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Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model

Discovered in 1841 in Brazil, carbonado was
named by Portuguese diamond prospectors
for its resemblance to charcoal (Leonardos,
1937; Dominguez, 1996). Carbonado was found later
in the Central African Republic. These two localities,
now separated by the Atlantic Ocean and situated on
the São Francisco and the Congo cratons, respec-
tively, previously shared a common geological set-
ting for more than a billion years (De Waele et al.,
2008) on the supercontinent of Rodinia (figure 1) and
its precursor Nuna, also known as Columbia.
Carbonado was prized by the French as a superior
polishing material. It was used for drilling during the
construction of the Panama Canal and formed part
of the U.S. strategic mineral stockpile as recently as
1990. At the height of alluvial mining in Brazil
(1850–1870), some 70,000 carats were produced by
an estimated 30,000 artisanal miners (Svisero, 1995).
A conservative estimate of the recovery from Brazil
and the Central African Republic is approximately 2
metric tons (Haggerty, 2014). Four of the five largest
diamonds reported from Brazil, ranging in weight
from 726 to 3,167 ct, are carbonado (Svisero, 1995).
The largest of the five, the Sergio, recovered in 1905,
is 61 ct heavier than the largest single-crystal dia-
mond ever reported (the 3,106 ct Cullinan rough).
While earlier investigations of carbonado focused
on physical and chemical properties and synthesis,
more recent studies have introduced dating tech-
niques, high-resolution microscopy, spectroscopy,
and an emphasis on origin (see Haggerty, 2014, for a
more comprehensive view). The present study offers
a detailed examination of about 800 carbonados
from Brazil and the Central African Republic (figure
2), ranging from <1 to 730 ct. These samples showed
no significant differences in their texture, superfi-
cial appearance, and physical and chemical proper-
ties (Haggerty, 2014). This article describes the
unusual textural features of carbonado, namely
their pores and the presence of glassy diamond as a
surface patina, with the aim of assessing the origin
of carbonado.
Stephen E. Haggerty
Carbonado diamond is found only in Brazil and the Central African Republic. These unusual diamond
aggregates are strongly bonded and porous, with melt-like glassy patinas unlike any conventional dia-
mond from kimberlites-lamproites, crustal collisional settings, or meteorite impact. Nearly two centuries
after carbonado’s discovery, a primary host rock compatible with the origin of conventional diamond at
high temperatures and pressures has yet to be identified. Models for its genesis are far-reaching and
range from terrestrial subduction to cosmic sources.
In Brief
• Carbonado, found only in Brazil and the Central
African Republic, is distinguished from conventional
diamond by its pores, patina, surfaces, and polycrys-
Although carbonado has been known since 1841, its
origin or host rock has yet to be identified.
The extraterrestrial model of carbonado origin, one of
five theories, posits that it formed from carbon-rich,
diamond-bearing stellar bodies that were transported
to Earth by meteorite about four billion years ago.
See end of article for About the Author and Acknowledgments.
GEMS & GEMOLOGY, Vol. 53, No. 2, pp. 168–179,
© 2017 Gemological Institute of America
Carbonado is typically found in five major size cate-
gories: >200 ct, 75–95 ct, 25–35 ct, 8–15 ct, and 0.25–
1.25 ct (see figure 5 of Haggerty, 2014). Sand-sized
particles (<1 mm) also occur, and melon-size objects
larger than the Sergio are reported but unconfirmed
(M. Ozwaldo, pers. comm., 1996). Carbonados are typ-
ically equidimensional (in millimeter to centimeter
Figure 1. Left: The Congo–São Francisco island in southwest Rodinia, at approximately 1.1 billion years ago (Ga),
is the only known site of carbonado that was originally deposited ca. 3.8 Ga on a possibly even smaller cratonic is-
land. Right: Separation of the microcontinent into two cratonic blocks, now Brazil and the Central African Repub-
lic, took place during the breakup of Gondwanaland about 180 million years ago. Continental masses in Rodinia
are underlain by ancient cratons approximately 2.5 to 4.0 Ga. Green zones are 1.1 Ga mountain belts, and the red
dots are granite intrusions (Torsvik, 2003).
India Australia
China Siberia
1.1 Ga belts
Figure 2. A: Scene from
Chapada Diamantina,
the carbonado site in
Bahia, Brazil. B: Boulder
of Tombodor conglomer-
ate, the carbonado host
rock. C: Polished con-
glomerate surface in a
streambed. D: Artisanal
mining of Brazilian allu-
vial carbonado. Photos
by Robert Weldon (A
and C) and Stephen E.
Haggerty (B and D).
sizes), although some are elongated (figure 3); they
are seldom rounded.
Carbonado is opaque, composed of randomly ori-
ented diamond crystallites that impede light refrac-
tion and increase absorption. Color varies from black
and putty gray to shades of brown (figure 3), deep pur-
ple to pink, rusty red, and the occasional olive green.
Pores (figure 4), an unusual glassy patina (figure 5),
highly irregular surfaces (figures 6 and 7), and poly-
crystallinity (figures 8 and 9) distinguish carbonado
from conventional diamonds.
Porosity. As the porosity of an object increases, its
apparent density decreases, because the voids take
up more and more of the volume. In carbonado, the
number of exposed micro diamond cutting points in-
creases with porosity. This was a sought-after prop-
erty that made carbonado more expensive by weight
than diamond at the turn of the twentieth century
(Haggerty, 2014). Densities as low as 2.8 g/cm3and
as high as 3.45 g/cm3, with most around 3.05 g/cm3
(Trueb and De Wys, 1969; Haggerty, 2014), are in
contrast to gem diamond at 3.52 g/cm3. Calculated
pore concentrations vary between 5% and 15% in
volume. The pores persist into the interior of the
carbonado and are either spherical or oblate. Some
are inferred to be interconnected (Ketcham and Koe-
berl, 2013), but the material’s permeability is very
low because the pores are free of infiltrating hydro -
thermal precipitates that abound in surface pores
(again, see figure 4). The spherical pores in car-
bonado are unlike those in other polycrystalline di-
amond such as framesite, where the open spaces are
at adjoining crystal faces and the shapes are irregu-
larly polyhedral. In other polycrystalline diamonds,
the open spaces are microns in width and either ra-
dial (in non-gem-quality ballas) or parallel (in fibrous
Figure 3. The carbonado in the left (118 ct) and center (16.2–52.2 ct) photos are from the Central African Republic,
and those on the right (10.8–15.1 ct) are from Brazil. Note the high density of pores, some of which are filled at the
surface by crustal infiltrates, and the metallic luster of the glassy melt-like patinas. Photos by Orasa Weldon. GIA
Collection numbers 40108–40119; gift of Stephen Haggerty.
Figure 4. Open pores in
carbonado (first five
photos) and pores cov-
ered by a surface
patina of nanodiamond
approximately 5 μm
thick (lower right).
Photos by Stephen E.
~1 cm
Patina. In carbonado, patina surfaces are pervasive
(figure 5). Pores in contact with surface patinas are
reduced in size, and at 50× magnification they can no
longer be distinguished. Glass-like in appearance and
similar to synthetic carbon glass (de Heer et al.,
2005), these veneers may be dimpled or furrowed,
with mounds and flow structures (figures 5 and 6).
These textures are akin to those seen in melts in vol-
canic rocks or in slags from metal processing. But in
carbonado, the veneers are diamond that appear to
have formed directly from the underlying porous
substrates, although diamond coating at a later time
is also possible. Contact boundaries between pore-
present and pore-absent surfaces are poorly defined,
except in cases where patina crusts have splintered
off where the contact is sharp, as seen in the lower-
right images of figure 6 and in figure 7. Secondary pits
and microcraters are pervasive and, in many cases,
younger than the patina (figure 7). While pores tend
to have sharp outlines (figure 4), craters are rounded
with bubbly surfaces or rimmed by smooth ridges
(again, see figures 6 and 7). The evidence of flow in
both types of voids implies differences in origin. Solid
melt marbles are typical. Microcraters, free of orna-
mentation, grade into texturally soft plastic walls
(figures 5 and 6). Slickensides, the striated surfaces
known to form on rocks that have been forced to
slide along a fracture surface at high pressure as in a
fault (figure 7), are of interest because these could
only have developed on frictional contact with a
body whose hardness was equivalent to another dia-
mond. On the other hand, the patina itself may rep-
resent frictional melting (e.g., de Heer et al., 2005;
Mitchell et al., 2016; Shumilova et al., 2016a,b; Shiell
et al., 2016). Standard diamond testers that measure
thermal conductivity give a sharp response to glassy
Figure 5. Typical melt-
like patinas and flow
ornamentation on car-
bonado. Photos by
Stephen E. Haggerty.
Figure 6. Melt marbles
and flow patterns on
carbonado. Photos by
Stephen E. Haggerty.
~1 cm
~1 cm
diamond surfaces, less so to the ridges and mounds.
The pore-rich surfaces are distinctly sluggish and er-
ratic in response, possibly due to crystal discontinu-
ities of microdiamond grain boundaries.
Octahedra, dodecahedra, tetrahexahedra, and fibrous
cubes, all typical of conventional diamond (e.g.,
Orlov, 1977), are not observed in carbonado. Poly-
crystalline cubes measuring 5 to approximately 20
µm are common. Encased in very fine diamond (<1–
5 μm), the matrix is tightly fused with angular inter-
stices and rounded pores (figure 8). Scanning electron
microscopy (SEM) images illustrate the distribution
of diamond cleavage surfaces, hopper crystals, skele-
tal crystallites, re-entrant intergrowths, and layers in
single crystals in the open-space pores of carbonado
(figures 8 and 9). Trueb and De Wys (1969) and Petro-
vsky et al. (2010) suggest that the closest analogy to
carbonado textures is in synthetically compressed
nanodiamond aggregates. Because these structures
are found in pores, a more reasonable comparison is
with vapor deposition of diamond. The preferred
crystal habit of these diamonds is cuboidal, either as
single solid cubes or as interpenetrating twins on
[111] that follow the fluorite twin law (figure 8). The
solid cubes are colorless and, although fine grained,
appear to be translucent. Diamond cubes and cuboc-
tahedra are routinely synthesized in metallic cata-
lysts at high pressure and temperature (Burns and
Davies, 1992) or by chemical vapor deposition (CVD)
under high vacuum and at plasma temperatures (Sato
and Kamo, 1992).
X-ray diffraction (XRD) data on crushed car-
bonado grains are similar to conventional diamonds.
Hardness is also similar, but there are data indicating
Figure 8. Brightly reflecting phenocrystic diamond cubes (top row) and twinned diamond clusters (circled)
in carbonado. SEM images are black and white. Photomicrographs by Stephen E. Haggerty; SEM images by
Sven P. Holbik.
Figure 7. Slickenside
patterns and melting of
vesicular carbonado
with later pits and cra-
tering. The melt layer is
about 20 μm thick.
Photos by Stephen E.
~1 cm
~1 cm
that carbonado is slightly harder (Haggerty, 2014). Its
toughness and tenacity, stemming from the random
orientation of microdiamonds, are clearly superior to
monocrystalline gem diamond, to the point that car-
bonado can only be cut by lasers.
Yet another unusual feature of carbonado is the
presence of an exotic array of metals (Fe, Ni, Cr, and
Ti), metal alloys (Fe-Ni, Fe-Cr, Ni-Cr, and W-Fe-Cr-V),
and very unusual minerals, specifically moissanite
(SiC) and osbornite (TiN). These phases occur as pri-
mary intergranular inclusions or as crystal-controlled
oriented intergrowths. They are only stable at the very
low oxidation states (Gorshkov et al., 1996; De et al.,
1998; Makeev et al., 2002; Jones et al., 2003) that
would occur deep within Earth’s mantle or other re-
ducing environments such as outer space. By contrast,
surface pores and fractures are filled by secondary,
low-temperature minerals such as quartz and highly
oxidized magnetite, goethite, florencite, and goyazite
(Trueb and Butterman, 1969), typical of a more oxi-
dized terrestrial surface growth environment.
Relative to mantle-derived diamonds, carbonado
is isotopically light, with δ13C = –24 to –31‰ (Ozima
et al., 1991; Shelkov et al., 1997; De et al., 2001). Ni-
trogen concentrations are low (~20 to 500 ppmw),
and δ15N ranges from –3.6 to 12.8‰ with an average
of 3.7‰ (Shelkov et al., 1997; Vicenzi and Heaney,
2001; Yokochi et al., 2008). The coupled isotopic dis-
tribution of C and N shows that the compositional
field for carbonado is distinctly different from that of
conventional diamonds (figure 10).
Figure 11 shows photoluminescence (PL) spectra
of carbonado, which are similar to those of irradiated
and heated CVD diamond (Clark et al., 1992). The
characteristic peaks at 1.945 eV and 2.156 eV are at-
tributed to nitrogen vacancy (NV) defects in type Ib
diamonds. Wang et al. (2009) report a substantial
amount of nonaggregated N in type Ib diamonds with
H2 and H3 defects. Hydrogen-containing defects (H1)
and NV defects are also reported by Nadolinny et al.
Cathodoluminescence of large (approximately 200
μm) monocrystals of diamond in carbonado exhibit or-
ange and green tones (Magee and Taylor, 1999; De et
al., 2001; Yokochi et al., 2008). However, blue lumi-
nescence in large diamonds, embedded in an orange
luminescent matrix of submicron diamond, are also
reported (Rondeau et al., 2008). The range in colors is
attributed to various N-V (nitrogen-vacancy) defects.
Synchrotron infrared measurements of carbonado
have shown the presence of single nitrogen (type Ib)
substitution and hydrogen (Garai et al., 2006), in con-
trast to aggregated N typical of conventional type Ia
diamonds that have undergone prolonged high P-T
annealing in the mantle.
Carbonado has been dated by Ozima and Tat-
sumoto (1997) and Sano et al. (2002), on samples de-
rived from conglomerates (again, see figure 2) that
Figure 9. SEM images of euhedral diamonds displaying a variety of morphologies in parallel growth typical of
vapor-deposited clusters in the open-space pores of carbonado. Images by Sven P. Holbik.
have been reworked over a period from at least 1.7
Ga to approximately 3.8 Ga (Pedreira and De Waele,
2008). It is relevant to note that, unlike the dating of
conventional diamond, which is based on trapped
mineral inclusions (garnet, pyroxene, and sulfides), the
age of carbonado discussed in this review was deter-
mined directly on diamond. Following a robust chem-
ical protocol of acid dissolution to remove all
nondiamond material, the cleansed carbonado was
subjected to two different instrumental methods of
analyses. Ozima and Tatsumoto (1997) used high-res-
olution mass spectrometry on carat-sized samples
from the Central African Republic, while Sano et al.
(2002) employed an ion probe that allowed for micron-
sized spot analyses on larger samples from Brazil. Both
studies report ages of 2.6–3.8 Ga on implanted radi-
ogenic lead. Although this method of age determina-
tion is unconventional, it is important to note that the
Archean result is consistent with trapped crustal in-
clusions (Sano et al., 2002) of zircon (1.7–3.6 Ga), rutile
(3.9 Ga), and quartz (3.2 Ga), and with the antiquity of
the basement in the São Francisco craton, which is
3.3–3.7 Ga (Barbosa and Sabate, 2004).
In summary, the chemical and physical character-
istics of carbonado point to marked similarities with
rapidly quenched type Ib diamonds and CVD dia-
mond, both of which contain significant hydrogen.
But there are also major differences: carbonado has
pores and patinas with distinctions in C and N iso-
topes, an absence of mantle minerals, and the pres-
ence of exotic metal inclusions. Carbonado is
unquestionably one of the most unusual forms of di-
amond ever reported. Because it has never been found
in typical diamond-bearing rocks, the many proposed
origins are varied, and none are uniformly accepted.
Theories on the genesis of carbonado fall into five
1. Meteoritic impact (Smith and Dawson, 1985)
2. Growth and sintering in the crust or mantle
(Burgess et al., 1998; Ishibashi et al., 2012; Chen
and Van Tendeloo, 1999; Heaney et al., 2005;
Kagi and Fukura, 2008; Ketcham and Koeberl,
3. Subduction (De Carli, 1997; Irifune et al., 2004)
4. Radioactive ion implantation of carbon sub-
strates (Kaminsky, 1991; Ozima et al., 1991;
Shibata et al., 1993; Kagi et al., 1994; Daulton
and Ozima, 1996; Ozima and Tatsumoto, 1997)
5. Extraterrestrial (Haggerty, 1996, 2014)
Figure 10. A paired stable
isotope plot of C vs. N for
conventional diamond (top)
and carbonado (bottom).
The compositional separa-
tion shows that carbonado
and deep Earth diamonds
are unrelated. Open sym-
bols are for eclogitic dia-
monds from Kim berley,
South Africa; filled symbols
are for diamonds from Jwa-
neng, Bot swana; both show
extreme variations. The
fields for peridotitic (typical
inclu sions are olivine,
clinopyroxene, and orthopy-
roxene), eclogitic (garnet
and clinopyroxene), and fi-
brous diamonds are from a
global database. Data for
conventional diamonds are
from Cartigny et al. (1998).
δ13C (‰)
δ15N (‰)
15105-5-15-20 0-10
Transitional types
Peridotitic eld
Approximate eld
for carbonado
Fibrous diamonds
(Gt + Cpx)
(Ol + Cpx + Opx)
“low δ13C” group
N = –3 to 12.8‰
C = –24 to –31‰
Meteoritic Impact. This model was based on a cor-
relation with the Bangui magnetic anomaly in the
Central African Republic. Originally thought to be a
buried iron meteorite, it was subsequently shown to
be a crustal-derived banded iron ore body (Regan and
Marsh, 1982), similar to the magnetic anomaly and
giant iron ore deposit in Kursk, Russia (Taylor et al.,
2014). Because the C-isotopic composition of car-
bonado is very light (δ13C = –21 to –34‰), the pres-
ence of biologically derived organic material in the
target rocks is assumed. The impact model is un-
likely because the C substrate, necessarily of
cyanobacteria at ~3.8 Ga, would have been inordi-
nately large (estimated at several cubic km and un-
contaminated by crustal material), to account for the
estimated two metric tons of carbonado recovered to
date (Haggerty, 2014). In addition, the known occur-
rences of meteorite-impact diamonds (Arizona,
United States; Ries, Germany; and Popigai, Russia)
are discrete microdiamonds rather than carbonado
(Frondel and Marvin, 1967; Hough et al., 1995;
Shelkov et al., 1997).
Growth and Sintering in the Crust or Mantle. Some
models propose catalytically assisted C-saturated
“fluids” in the crust or the mantle. Such fluids pro-
vide a source of carbon and a medium capable of dras-
tically decreasing the P-T stability limits of diamond
from the traditional 5–6 GPa and 1200–1300°C, at a
depth of 200 km or more (Shirey and Shigley, 2013).
These “fluids” are hydrous, supercritical (i.e., beyond
the point of coexisting fluid + vapor), and intensely
oxidized so that diamond crystallization is unlikely,
and diamond survival even less so. An analogy with
loosely aggregated framesite, found in mantle-derived
kimberlites, has also been suggested, but is unsatis-
factory because the diamonds are semiprecious, free
of pores and patina, and lack the highly reduced min-
eral suite of metals, carbides, and nitrides.
Subduction. Although carbonado is present in meta-
conglomerates (again, see figure 2), these robustly ce-
mented diamonds are very different from the
ultra-high-pressure, subducted, metamorphic dia-
monds found in continental collision zones in Nor-
way, China, Kazakhstan, Greece, and Germany
(Ogasawara, 2005). The diamonds at these localities
are single crystals and are armored by zircon, garnet,
pyroxene, and amphibole that acted as insulating
capsules. Sintering would be necessary to form car-
bonado. This is possible at high pressures and tem-
peratures in the mantle, but the process would have
incorporated one or more mantle minerals such as
olivine, garnet, pyroxene, and spinel, none of which
are observed. Moreover, the inferred subducted plates
are oceanic and basaltic in composition and on trans-
formation at high P-T would produce large concen-
trations of garnet + pyroxene (namely eclogite),
which again is not encountered. Transport to Earth’s
surface is either not considered or is tentatively as-
cribed to deep mantle volcanic plumes in both the
subduction and radiation models (below).
Radioactive Ion Implantation of Carbon Substrates.
Radiation-induced diamond is on the scale of nanome-
ters and cannot account for larger diamonds in the mi-
crometer to millimeter size range found in carbonado.
Once diamond is formed, low-energy implantation al-
ters the atomic structure and turns the diamond
green; high-energy ion doses produce graphite rather
than additional diamond (Kalish and Prawer, 1995).
There were no coal deposits at 2–3 Ga, and the radia-
Figure 11. PL spectra illustrating the similarity be-
tween carbonado (A and B) and CVD diamond (C)
and CVD diamond that has been heated to 1000°C
(D). Modified from Clark et al. (1992).
2.156 eV
Carbonado peak
2.156 eV
CVD peak
1.945 eV
1.681 eV
2.156 eV 2.463 eV
tion-induced diamonds recovered from very rare car-
buranium (U-rich hydrocarbon) are low in abundance
and nanometer in size. Proposals of radiation sinter-
ing, and even pore formation, are equally untenable.
Extraterrestrial Origin. The extraterrestrial (ET)
model was initially proposed because traditional
earthbound scenarios failed to account for major
characteristics of carbonado, namely diamond poros-
ity, patina, polycrystallinity, rarity, and location
(Haggerty, 2014). Pores are incompatible with high-
pressure environments; therefore, carbonado cannot
have formed under the same conditions in which con-
ventional diamonds form in the mantle at depths of
approximately 200 km. The pores in carbonado
(again, see figures 3 and 4) are similar to vesicles in
basalts that degassed at low pressures under near-sur-
face conditions from a molten or semi-molten
magma. This rules out an origin for carbonado in the
crust or the mantle, because liquefaction of carbon
is not readily accomplished. In fact, diamond is solid
in Earth’s core (6,380 km and approximately 350 GPa
and 7000 K; Bundy et al., 1996; Oganov et al., 2013).
Consequently, none of the interpreted melt-like fea-
tures in carbonado (figures 5–7) can possibly be of ter-
restrial origin. Furthermore, not a single carbonado
has been reported from kimberlite-lamproite suites
in the nearly 700 metric tons of diamond mined
since about 1900 (Levinson et al., 1992). As noted
above, carbonado differs from conventional diamond
in several respects:
1. Hydrogen is prominent and N is dispersed,
which is the case for <1% of conventional dia-
monds (i.e., type Ib).
2. Combined N and C isotopes are distinctly not
terrestrial (figure 10).
3. There are remarkable similarities to diamonds
formed by carbon vapor deposition (figure 11),
a process that requires vacuum conditions and
plasma temperatures that cannot possibly be
accomplished in any natural environment on
4. Carbonado lacks the characteristic suite of di-
amond inclusion minerals such as Cr-garnet,
Na-Al-pyroxene, Mg-olivine, Mg-chromite, and
Fe-Ni-sulfides, and is instead replaced by ex-
otic, reduced metal alloys and minerals.
The ET scenario posits that carbonado originated
from carbon-rich, diamond-bearing stellar bodies
and/or disrupted C-bearing planets (Haggerty, 2014).
All of the characteristic features of carbonado are sat-
isfied: CVD diamond is the sintering glue to micro-
diamonds in carbonado; the loss of interstellar H
produced the pores, and the patina and flow textures
are stellar or interstellar high-vacuum melt products.
The model further proposes that carbonado was
transported to Earth as a large diamond meteorite or
as smaller diamond “plums” in a carbonaceous me-
teoritic matrix, possibly during the Late Heavy Bom-
bardment (3.8–4.2 Ga), in which the inner solar
system was pummeled by meteorites (Fassett and
Minton, 2013; Abramov et al., 2013). The numerous
craters on the moon are considered evidence of the
bombardment (Marchi et al., 2013). The theoretical
age of this event corresponds to the oldest age deter-
mined for carbonado (3.8 ± 1.8 Ga). This would ac-
count for its rarity as a single known occurrence on
the São Francisco and Congo cratons, which were
once joined geologically as the supercontinents of
Nuna and Rodinia. Carbonado was undoubtedly
widespread during the bombardment, but the car-
bonado falls were largely into the expansive oceans
that existed at that time. Supercontinent disruption
and subduction followed, leaving only the preserved
remnants of carbonado on an island that is today split
between Brazil and the Central African Republic.
The recent discovery of patches of sub-micron di-
amonds in Libyan desert glass, a high-silica natural
glass that is thought to be of cometary origin (Kramers
et al., 2013), lends credence to the ET model for car-
bonado. This view is supported by the growing lines
of evidence for (1) synthetically produced diamond-
like glass (Shumilova et al., 2016a, b); (2) nanodiamond
encased in glassy carbon shells in the interstellar
media (Yastrebov and Smith, 2009); and (3) glassy car-
bon and nanodiamond produced experimentally
(Shiell et al., 2016) and in supernova shock waves
(Stroud et al., 2011). Another supporting fact is the dis-
covery of asteroid 2008 TC3, which was tracked upon
entering Earth’s atmosphere and landed in North
Sudan as a fragmented, diamond-bearing ureilite
(Miyahara et al., 2015). Unusual in several respects,
the meteorite contains diamonds measuring approxi-
mately 100 μm. These are exceptionally large for ure-
ilites, whose diamonds typically measure 1–5 μm, and
substantially larger than nanodiamonds of pre-solar
origin in carbonaceous chondrites. These reports are
complemented by the unexpected discovery that Mer-
cury has a crust of graphite, now covered by volcanic
rocks but exposed in meteorite craters (Peplowski et
al., 2016), that may prove to be diamond bearing.
Carbonado (figure 12) is the most unusual form of
diamond on Earth. Despite many mineralogical
clues not observed in conventional diamonds, its
mode of origin remains largely unexplained. Discov-
ering the origin of carbonado would herald a whole
new mode of diamond formation and could repre-
sent a remarkable form of extraterrestrial carbon de-
livery to Earth. The extraterrestrial model, although
conceptual and supported by astrophysical data, will
only be vindicated by the discovery of carbonado in
the asteroid belt by remote sensing, or by an ob-
served diamond meteorite fall that is dark in color,
porous, and patinaed.
Prof. Haggerty is distinguished research professor in the Depart-
ment of Earth and Environment at Florida International University
in Miami.
Fieldwork for this study was supported by a faculty research
grant from the University of Massachusetts Amherst, and by De
Beers. Laboratory work was supported by the National Science
Foundation and Florida International University. Thanks to Jose
Ricardo Pisani and the late Jeff Watkins, who provided enor-
mous logistical help and hospitality during fieldwork in Brazil.
Thanks also to my many colleagues and critics, from whom I’ve
benefited enormously in active discussions on the controversial
issues surrounding the origin of carbonado. And lastly to the re-
viewers for detailed and constructive comments that led to im-
provements in presentation. To all I express my sincere
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Figure 12. The origin of
carbonado diamond (far
right) has yet to be defin-
itively established. Un-
covering their formation
would represent a scien-
tific breakthrough. Left
to right: The 9.49 ct yel-
low diamond octahedron
is a gift of the Oppen-
heimer Student Collec-
tion. The 109.47 ct
diamond bort is a gift of
Richard Vainer. The
118.01 ct carbonado, a
gift of Stephen Haggerty,
is from the Central
African Republic. GIA
Collection nos. 11953,
31602, and 40108. Photo
by Robert Weldon/GIA.
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... Polycrystalline Diamonds from Kimberlites 169 and alloyed phases (Heaney et al. 2005;Haggerty 2014). The mixture of uncommon syngenetic and epigenetic inclusions, including crustal inclusions, have been posing a challenge to develop models explaining their genesis which encompass a wide range of scenarios, including extraterrestrial origins (Heaney et al. 2005;Haggerty 2014). ...
... Polycrystalline Diamonds from Kimberlites 169 and alloyed phases (Heaney et al. 2005;Haggerty 2014). The mixture of uncommon syngenetic and epigenetic inclusions, including crustal inclusions, have been posing a challenge to develop models explaining their genesis which encompass a wide range of scenarios, including extraterrestrial origins (Heaney et al. 2005;Haggerty 2014). As carbonados are geologically much less constrained than polycrystalline diamond aggregates from kimberlites there is a risk that the new findings for the kimberlitic PDAs do not apply to carbonado. ...
... Consequently, a summary of the carbonado literature in this chapter would contribute little that has not been said before. For details on carbonado, we direct the reader to the extensive reviews existing in the literature for carbonados, such as Haggerty (2014Haggerty ( , 2017 and Heaney et al. (2005). ...
... Since this porous/ channel structure is a characteristic of this type of impact diamond compared to diamonds from any other origin, whether kimberlitic, impact or any other, it can thus be considered as a typomorphic feature of after-coal impact diamonds. Among the known types of diamond, only carbonado is characterised by porosity, which has been described in numerous other works (DeCarli 1998;Gorshkov et al. 2000;Haggerty 2014Haggerty , 2017Ketcham and Koeberl 2013;McCall 2009;Petrovsky et al. 2010;Piazolo et al. 2016). According to Haggerty (2014Haggerty ( , 2017, carbonado porosity varies within the range 5-15%, which is a very important specific property that also corresponds to that observed within the after-coal impact diamonds. ...
... Among the known types of diamond, only carbonado is characterised by porosity, which has been described in numerous other works (DeCarli 1998;Gorshkov et al. 2000;Haggerty 2014Haggerty , 2017Ketcham and Koeberl 2013;McCall 2009;Petrovsky et al. 2010;Piazolo et al. 2016). According to Haggerty (2014Haggerty ( , 2017, carbonado porosity varies within the range 5-15%, which is a very important specific property that also corresponds to that observed within the after-coal impact diamonds. ...
... For many decades, the problem of carbonado's origin has been debated (Haggerty 2014(Haggerty , 2017McCall 2009;Kagi et al. 2007;Kaminsky 1984Kaminsky , 1987Kaminsky , 1991Kaminskiy et al. 1978Kaminskiy et al. , 1979Smith and Dawson 1985), but its genesis still remains enigmatic. The results of these current studies on the after-coal impact diamonds are similar in many respects to the characteristic features of carbonado. ...
Full-text available
Complementary nano- and atomic-scale data from SEM, FIB, HRTEM, and EELS observations of after-coal impact diamonds from the giant Kara astrobleme are described, presenting their particular nano-sized porous polycrystalline structure, which consists of well-shaped single 20-30 nm nanocrystals that are free of deformation defects and do not contain lonsdaleite. The porous micro- and nanostructure is a special typomorphic feature of after-coal diamonds that suggests a crystallisation mechanism through short distance diffusion. The data for the after-coal impact diamonds presented here demonstrate their distinguishing characteristics from after-graphite impact diamonds, and have some similarity with the enigmatic carbonado, providing new insights to the origin of the latter.
... Original suspicions that the stone might be an unusual variety of carbonado diamond, a type of polycrystalline diamond found in Brazil and the Central African Republic (e.g. Trueb and de Wys, 1969;De et al., 1998;Haggerty, 2014) were set aside by d 13 C values of À0.2‰ and À1.1‰ (Kramers et al., 2013) in contrast to the values between À30‰ and À12‰ obtained on carbonados (Shelkov et al., 1997;De et al., 2001). Further, Kramers et al. (2013) and Avice et al. (2015) reported 40 Ar/ 36 Ar ratios ($39 and $0.5, respectively) far lower than the lowest present-day terrestrial value, which is the atmospheric ratio, 298.56 ± 0.31 (Lee et al., 2006). ...
... It is of interest to revisit the comparison with carbonado diamonds (see introduction) in the light of the above observations. Carbonados are extremely hard, tough and cohesive polycrystalline diamond aggregates, consisting of partly interlocked grains ranging from $0.5 to $50 lm in size (Trueb and de Wys, 1969;De et al., 1998;Haggerty, 2014). They are mined from alluvial occurrences in Brazil and the Central African Republic, where they are derived from Proterozoic conglomerates. ...
... They are mined from alluvial occurrences in Brazil and the Central African Republic, where they are derived from Proterozoic conglomerates. Specimens range from 0.05 to 20 g in mass with exceptional larger ones; the largest recorded being 633 g (Haggerty, 2014). They are invariably porous, with pores between 10 and 100 lm in size. ...
The stone named “Hypatia” found in the Libyan Desert Glass area of southwest Egypt is carbon-dominated and rich in microdiamonds. Previous noble gas and nitrogen isotope studies suggest an extraterrestrial origin. We report on a reconnaissance study of the carbonaceous matrix of this stone and the phases enclosed in it. This focused on areas not affected by numerous transecting fractures mostly filled with secondary minerals. The work employed scanning electron microscopy (SEM) with energy-dispersive (EDS) and wavelength-dispersive (WDS) electron microprobe (EMPA) analysis, Proton Induced X-ray Emission (PIXE) spectrometry and micro-Raman spectroscopy. We found that carbonaceous matrices of two types occur irregularly intermingled on the 50–500 μm scale: Matrix-1, consisting of almost pure carbonaceous matter, and Matrix-2, containing Fe, Ni, P and S at abundances analyzable by microprobe. Matrix-2 contains the following phases as inclusions: (i) (Fe,Ni) sulphide occurring in cloud-like concentrations of sub-μm grains, in domains of the matrix that are enriched in Fe and S. These domains have (Fe + Ni)/S (atomic) = 1.51 ± 0.24 and Ni/Fe = 0.086 ± 0.061 (both 1SD); (ii) grains up to ∼5 μm in size of moissanite (SiC); (iii) Ni-phosphide compound grains up to 60 μm across that appear cryptocrystalline or amorphous and have (Ni + Fe)/P (atomic) = 5.6. ± 1.7 and Ni/Fe = 74 ± 29 (both 1SD), where both these ratios are much higher than any known Ni-phosphide minerals; (iv) rare grains (observed only once) of graphite, metallic Al, Fe and Ag, and a phase consisting of Ag, P and I. In Matrix-2, Raman spectroscopy shows a prominent narrow diamond band at 1340 cm⁻¹. In Matrix-1 the D and G bands of disordered carbon are dominant, but a minor diamond band is ubiquitous, accounting for the uniform hardness of the material. The D and G bands have average full width at half maximum (FWHM) values of 295 ± 19 and 115 ± 19 cm⁻¹, respectively, and the D/G intensity ratio is 0.75 ± 0.09 (both 1SD). These values are similar to those of the most primitive solar system carbonaceous matter. The diamond phase is considered to be a product of shock. The (Fe, Ni) sulphide phase is probably pyrrhotite and a shock origin is likewise proposed for it. Moissanite is frequently associated with the Ni-phosphide phase, and a presolar origin for both is suggested. The lack of recrystallization of the Ni-phosphide phase suggests that the Hypatia stone did not experience long-lasting thermal metamorphism, in accord with the Raman D-G band characteristics. A lack of silicate matter sets the stone apart from interplanetary dust particles and known cometary material. This, along with the dual intermingled matrices internal to it, could indicate a high degree of heterogeneity in the early solar nebula.
... Among the many other forms of natural diamond, carbonado, a sintered type of polycrystalline diamond, remains the most enigmatic. Carbonados may form during meteorite impact, in Earth's mantle or in extraterrestrial environments (see Garai et al. 2006;Kagi and Fukura 2008;Cartigny 2010;Haggerty 2014 and reference therein). (110-250 km). ...
... The early findings of diamond in meteorites stimulated experimental shock investigations (DeCarli and Jamieson, 1961;Hough et al., 1995;Koeberl et al., 1997;Masaitis, 1998;El Goresy et al., 2001), and the existence of extraterrestrial diamonds in meteorites and carbonados has been discovered by spectroscopic methods (e.g. Huss, 1990;Koscheev et al., 2001;Parthasarathy et al., 2005;Garai et al., 2006;Gucsik et al., 2008Gucsik et al., , 2012Haggerty, 2014). Carbon in ordinary chondrites has been generally found to consist of components such as graphite and diamond (Grady, 2000). ...
Full-text available
We present here the evidence for the presence of organic matters in Dergaon, Mahadevpur and Natun Balijan ordinary chondrites using Fourier transforminfrared and micro-Raman spectroscopic technique. The Fourier transforminfrared spectrum of these ordinary chondrites in the range 2700 3000 cm-1 indicates the presence of CH3 asymmetric stretching, and CH2symmetric and asymmetric stretching modes due to aliphatic hydrocarbons. The micro-Raman spectrum exhibits the diamond and graphite peaks correspondingly at 1331 cm-1 , 1349 cm-1 and 1588 1618 cm-1. The full wave at half maximum value correspondingly 120 cm-1 , 70 cm-1 and 17.5 cm-1 for Dergaon, Mahadevpur and Natun Balijan, indicate the nature of disordered phase involved shock metamorphism in the meteorites. The diamond and graphite peaks intensity ratios of ~1.121, ~1.075 and ~0.532, correspondingly for Dergaon, Mahadevpur and Natun Balijan, indicates the disordered nature of graphite. This study has strong implications in understanding of the origin of organic matters in extra-terrestrial materials and origin of extraterrestrial life.
... Our experience in multivariate analysis of LIBS spectra leads us to believe that 30 samples is the minimum required to encompass the heterogeneity in natural sample sets. All stones were visibly inclusion-free, except the Brazilian Minerals 2020, 10, 916 3 of 12 carbonado diamonds, which are polycrystalline aggregate diamonds with numerous inclusions of amorphous carbon, graphite, metals, metal alloys, and minerals [41]. Acquired from industry partners, the diamonds are from a variety of geologic sources (Table 1), including placer deposits, lamproites, and kimberlites, two of which are within 100 km of each other. ...
Full-text available
The country or mine of origin is an important economic and societal issue inherent in the diamond industry. Consumers increasingly want to know the provenance of their diamonds to ensure their purchase does not support inhumane working conditions. Governments around the world reduce the flow of conflict diamonds via paper certificates through the Kimberley Process, a United Nations mandate. However, certificates can be subject to fraud and do not provide a failsafe solution to stopping the flow of illicit diamonds. A solution tied to the diamonds themselves that can withstand the cutting and manufacturing process is required. Here, we show that multivariate analysis of LIBS (laser-induced breakdown spectroscopy) diamond spectra predicts the mine of origin at greater than 95% accuracy, distinguishes between natural and synthetic stones, and distinguishes between synthetic stones manufactured in different laboratories by different methods. Two types of spectral features, elemental emission peaks and emission clusters from C-N and C-C molecules, are significant in the analysis, indicating that the provenance signal is contained in the carbon structure itself rather than in inclusions.
... Carbonado is a rare and enigmatic form of diamondlike carbon. As summarized by Haggerty (2014), it is 3.8 Ga old, polycrystalline, and porous but strongly bonded and super hard. It contains silicate and other mineral inclusions consistent with crustal rocks and their alteration products, and inclusions of metals, SiC, FeC, TiN, and new ternary Cu, Ti nitrides. ...
Geologic and planetary processes are punctuated by sudden cataclysmic events, and planetary evolution is irrevocably changed by impacts and intense seismic and magmatic/volcanic activity. Such events often are associated with or generate high temperature, high pressure, and low oxygen fugacity. Their traces in the accessible geologic record are not pristine but altered by subsequent petrologic reactions. Evidence from the thermochemistry of synthetic materials, largely studied in a materials science context, in Si‐O‐C and M‐Si‐O‐C‐H systems under reducing conditions can be used to propose some possible rare but significant reactions, together called a geologic Si‐O‐C pathway, involving carbon‐containing silicate melts, glasses, and amorphous materials. The substitution of carbon for oxygen in the first coordination shell of silicon provides a reducing local environment for the formation of metals, carbides, and silicides. Grains of these refractory compounds may persist long after the main carbon‐containing silicate phase has transformed and disappeared. Such relict refractory materials may be markers of impact events and unusual volcanism. Anomalies in minor phases, trace elements, and textures in settings ranging from ultra‐high pressure metamorphic rocks to impact craters to carbonado diamonds may be linked to the transient presence of carbon‐rich silicate phases generated under reducing conditions from initially carbon‐rich target rocks and/or impactors.
... These highly porous dark nodules possess a narrow range of isotopically light carbon (δ 13 C −31 to −24 ), an inclusion suite reflecting crustal pressure-temperature conditions (e.g., kaolinite and florencite -a REE-rich aluminophosphate commonly formed from the hydrothermal breakdown of monazite) and unusually enriched in REEs and actinides filling the pore spaces, a loosely constrained crystallization age between 2.6 and 3.8 Ga (Ozima and Tatsumoto, 1997;Sano et al., 2002), and other atypical features. This unusual assortment of properties has led to a variety of formation theories, from extra-solar to deep mantle (Haggerty, 2014). The pore network in some carbonado specimens has been shown to be interconnected based on the successful removal of all inclusion-hosted magnetism after intense acid leaching (Dismukes et al., 1988). ...
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In this study, we utilize high-resolution X-ray computed tomography (XCT) to track the progress of a leaching process within a pore network. Dissolution and leaching are difficult processes to observe with combined temporal and spatial context, particularly when dissolving material within a non-reactive pore network, and XCT is a uniquely suited technique for observing dissolution in situ, and extracting quantitative data on pore networks and the material in them in 3D. We XCT image two samples of porous diamond (carbonado) during a sequential acid leaching procedure designed to remove a diverse assemblage of pore-filling minerals. This experiment provides a unique perspective for observing mineral dissolution in 4D, allowing us to identify differences in mineral inclusions and pore network topology between carbonado samples based on dissimilar dissolution styles and rates. We are also able to observe the formation of fluorides during acid digestion, which can persist throughout acid leaching procedures and drastically affect yields for geochemical measurements of certain elements, most importantly REEs, U, Th, and Pb. We test various approaches to measuring porosity, finding that methods based on quantitative interpretation of CT numbers as partial porosity give more accurate results than purely binary segmentation, and that attempts to segment the pore network using visual criteria are scattered and unreliable. We document how image quality can be locally affected by material properties, with filled pores measurably blurrier than empty ones. Such local variation in point-spread function is important when segmenting XCT data for the purposes of quantification. Finally, we demonstrate that by comparing mass and X-ray attenuation loss it is possible to estimate the relative heavy-metal content of the leached material.
... According to this model, the extremely fast quenching rate due to impurity enrichment will lead to a passive boundary layer that will reduce the diffusion rate to stimulate a radial (spherulitic) growth of the crystals (Lux et al. 1997). Besides, the occurrence of such spherulitic polycrystalline diamonds is likely restricted to the oldest parts of the continental crust (cratons, generally >2.0 Ga) (Haggerty 2014), and the long resident time would allow nitrogen to be fully aggregated. Based on the hailstone (rapid growth) model, their formation might take place as a result of the interaction of hot fluids with cooled materials that enables rapid diamond growth due to significantly undercooling environments. ...
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The milky appearance shown by certain type IaB diamonds has been subjected to several recent studies, but the origin of this feature is not fully understood. Here several type IaB diamonds with a milky appearance have been studied by cathodoluminescence (CL), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). CL of several hazy type IaB diamonds shows scattered or orientated micro-sized spots or short linear luminescence features. TEM observation revealed that those spots and linear features are caused by dislocation loops that are likely responsible for the hazy appearance of the host diamonds. It is also shown that type IaB diamonds with a cloudy appearance contain nano-sized inclusions with negative crystals of octahedral shape. Some of these negative crystals contain a precipitate that can be explained by a compressed disordered cubic δ-N2 phase observed by high-resolution TEM. In one of the milky IaB diamonds with platelet defects, polycrystalline areas composed of columnar diamond crystals elongated radially in [110], similar to ballas diamond, were revealed by EBSD. Taking into account these observations, it is suggested that the dislocation loops, nano-sized inclusions (negative crystals) and/or characteristic grain boundaries of the radiating fibrous crystals would be the origins for the milky appearance of the type IaB diamonds studied here. Those results add a complementary explanation that accounts for the milkiness of type IaB diamonds studied before.
Since they were first discovered in the 1930s, the diamond placers that are dispersed intermittently along Cameroon's eastern frontier with the Central African Republic have been the subject of numerous exploration campaigns. These small placers, mainly confined to Quaternary sediments, have never been mined on a large and commercial scale, but have instead been, and continue to be, exploited by the artisanal mining sector. From exploration in the 1980s, the Mobilong alluvial field was recognised to have economic potential based on a manually directed exploration campaign, but gravel sample volumes used for grade estimation are considered to be unrepresentatively small in this study. Recent mechanical mining and bulk trench sampling of this alluvial field, however, yielded greater gravel volumes, which allowed for a modern reassessment of the placer's sedimentology and diamond potential. It demonstrated that this alluvial field is overall a low-volume, small-diamond and uneconomic placer, with higher diamond concentrations in localised areas. The low potential is attributed to the limited availability of diamonds from secondary detrital sources, coupled with the inefficiency of fluvial mechanisms to promote diamond concentration. With unknown kimberlite occurrences in Cameroon and the neighbouring Central African Republic, Proterozoic and Cretaceous successions are considered to be detrital hosts of diamonds; a re-evaluation of these provide insight into their diamond contribution to the Mobilong alluvial field.
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Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.
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During earthquakes, melt produced by frictional heating can accumulate on slip surfaces and dramatically weaken faults by melt lubrication. Once seismic slip slows and arrests, the melt cools and solidifies to form pseudotachylytes, the presence of which is commonly used by geologists to infer earthquake slip on exhumed ancient faults. Field evidence suggests that solidified melts may weld seismic faults, resulting in subsequent seismic ruptures propagating on neighboring pseudotachylyte-free faults or joints and thus leading to long-term fault slip delocalization for successive ruptures. We performed triaxial deformation experiments on natural pseudotachylyte-bearing rocks, and show that cooled frictional melt effectively welds fault surfaces together and gives faults cohesive strength comparable to that of an intact rock. Consistent with the field-based speculations, further shear is not favored on the same slip surface, but subsequent failure is accommodated on a new subparallel fault forming on an off-fault preexisting heterogeneity. A simple model of the temperature distribution in and around a pseudotachylyte following slip cessation indicates that frictional melts cool to below their solidus in tens of seconds, implying strength recovery over a similar time scale.
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Mercury's global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury's lowest-reflectance spectral unit, the low-reflectance material. This low-reflectance material is generally found in large impact craters and their ejecta, which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury's surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra of low-reflectance material is carbon, at an abundance that is 1-3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet's crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean, and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury's globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.
Conference Paper
Diamonds from Jagersfontein kimberlite with isotopically light carbon include majoritic garnets and associated minerals that may be superdeep, perhaps from the asthenosphere or transition zone (Tappert et al, 2005). Metals and carbides are very rare in natural diamonds but have been reported in pyroxenitic garnet inclusions within polycrystalline diamond from Venetia kimberlite (Jacob et al, 2003; Fe3C “cohenite”). We describe the occurrence of metallic and carbide inclusions together with their host diamonds from Jagersfontein.
An outgrowth of the LPI Conference on the Origin of the Earth, held in Berkeley, California in December 1988, a strong theme is the Giant Impact Hypothesis and it has been recognised that the origin of the Moon by giant impact has serious implications for the early history of the Earth. The book is divided into six sections. The first three sections deal with accretion and giant impact and the next three discuss the establishment of chemical and physical reservoirs. The book begins with a discussion of terrestrial accretion within the more general context of the origin of the solar nebula. The discussions of accretion then focus on the effects of a giant impact and whether there was a terrestrial magma ocean. The final three sections deal with the formation of the reservoirs that we are familiar with today: the core, the continental crust, and the atmosphere/hydrosphere. Each of the papers are abstracted separately. A subject index is included. -after Editors