. Specimens
may show abrupt color transitions or complex variegations (Figure 24). LA-ICP-MS analyses show
that Fe is the dominant trace element in both red and green phases, suggesting that the color
variations result from localized oxidation /reduction effects.
A different type of oxidation occurs in Triassic wood from the Chinle Formation in Arizona,
USA. Specimens, including intact logs and small wood fragments, commonly show red exterior zones
indicative of the presence of iron oxides. This red coloration commonly extends into the interior
regions along fractures. This phenomenon is evidence that the red iron infiltration occurred late in
the diagenetic history, when the wood had been permineralized to a state of brittleness. However,
the “fuzzy” color patterns bordering the fractures indicates that the wood still retained some degree
of permeability (Figures 22 and 23). Figure 25 shows an Oregon specimen where iron-bearing
solutions absorbed along wood grain show three color zones. LA-ICP-MS analyses show that Chinle
Formation color variations are caused by the abundance of Fe, with the exception of bright green
wood colored by Cr. Color variations are presumably caused by differences in concentration and
oxidation state of Fe, but the Arizona wood color patterns show considerably more gradation than
specimens from Hampton Butte, Oregon, USA. Perhaps these color transitions originated in early
stages of permineralization, when precipitation, remobilization, and oxidation/reduction effects
occurred at a time when the wood retained a degree of longitudinal permeability (Figures 26 and 27).
Figure 23.
(
A
), Geometric color patterns caused when areas of Miocene wood have been isolated by
chalcedony-filled fractures);
(B
) Close-up view of distinct color boundaries controlled by silicified
fractures, marked with arrows. Specimen from Rogers Mountain, Lynn County, Oregon, USA.
8.7. Oxidation/Reduction
In the above examples, percolation of groundwater through permeable wood produced color
changes by local precipitation or dissolution of mineral pigments. However, color changes may be

Geosciences 2016, 6, 25 17 of 24
caused by changes in redox conditions. In this case, the concentration of mineral pigments may remain
constant, but changes in their oxidation states may cause color change. This color change is evident in
specimens from Hampton Butte, Oregon, USA. Despite a long history of avocational collecting, the
locality has not been studied in detail. Silicified logs show evidence of decay prior to fossilization;
permineralized tissues commonly are highly brecciated, showing multiple episodes of silica deposition.
Specimens may be light brown, dark green, or bright red, with void spaces filled with white quartz or
chalcedony. Variations in structure and coloration are evidence of a complex diagenetic history, but
green is the most common color for Hampton Butte speciemns. Specimens may show abrupt color
transitions or complex variegations (Figure 24). LA-ICP-MS analyses show that Fe is the dominant
trace element in both red and green phases, suggesting that the color variations result from localized
oxidation /reduction effects.
Geosciences 2016, 6, 25 17 of 23
Figure 24. Eocene wood from Hampton Butte, Deschutes County, Oregon, USA, commonly contains
green/red patterns caused by localized oxidation/reduction of Fe. (A, Light colored zones are
chalcedony deposited during a late stage of mineralization; (B–D), Red and green zones have well-
defined contacts; (E) variegated pattern comprised of green wood, red oxidation, and translucent
chalcedony; (F), Red and green zones show complex intermixing.
Figure 25. Iron-bearing solutions infiltrating along fibers in Miocene wood from central Oregon, USA,
show a redox gradient that produces well-defined color zones.
Figure 26. Triassic “Araucarioxylon” wood from Chinle Formation, Holbrook, Navajo County,
Arizona, USA. Red oxidized exterior, showing internal infiltration along perimeter fractures.
Figure 24.
Eocene wood from Hampton Butte, Deschutes County, Oregon, USA, commonly contains
green/red patterns caused by localized oxidation/reduction of Fe. (
A
, Light colored zones are
chalcedony deposited during a late stage of mineralization; (
B
–
D
), Red and green zones have
well-defined contacts;
(E
) variegated pattern comprised of green wood, red oxidation, and translucent
chalcedony; (F), Red and green zones show complex intermixing.
A different type of oxidation occurs in Triassic wood from the Chinle Formation in Arizona,
USA. Specimens, including intact logs and small wood fragments, commonly show red exterior zones
indicative of the presence of iron oxides. This red coloration commonly extends into the interior
regions along fractures. This phenomenon is evidence that the red iron infiltration occurred late in
the diagenetic history, when the wood had been permineralized to a state of brittleness. However, the
“fuzzy” color patterns bordering the fractures indicates that the wood still retained some degree of
permeability (Figures 22 and 23). Figure 25 shows an Oregon specimen where iron-bearing solutions
absorbed along wood grain show three color zones. LA-ICP-MS analyses show that Chinle Formation
color variations are caused by the abundance of Fe, with the exception of bright green wood colored by
Cr. Color variations are presumably caused by differences in concentration and oxidation state of Fe,
but the Arizona wood color patterns show considerably more gradation than specimens from Hampton
Butte, Oregon, USA. Perhaps these color transitions originated in early stages of permineralization,
when precipitation, remobilization, and oxidation/reduction effects occurred at a time when the wood
retained a degree of longitudinal permeability (Figures 26 and 27).

Geosciences 2016, 6, 25 18 of 24
Geosciences 2016, 6, 25 17 of 23
Figure 24. Eocene wood from Hampton Butte, Deschutes County, Oregon, USA, commonly contains
green/red patterns caused by localized oxidation/reduction of Fe. (A, Light colored zones are
chalcedony deposited during a late stage of mineralization; (B–D), Red and green zones have well-
defined contacts; (E) variegated pattern comprised of green wood, red oxidation, and translucent
chalcedony; (F), Red and green zones show complex intermixing.
Figure 25. Iron-bearing solutions infiltrating along fibers in Miocene wood from central Oregon, USA,
show a redox gradient that produces well-defined color zones.
Figure 26. Triassic “Araucarioxylon” wood from Chinle Formation, Holbrook, Navajo County,
Arizona, USA. Red oxidized exterior, showing internal infiltration along perimeter fractures.
Figure 25.
Iron-bearing solutions infiltrating along fibers in Miocene wood from central Oregon, USA,
show a redox gradient that produces well-defined color zones.
Geosciences 2016, 6, 25 17 of 23
Figure 24. Eocene wood from Hampton Butte, Deschutes County, Oregon, USA, commonly contains
green/red patterns caused by localized oxidation/reduction of Fe. (A, Light colored zones are
chalcedony deposited during a late stage of mineralization; (B–D), Red and green zones have well-
defined contacts; (E) variegated pattern comprised of green wood, red oxidation, and translucent
chalcedony; (F), Red and green zones show complex intermixing.
Figure 25. Iron-bearing solutions infiltrating along fibers in Miocene wood from central Oregon, USA,
show a redox gradient that produces well-defined color zones.
Figure 26. Triassic “Araucarioxylon” wood from Chinle Formation, Holbrook, Navajo County,
Arizona, USA. Red oxidized exterior, showing internal infiltration along perimeter fractures.
Figure 26.
Triassic “Araucarioxylon” wood from Chinle Formation, Holbrook, Navajo County, Arizona,
USA. Red oxidized exterior, showing internal infiltration along perimeter fractures.
Geosciences 2016, 6, 25 18 of 23
Figure 27. Longitudinal view of Araucarioxylon wood from Holbrook, Arizona, USA, showing diffuse
color transformations developed along wood grain directions.
8.8. Relict Organic Matter
The amount of original organic matter that remains after permineralization is variable, but
generally rather small [28]. However, at some localities, relict organic constituents may be a major
cause of silicified wood color. Most commonly, the original consituents of wood, primarily lignin and
hemicellulose, have been altered to carbonaceous degradation products, producing dark brown hues
(Figure 28). These compounds can be detected in several simple ways. One method is to heat the
powdered fossil wood under aerobic conditions at 450 °C for a few hours. Combustion of organic
matter causes the color of the powder to lighten to tan or white. In woods silicified with quartz or
chalcedony, the percentage of original wood that remained after fossilization can be quantitatively
estimated [28]. Soaking the specimen in 8% sodium hypochlorate solution for a few hours also causes
bleaching The presence of organic matter can also be evaluated using X-ray fluorescence
spectrometry in combination with scanning electron microscopy (SEM/EDS), based on the magnitude
of the carbon X-ray peak.
Figure 28. Eocene Sequoiaoxylon wood from Tom Miner Basin, Gallatin County, Montana, USA. Wood
fragmented by fungal rot (brown rot) prior to fossilization is dark because of relict carbon.
Chalcedony filling voids is white or gray.
9. Conclusions
LA-ICP-MS analyses of trace elements shows that most pastel and bright colors are caused by
Fe in varying abundances and oxidation states. Iron is likely present as nanoparticle-sized
ferruginous minerals. Dark green colors may result from ferrous iron minerals, but bright green hues
Figure 27.
Longitudinal view of Araucarioxylon wood from Holbrook, Arizona, USA, showing diffuse
color transformations developed along wood grain directions.

Geosciences 2016, 6, 25 19 of 24
8.8. Relict Organic Matter
The amount of original organic matter that remains after permineralization is variable, but
generally rather small [
28
]. However, at some localities, relict organic constituents may be a major
cause of silicified wood color. Most commonly, the original consituents of wood, primarily lignin and
hemicellulose, have been altered to carbonaceous degradation products, producing dark brown hues
(Figure 28). These compounds can be detected in several simple ways. One method is to heat the
powdered fossil wood under aerobic conditions at 450
˝
C for a few hours. Combustion of organic
matter causes the color of the powder to lighten to tan or white. In woods silicified with quartz or
chalcedony, the percentage of original wood that remained after fossilization can be quantitatively
estimated [
28
]. Soaking the specimen in 8% sodium hypochlorate solution for a few hours also causes
bleaching The presence of organic matter can also be evaluated using X-ray fluorescence spectrometry
in combination with scanning electron microscopy (SEM/EDS), based on the magnitude of the carbon
X-ray peak.
Geosciences 2016, 6, 25 18 of 23
Figure 27. Longitudinal view of Araucarioxylon wood from Holbrook, Arizona, USA, showing diffuse
color transformations developed along wood grain directions.
8.8. Relict Organic Matter
The amount of original organic matter that remains after permineralization is variable, but
generally rather small [28]. However, at some localities, relict organic constituents may be a major
cause of silicified wood color. Most commonly, the original consituents of wood, primarily lignin and
hemicellulose, have been altered to carbonaceous degradation products, producing dark brown hues
(Figure 28). These compounds can be detected in several simple ways. One method is to heat the
powdered fossil wood under aerobic conditions at 450 °C for a few hours. Combustion of organic
matter causes the color of the powder to lighten to tan or white. In woods silicified with quartz or
chalcedony, the percentage of original wood that remained after fossilization can be quantitatively
estimated [28]. Soaking the specimen in 8% sodium hypochlorate solution for a few hours also causes
bleaching The presence of organic matter can also be evaluated using X-ray fluorescence
spectrometry in combination with scanning electron microscopy (SEM/EDS), based on the magnitude
of the carbon X-ray peak.
Figure 28. Eocene Sequoiaoxylon wood from Tom Miner Basin, Gallatin County, Montana, USA. Wood
fragmented by fungal rot (brown rot) prior to fossilization is dark because of relict carbon.
Chalcedony filling voids is white or gray.
9. Conclusions
LA-ICP-MS analyses of trace elements shows that most pastel and bright colors are caused by
Fe in varying abundances and oxidation states. Iron is likely present as nanoparticle-sized
ferruginous minerals. Dark green colors may result from ferrous iron minerals, but bright green hues
Figure 28.
Eocene Sequoiaoxylon wood from Tom Miner Basin, Gallatin County, Montana, USA. Wood
fragmented by fungal rot (brown rot) prior to fossilization is dark because of relict carbon. Chalcedony
filling voids is white or gray.
9. Conclusions
LA-ICP-MS analyses of trace elements shows that most pastel and bright colors are caused by Fe
in varying abundances and oxidation states. Iron is likely present as nanoparticle-sized ferruginous
minerals. Dark green colors may result from ferrous iron minerals, but bright green hues correlate with
elevated levels of chromium. Relict organic matter may influence silicified wood color, particularly in
brown specimens.
The distribution of colorants within a single specimen may be caused by a variety of factors, which
sometimes combine to produce patterns of great complexity (Figure 29). Because wood commonly
becomes mineralized in a series of steps, changes in groundwater geochemistry may cause varying
colors to be precipitated during successive episodes of mineralization. During early stages, wood
may retain directional permeabilty because of cellular architecture. Later, penetration of groundwater
along fractures or other open spaces may cause additional colors to appear. Neither the geologic age of
the specimens or their taxonomy appear to be important for determining petrifaction colors. More
important are the availability of dissolved trace metals in percolating groundwater and a mineralization
sequence that proceeds in sucessive stages.

Geosciences 2016, 6, 25 20 of 24
Geosciences 2016, 6, 25 19 of 23
correlate with elevated levels of chromium. Relict organic matter may influence silicified wood color,
particularly in brown specimens.
The distribution of colorants within a single specimen may be caused by a variety of factors,
which sometimes combine to produce patterns of great complexity (Figure 29). Because wood
commonly becomes mineralized in a series of steps, changes in groundwater geochemistry may cause
varying colors to be precipitated during successive episodes of mineralization. During early stages,
wood may retain directional permeabilty because of cellular architecture. Later, penetration of
groundwater along fractures or other open spaces may cause additional colors to appear. Neither the
geologic age of the specimens or their taxonomy appear to be important for determining petrifaction
colors. More important are the availability of dissolved trace metals in percolating groundwater and
a mineralization sequence that proceeds in sucessive stages.
Figure 29. Triassic conifer wood from Madagascar. Factors that produced complex coloration include
infiltration of silica along wood grain, red/green oxidation/reduction, entry of groundwater along
fractures and rot pockets, and bleaching of exterior surfaces.
This report is based on analysis and observation from fossil wood specimens that span a wide
range of ages and geographic occurrences, but these specimens provide only a miniscule
representation of fossil forests that occur throughout the world. Our data and interpretations will
hopefully provide a starting point for research by other investigators. Possibilities for future research
include quantitative chemical analyses of specimens from other localities and interpretation of the
origin of color patterns. Colorant dispersal mechanisms proposed in this report could also be
investigated under experimental conditions in the laboratory.
Acknowledgments: Western Washington University Advanced Materials Science and Engineering Center lab
managers Polly Berseth and Kyle Mikkelsen provided technical assistance and access to the LA-ICP-MS
laboratory. Specimens were provided for study by Richard Dayvault, Richard Rantz, and Jim Mills. Leah Boam
polished many samples. Jim Mills and Mike Viney provided helpful comments on an early draft. This paper is
dedicated to the memory of Richard Dayvault (1948-2015), a geochemist and petrified wood expert who for
many years generously shared his wealth of knowledge with avocational collectors and professional
geoscientists.
Author Contributions: Both authors participated in experimental design, data acquisition and interpretation,
and preparation of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
Figure 29.
Triassic conifer wood from Madagascar. Factors that produced complex coloration include
infiltration of silica along wood grain, red/green oxidation/reduction, entry of groundwater along
fractures and rot pockets, and bleaching of exterior surfaces.
This report is based on analysis and observation from fossil wood specimens that span a wide
range of ages and geographic occurrences, but these specimens provide only a miniscule representation
of fossil forests that occur throughout the world. Our data and interpretations will hopefully provide a
starting point for research by other investigators. Possibilities for future research include quantitative
chemical analyses of specimens from other localities and interpretation of the origin of color patterns.
Colorant dispersal mechanisms proposed in this report could also be investigated under experimental
conditions in the laboratory.
Acknowledgments:
Western Washington University Advanced Materials Science and Engineering Center lab
managers Polly Berseth and Kyle Mikkelsen provided technical assistance and access to the LA-ICP-MS laboratory.
Specimens were provided for study by Richard Dayvault, Richard Rantz, and Jim Mills. Leah Boam polished
many samples. Jim Mills and Mike Viney provided helpful comments on an early draft. This paper is dedicated
to the memory of Richard Dayvault (1948-2015), a geochemist and petrified wood expert who for many years
generously shared his wealth of knowledge with avocational collectors and professional geoscientists.
Author Contributions:
Both authors participated in experimental design, data acquisition and interpretation, and
preparation of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

Geosciences 2016, 6, 25 21 of 24
Appendix
Table A1. Wood color and trace element content (ppm).
Location Age Color Pantone Color # Ti V Cr Mn Fe Co Ni Cu U
Green Wood
Zimbabwe Triassic Light green 7494C 57 51 10,400 37 2465 12 28 34 1
Winslow AZ 1 Triassic Light green 625C 12 140 9600 38 293 9 8 7 3
Winslow AZ 2 Triassic Light green 624C 427 209 13,000 263 9900 80 176 69 2
Nye Co. NV Miocene Light green 7494C 41 41 6 33 2413 0.2 2 52 51
Holbrook AZ Triassic Dark green 5545C 244 24 10 46 3570 3 16 105 0.1
Nazlini AZ Triassic Dark green 5535C 2280 122 142 1690 16,100 49 30 102 9
Hampton Butte OR Miocene Dark green 560c 141 2 7 30 300 1 11 59 0.3
Hampton Butte OR Miocene Dark green 555c 326 46 31 87 14,280 5 21 33 0.2
Winslow AZ Triassic Dark green 560C 28 63 9030 16 974 10 39 29 2
Red Wood
Holbrook AZ Triassic Red 491C 10 3 4 67 2030 0.1 7 15 8
Holbrook AZ Triassc Red 483C 37 5 1 47 1430 0.3 7 32 -
Nazlini AZ Triassic Red 491C 16 2 5 120 2520 0.3 8 11 15
Hampton Butte OR Miocene Red 499C 16 57 2 7 723 1 6 24 3
Texas Spring NV Miocene Red 484C 4 4 4 18 92 0.1 4 7 4
Madagascar Jurassic Brownish red 484C
Rainbow Colors
Nazlini AZ Triassic Orange 131C 9 1 1 40 1035 0.2 2 6 6
Nazlini AZ Triassic Yellow 127C 3 1 4 22 248 1 5 10 1
Holbrook AZ Triassic Yellow 128C 26 3 1 18 510 0.2 6 37 -
Holbrook AZ Triassic Light purple 505C 19 2 0.3 14 413 0.1 4 14 -
Holbrook AZ Triassic Dark purple 5205C 18 3 5 33 750 0.4 18 37 -
Nazlini AZ Triassic Orange 1385C 9 1 1 40 1035 0.2 2 6 6
Nazlini AZ Triassic Yellow 1215C 3 1 4 22 248 1 5 10 1
Nazlini AZ Triassic Pale pink 217C 2 1 3 9 495 0.0 4 5 0.0

Geosciences 2016, 6, 25 22 of 24
Table A1. Cont.
Location Age Color Pantone Color # Ti V Cr Mn Fe Co Ni Cu U
White, Clear
Nazlini AZ Triassic Clear 1C 8 1 1 3 35 0.1 3 5 8
Holbrook AZ Triassic Clear 1C
Nye Co. NV Miocene Clear 1C 18 2 1 0 0 0 5 28 -
Holbrook AZ Triassic Clear 1C 8 1 1 3 35 0.1 3 5 0.0
Nye Co. NV Miocene White 1C 20 4 7 20 171 0.2 5 13 5
Texas Spring NV Miocene White 7499C 18 3 2 0 21 0 14 26 -
Holbrook Az Triassic Pinkish white 5175C 22 2 2 12 265 0.2 8 36 14
Brown, Black:
Hampton Butte OR Miocene Medium brown 463C 2330 65 33 72 9046 5 23 36 1
Nye Co. NV Miocene Medium brown 463C
Nye Co. NV Miocene Black 447C 28 8 3 9 250 0.3 5 16 3
Holbrook AZ Triassic Black stripe 6C 3 9 23 15 126 0.1 6 10 6
Nye Co. NV Miocene Gold 19 2 3 0 342 0 9 13 -
Texas Spring NV Miocene Dark brown 7519C 6 7 2 2 43 0.1 8 11 5
Nye Co NV Miocene Dark brown 356 847 167 9 5870 6 8 24
Goose Creek NV Miocene Medium Brown 464C 5 1 4 14 133 0.1 9 18 11
Goose Creek NV Miocene Tan 466C 23 2 8 23 225 0.4 22 48 1
Hampton Butte OR Miocene Golden brown 463C 16 57 2 7 723 1 6 24 3

Geosciences 2016, 6, 25 23 of 24
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