Opals from Slovakia ("Hungarian" opals): A re-assessment of the conditions of formation

Abstract

Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm(-1)) and microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high 6180 for Slovakian and Australian opals (approximate to 31 parts per thousand) consistent with low temperatures of formation (lower than 45 degreesC; by contrast, Mexican opals-CT show a lower delta(18)O at 13 parts per thousand consistent with a formation at a higher temperature, possibly up to 190 degreesC.

Full text

1. Introduction
The "Hungarian" opal deposits (now in Slovakia) were
the unique source of gem opal in Europe from the Roman
times to XIX
th
century (Leechman, 1965; Webster, 1975;
Bariand & Poirot, 1985). However it has been little studied
compared to other deposits such as Australia or Mexico
discovered in the XIX
th
century and still actively mined.
This is mainly due to the fact that Slovakian mines closed
in 1922 (Kali
čiak et al.,1976) prior to the development of
many modern means of chemical and physical analysis.
Yet, as a once important deposit, it constitutes a key area to
understand the formation of opal deposits.
This study focuses on opal properties and its relation-
ship with the genesis of the Slovakian opals, compared to
other deposits in the world. As the main outcrops have
considerably deteriorated, private and public collections, in
museums and other institutions, are the only remaining
reliable sources for Slovakian specimens. These specimens
are studied within the framework of literature regarding the
geology of the Slovakian opal deposit.
Gemmological properties, XRD pattern, Raman scat-
tering and microstructure of numerous Slovakian opals are
investigated. They are compared with data on some
samples from others classical gem opal localities:
Australia, Mexico, or Brazil. In addition, some preliminary
results on isotopic composition are presented and used to
constrain a genesis model for Slovakian opal.
2. Background
2.1 Geographic provenance and history of "Hungarian"
opals.
The source of the well-known "Hungarian opals" is now
located in south eastern Slovakia, in the Dubník area near
Ko
šice, in the Libanka-Šimonka mounts (Webster, 1975).
These opals are known as Hungarian opals because in the
past this region was part of Hungary until the end of World
War I. Although there isn’t any written document available
about the extent of the medieval opal mining of the area,
the importance of this deposit is attested by the large
amount of historical pieces extracted from these mines. The
maximum of the documented mining activity occurred in
the mid-XIX
th
century when an annual production of
twenty to thirty thousand carats per year was recorded
(Ďud’a & Molnár, 1992).
The field is called “Červenica-Dubník region” (Ďud’a &
Molnár, 1992) and also Vörösvágás or Veresvágás in
Eur. J. Mineral.
2004, 16, 789-799
Opals from Slovakia ("Hungarian" opals): a re-assessment
of the conditions of formation
BENJAMIN
RONDEAU
1,
*, EMMANUEL FRITSCH
2
, MICHEL GUIRAUD
1
and CHRISTOPHE
RENAC
3
1
Muséum National d’Histoire Naturelle (MNHN), Département Histoire de la Terre – USM 0201 Minéralogie
61 rue Buffon, F-75005 Paris, France
2
Institut des Matériaux Jean Rouxel (IMN), Laboratoire de Physique Cristalline, Equipe Matériaux Absorbants
et Photoluminescents, 2, rue de la Houssinière – BP 32229, F-44322 Nantes Cedex 3, France
3
Département de Géologie - Université Jean Monet, 23 rue du Dr. P. Michelon, F-42023 St Etienne, France
Abstract: Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic
event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm
-1
) and
microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary
deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical
properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high δ
18
O
for Slovakian and Australian opals (≈ 31 ‰) consistent with low temperatures of formation (lower than 45°C); by contrast,
Mexican opals-CT show a lower δ
18
O at 13 ‰ consistent with a formation at a higher temperature, possibly up to 190°C.
Key-words: Slovakia, gem opal, Raman spectroscopy, oxygen isotopes, opal classification.
0935-1221/04/0016-0789 $ 4.95
© 2004 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart
DOI: 10.1127/0935-1221/2004/0016-0789
*E-mail: rondeau@mnhn.fr

B. Rondeau, E. Fritsch, M. Guiraud, C. Renac
Hungarian. It lies in the Slanské mountains (Harman &
Chovanec, 1981) (Fig. 1). The locality is described as follows
by Beudant (1822): “village of Cservenicza (…), about two
miles of Kaschau (Kassa in Hungarian), in the trachytic group
lying from Tokaj to Epériés ”.
2.2 Other opal deposits mined in the region.
The Dubník deposit was the most important source of
opal in Europe until the discovery of Mexican and
Australian deposits in the XIXth century. However some
smaller deposits in the Tokaj Mountains, NE Hungary
(which are the southern continuation of the Slaské
Mountains) have been mined in the past and may have
provided some small samples labelled as "Hungarian opal":
*The small deposits of Telkibánya was famous in the
late XIXth century for having provided some samples of
fire opal and honey opal (called Telkebanyerstein); in addi-
tion some rare play-of-color opal may have also been
extracted (Takács,1994; Szakáll & Jánosi, 1996). The
material was brittle, and the exploitation rapidly stopped.
Historical details of this deposit are given in Papp (1994).
*The Monok area in the Szerencs Hills is also known to
be opal-rich (Szakáll & Kovács, 1998): transparent and play-
of-color opal are found in the Hosszú Hills, and milk opal
and jasper-opal in the Ingvár Hills. Precious opal in the Tokaj
Mountains are found in small vesicles of perlitic rhyolite.
These opals can be called Hungarian opals in the modern
sense of the name. However the vast majority of ancient opal
samples, including the largest historical samples such as
those in the French National Museum of Natural History
(MNHN), have been mined in the Dubník area, that is now in
Slovakia.
2.3 Geological settings of Dubník opal deposit:
Useful samples for analysis are those kept in mineralogical
collections because the Slovakian mines are closed today and
therefore the precise field relationships and textural relation-
ship with host-rock can no longer be correctly observed;
however, detailed geological and textural descriptions have
been written in the past when the deposit was mined, and recent
geological descriptions provide complete information at the
regional scale (Kali
č
iak et al., 1976; Ďud’a & Molnár, 1992).
Geological settings are thus available from literature data
.
Slovakian opals have been considered as formed in a
volcanic environment (e.g. Webster, 1975; Horton, 2002). The
locality is located in the volcanic complex of Pre
š
ov-Tokaj
mountains (Dudek et al., 1966; Ilavsk´y et al., 1966), also
known as the Zlatá Ban
∨∨
a layered volcanic complex (Ďud’a &
Molnár, 1992). All volcanic formations are 10 to 15 million
years old (Zelenka, 1994). They are of andesitic composition
(previously called trachyte in Beudant, 1822), with minor
mineralogical variations. The opal mineralization is described
as the last hydrothermal stage of this volcanism (Harman &
Chovanec, 1981). Two hydrothermal stages are distinguished
by Kali
č
iak et al. (1976) from mineral associations: pyrite,
antimonite, marcasite have crystallized at high temperature.
Marcasite, chalcedony and all opal varieties formed at lower
temperature. Limonite and then white hydrophane crystal-
lized during a last supergene stage, accompanied by a series
of secondary metallic minerals.
A geological map and cross section after Kali
č
iak et al.
(1976) are shown in fig. 1. The opal host-rock is a coarse
conglomerate involving andesitic blocks or gravels of various
sizes, developed by tectonic constraints: opal occurrence is
disseminated along a major tectonic faults set, roughly oriented
NNW-SSE, responsible for a small horizontal displacement. As
described by Beudant (1822), "Opals appear everywhere in the
conglomerate levels as veins, veinlets and nests of different
width". Breccia, which underline the faulted zone, are about 80
to 100 meters thick and 1000 meters in extension. These
descriptions indicate that opal formation is linked to a tectonic-
controlled stage and not to the volcanism itself.
3. Materials and methods
3.1 Samples
We focus our study on collection's samples for which
the geographic origin is known with certainty. Most of the
790
Fig. 1. Geological map and section across opal deposits of the
Dubník region (modified after Kaličiak et al., 1976). The star in the
inset shows the location of the Dubník Area in Slovakia. Opalized
zones underline the main tectonic faults and are independent of the
andesitic facies.

Formation conditions of Slovakian opals
791
Fig. 2. Photographs of the studied jewels.
The nine cabochons (upper left, ref nu
87.34a to i) belongs to the French Crown
Jewels (samples used by King Louis
XIV). The oval cabochon (right, ref nu
87.44) was bought for King Louis XVIII
and used for King Charles X sacrament.
The six cabochons (down left) are
samples of the René-Just Haüy collec-
tion (opals collected before 1822).
Fig. 3. Typical white-to-colourless play-of-colour Slovakian opal on andesitic matrix. Sample nu MNHN 98.427.
Fig. 4. Photomicrographs of sample OP284. Three
pictures are taken with the SEM on a fresh surface
and one under the optical microscope on a polished
surface. Slovakian opals are made of silica spheres
(a) about 175 nm in diameter regularly stacked in
transparent, play-of-colour samples. Some twin-
ning planes are observed in the three dimensional
packing of silica spheres following the <111>
directions (b and c, dashed line). They define some
bands of homogeneous packing orientation that
intersect each other with angles of about 60°.
These bands are seen as bands of different colours
in the optical microscope (d).

B. Rondeau, E. Fritsch, M. Guiraud, C. Renac792
Table 1. Description of the historical gemstones studied and their gemological properties. All come from Slovakia without any further
precision.
Sample n° Colour Dimensions (mm) LWUV fluorescence Phosphorescence SWUV fluorescence Phosphorescence Index of
Refraction
MNHN. Series of nine oval cabochons mounted in a golden circle, from the ancient French Crown collection. Date of entry: 1887.
87.34a Black opal, vivid play-of-colour 14 * 10.5 * 10 White, intense Greenish yellow, moderate,20 s. Yellowish white, moderate Greenish yellow, moderate, 20 s. 1.4
87.34b
Translucent opal, vivid play-of-colour
11 * 11 * 7 White, intense Greenish yellow, moderate Yellowish white, 20 s. White, moderate, 1.44
mostly green and blue moderate, 20 s.
87.34c Grayish opal, mostly green play-of- 14 * 11 * 6.5 White, moderate
Greenish yellow, very weak, 10 s.
Yellowish white, moderate Greenish yellow, very weak, 10 s. 1.45
colour
87.34d Grayish-white opal, mostly green 11.5 * 11.5 * 6 White, weak
Greenish yellow, very weak, 10 s.
Yellowish white, moderate White, very weak, 10 s. 1.45
play-of-colour
87.34e Translucent white opal, multicolour 11 * 7.5 * 5.5 n.a. inert n.a. inert 1.44
play-of-colour
87.34f Translucent white opal, mostly green 9 * 7.5 * 5 n.a. inert n.a. inert 1.45
play-of-colour
87.34g Grayish opal, blue and 11.5 * 7 * 4 White, weak Yellowish white, moderate
Greenish yellow, very weak, < 10 s.
1.45
green play-of-color
87.34h Grayish opal, multicolour 7 * 5.5 * 4 n.a. inert n.a. inert 1.44
play-of-colour
87.34i Grayish opal, multicolor 7 * 5.5 * 3.5 n.a. inert n.a. inert 1.45
play-of-colour
MNHN. King Charles X great opal, 77 carats. Sample initially bought by King Louis XVIII. Date of entry: 1887. Oval cabochon mounted in a silver circle with 48 brilliant-cut diamonds.
87.44 Zoned white to transparent opal 34.5 * 25 * 13 Greenish-white, Greenish-yellow,
Greenish-white,weak, zonation
Greenish-yellow, weak, 30 s. 1.44
disposed in layers, vivid play-of-colour.
moderate weak, 10 s. parallel to the layering
MNHN. Series of six oval cabochons mounted on a golden circle soldered to a pin, from R.-J. Haüy’s collection. Date of entry: prior to 1822.
H 7152 White opal, pale play-of-colour 11.5 * 7 * 9 inert inert Whitish-yellow, weak Whitish-yellow, weak, 10 s. 1.44 - 1.45
H 7154 White opal, pale play-of-colour 9 * 6.5 * 2 Yellow, intense inert Whitish-yellow, moderate Whitish-yellow, weak, 10 s. 1.43
H 7155 Grayish-white, few play-of-colour 6 * 5 * 2.5
Greenish-yellow, moderate
inert Whitish-yellow, weak Whitish-yellow, weak, 15 s. 1.44
H 7159 Grayish-white, vivid play-of-colour 6.5 * 4 * 2
Greenish-white, moderate
inert Whitish-yellow, weak inert 1.45 - 1.46
H 7160 Translucent, vivid play- of-colour 4.5 * 4 * 2.5 inert inert inert inert 1.46
H 7161 Translucent, vivid play-of-colour 8 * 4.5 * 2 Greenish-white, weak inert Whitish-yellow, weak inert 1.44 - 1.45
mostly blue and violet
J.S. Series of oval cabochon. All have a density of 2.10
OP 294 Milky opal, play-of-colour 8 * 7 * 3.5 1.43
OP 295 White opal, few play-of-colour 7.5 * 5.5 * 3.5 1.43
OP 296 White, few play-of-colour 6.7 * 6.8 * 4 1.43
OP 297 Transparent, vivid play-of-colour 6.3 * 4.2 * 4 1.44
OP 298 White, few play-of-colour 7 * 7 * 4.2 1.42
OP 299 Fire opal, orange with few play-of-
colour. 4.01 ct.
OP 300 Fire opal, yellow, no play-of-colour.
5 ct.
OP 346 Play-of-colour fire opal, 23 ct.
MNHN: French National Museum of Natural History. J.S.: loan Jurgen Schutz. LWUV: long wave ultraviolet radiation. SWUV: short wave
ultraviolet radiation. Fluorescence is sometimes non available (n.a.) because of an intense greenish-yellow luminescence due to the glue.
Density and weight are not available because all jewels are mounted. No visible inclusions have been observed.
Table 2. Description and physical properties of rough samples studied, from various localities.
Collection Sample n° History or original source Geographic locality Description (including colour)
provenance:
MNHN 0.18 Old "King Cabinet" sample (prior to 1791) Slovakia Colourless, play-of-colour opal on matrix
MNHN 61.275 Entered the collection Slovakia play-of-colour Precious to white opal on matrix, vivid in 1861.
MNHN 7.165 Old " King Cabinet " sample (prior to 1791) Probably Slovakia Polished gravel of colourless, play-of-colour opal.55.4 ct, 29 * 20 * 19 mm.
MNHN 203.15 Ancient sample from the Reserve collection Cervenica, Slovakia White, play-of-colour opal on matrix
MNHN 203.16 Old sample from the Reserve collection
Košice, Slovakia
Millimetric droplets of play-of-colour opal in sulfurous matrix
MNHN 2.828 Old sample of Mr. Weiss collection Pecklin, Slovakia Milk opal on ferruginous matrix
MNHN 86.2 Entered the collection in 1886. Kaschau, Slovakia Translucent and white, opaque opal on sulfurous matrix
MNHN 98.427 Entered the collection in 1898. Košice, Slovakia hite to translucent opal on matrix
CRG OP 280 J. Hyrsl Dubník, Slovakia Vivid play-of-colour in white, opaque opal
CRG OP 281 J. Hyrsl Slovakia Play-of-colour translucent opal on matrix
CRG OP 282 J. Hyrsl Slovakia Milk opal , no play-of-colour
CRG OP 283 J. Hyrsl Slovakia Transparent opal on matrix
CRG OP 284 J. Hyrsl Slovakia White, play-of-colour opal on matrix
ROM OP 338 Veresvagas, Slovakia Brown, opaque opal, no play-of-colour
M16818
ROM OP 347 Slovakia Play-of-colour opal
M16829
CRG OP 547 J. Hyrsl Slovakia Translucent opal on matrix, no play-of-colour
CRG OP548 J. Hyrsl Slovakia White opal, no play-of-colour
CRG OP250 Pedro II, Piaui, Brazil Grayish precious opal, vivid play-of-colour; 6.82 ct
CRG OP 251 F. Notari Pedro II, Piaui, Brazil Precious opal, vivid play-of-colour; 90 * 40 mm.
CRG OpalBREF J.-C. Samama Pedro II, Piaui, Brazil White opal , vivid play-of-colour
CRG OpalBRF1 J.-C. Samama Pedro II, Piaui, Brazil White opal , vivid play-of-colour
MNHN OP AUS J.M. Fourcault Coober Peddy, Australia White opal, few play-of-colour
CRG OP 236 M. Tremonti Lightning Ridge, Australia Grayish opal, violet play-of-colour
CRG OP 243 M. Tremonti Allah, Australia Grayish opal, blue play-of-colour
CRG OP 246 M. Tremonti Lightning Ridge, Australia Grayish opal, vivid play-of-colour
CRG OP 387 Tintenbar, Australia Pale yellow opal
CRG OP 388 Tintenbar, Australia Translucent, few play-of-colour
CRG OP398 Tintenbar, Australia Play-of-colour opal
CRG OP422 Tintenbar, Australia Play-of-colour opal
CRG JAL-10 Jalisco, Mexico Brownish orange, translucent fire opal
CRG JAL-7 Jalisco, Mexico Brownish orange, translucent fire opal
MNHN 203.21 Mexico Pale fire opal with play-of-colour
CRG OP 355 Queretaro, Mexico Bluish-white opal
CRG OP 362 Don Cabrera Queretaro, Mexico Brownish orange, translucent fire opal
MNHN: French National Museum of Natural History; ROM: Royal Ontario Museum, Canada; CRG: Centre de Recherche en
Gemmologie, University of Nantes, France. MNHN samples are presented following the entrance chronological order.

samples
studied are from Dubník, Slovakia. Faceted
gemstones and rough samples are presented separately on
Tables 1 and 2 respectively. Useful indications as history,
colour description and dimensions are given for the
gemstones (also represented on Fig. 2). Most of them are
historical jewels kept in the mineralogy collection of the
French National Museum of Natural History, Paris (MNHN).
One is very famous: the great opal n° 87.44 was first bought
for King Louis XVIII personal collection, and then used for
King Charles X sacrament in 1824 (Schubnel et al., 1998). It
was remounted around 1853 for empress Eugénie in a silver
circle with forty-eight brilliant-cut diamonds. The other
following historical jewels are less famous but demonstrate
the importance of the Slovakian opal specimens in history. A
series of nine oval cabochons mounted as cufflinks are kept
under n° 87.34: these jewels were parts of ancient French
Crown Collection and entered the Museum’s collection in
1887 as a permanent deposit from the French State. The
biggest (n° 87.34a) shows a spectacularly vivid play-of-
colour with a black body colour. Six small cabochons from
the personal collection of French early mineralogist René-
Just Haüy (n° H7152 to H7161) have also been selected for
their unambiguous geographical provenance (late
"Hungary"). Some rare fire opal cabochons from Slovakia
have been loaned by the Royal Ontario Museum.
A series of rough samples are used for destructive studies
(XRD pattern, geochemistry and microstructure observation)
as well as for the non destructive Raman scattering study.
They are presented in Table 2. Figure 3 shows a typical white
to colourless, play-of-color Slovakian opal on andesitic lava.
Additional samples of typical gem opal localities of
Mexico, Australia and Brazil are studied for comparison with
Slovakian opals, especially when specific data are not avail-
able in the literature.
3.2 Methods
* We used standard gemological testing equipment
including an ultraviolet lamp with shortwave (254 nm) and
longwave (365 nm) radiation for luminescence observation
in a darkened room, a classical pycnometer for density
measurement and a TOPCON refractometer for indices of
refraction measurements.
* Microstructure pictures observed on polished surfaces
were taken on an optical Olympus microscope with
reflected light, samples being included in an epoxy prepa-
ration and polished with an abrasion powder of 3 µm
monocrystalline diamond.
* XRD pattern are obtained by the classical powder
method, using an INEL CPS120 multichannel detector.
Analytical conditions: a Cu X-ray source was obtained
under a current of 20 mA and 40 kV voltage, the Kα1 line
(λ = 1.5405 Å) being isolated thanks to a Ge crystal
monochromator. Acquisition time was on average 4500
seconds.
* Raman scattering was measured on two instruments:
first, a Fourier Transform Bruker RFS100 spectrometer,
with a Nd:YAG laser excitation at 1064 nm, a power of
300 mW, and a 4 cm
-1
resolution; each spectrum is an
accumulation of 1000 scans. Second, a Jobin-Yvon
T64000 dispersive spectrometer, with the 514 nm excita-
tion of an ionized argon laser, a power of 600 mW, at a
resolution of 1 cm
-1
.
* Electron micrographs were acquired with a field
effect JEOL 6400 scanning electron microscope using a
1nA current with a 7 kV voltage.
* Off line extraction for oxygen isotopes measurements
were achieved by BrF
5
fluorination (Bigeleisen et al.,
1952; Clayton & Mayeda, 1963). Samples were heated at
250°C under vacuum for 2 hours (Brandriss et al., 1998)
before fluorination. Gas and isotopes ratios have been
measured using an ISOPRIME mass spectrometer with off
line standards (NIST 28, MQ: δ
18
O = 10.1 ± 0.3 ‰ based
on 15 replications) for oxygen and then normalized (see
details in Coplen, 1988). Temperature of formation of opal
were re-calculated using different equilibrium equations
corresponding to three ranges of temperature (respectively
Kawabe, 1978, at ca. 100°C; Kita et al., 1985, from 93 to
33°C; Juillet-Leclerc & Labeyrie, 1987, and Brandriss et
al., 1998, for lower temperatures) and different assump-
tions on initial fluid composition. Only temperatures
consistent with the definition range of the used equation
are presented below.
4. Results
4.1 Gemological properties
Gemological properties have been detailed for the
Slovakian samples only (see Table 1). All measurements of
density and index of refraction (RI) are in the usual range
for opal, with a density of 2.10 (measured on several
rough, unfacetted homogeneous samples) and a RI ranging
from 1.43 to 1.46. The low RI of 1.43 is measured on
white, porous samples.
Fluorescence under longwave (LW) is typically white
to greenish-white, and yellowish-white under shortwave
(SW), as expected for opal (Fritsch et al., 2001). A
moderate, greenish-yellow phosphorescence is observed in
Formation conditions of Slovakian opals
793
Fig. 5. Photomicrograph of microstructure in sample OP548 taken
with the SEM on a fresh surface. Silica spheres of about 250 nm in
diameter are disorderly packed as in most white, opaque opals.
Note the cylindrical silica bridges that link some vicinal spheres.

B. Rondeau, E. Fritsch, M. Guiraud, C. Renac
some cases. These emissions are essentially related to
specific surface (Fritsch et al., 2001).
All translucent samples show a yellow to orange-red
transmitted body colour under the binocular microscope, as
the complementary bluish hues are diffracted or scattered at
other angles (preferentially at 90°) due to the play-of-colour
phenomenon. The largest samples (such as the great opal of
King Louis XVIII, n° 87.44, or the rough sample on matrix
n° 98.427, see Fig. 3) often show a lamellar structure with
transparent layers alternating with whitish layers of milli-
metric dimension; the distribution of play-of-colour zones
also follows this layering.
4.2 Microstructure
1- In all samples investigated, silica spheres with a diam-
eter ranging from 125 to 270 nm are observed using the SEM
(Fig. 4a). These measurements are consistent with those
presented in Dódony & Takács (1980). This is a classical
feature for gem opal-A (Jones et al., 1964; Sanders, 1964;
Darragh et al., 1976; Gauthier, 1985) as the geometric
arrangement of silica spheres (Fig. 4a) of such diameter is
responsible for the visible light diffraction. We note that the
diameter of silica spheres in Slovakian opals is similar to that
of Australian (Jones et al., 1964) and Brazilian opals
(Gauthier et al., 1995), as noted earlier by Harman &
Chovanec (1981).
2- However, the diameter of the spheres differs with
respect to the type of opal: from a dozen of measurements,
diameters range from125 to 208 nm in translucent opals,
whereas white, opaque opals show larger spheres ranging
from 230 to 270 nm.
3- Some geometrical sectors of different orientations are
observed within the silica spheres three-dimensional network
(Fig. 4). The bands observed with the SEM are seen as
several sets of parallel bands of different diffraction colours
under the optical microscope. The colour of each band
depends on its orientation and thus of the orientation of the
silica spheres network. The bands are about 500 µm thick and
always intersect with one another at angles of about 60°. This
is explained by a polysynthetic twinning phenomenon that
could have been developed during silica spheres deposition
(growth twinning) or after growth (mechanical twinning) and
caused by a small displacement along "crystallographic"
planes in the spheres lattice. These coloured bands are
possibly oriented following the <111> planes of the stacking
sequence, consistent with the fcc arrangement of silica
spheres classically described for natural (Sanders, 1968) and
synthetic opals (Cheng et al., 1999; Baryshev et al., 2003)
4- A curious structure is observed on Fig. 5, which has
not been previously described to our knowledge: silica
spheres are sufficiently distant so not all are in contact with
one another. In such a case, a cylindrical "bridge" of silica
links two vicinal spheres. This could be the result of a capil-
larity phenomenon involving silica deposition from a silica-
rich water circulation.
4.3 X-ray diffraction pattern
Seventeen XRD diagrams have been acquired on the
Slovakian samples. Figure 6 shows two of them which are
typical. all opals display an amorphous pattern typical for
opal-A, according to Jones & Segnit (1971), with a major
broad band around 4.1 Å and three additional very broad
bands around 1.9, 1.4 and 1.2 Å. This is in agreement with the
earlier work of Harman & Chovanec (1981) on some other
794
Fig. 6. Two typical XRD diffractograms expressed as a function of
d-spacing (distances between lattice planes, calculated from
diffraction angles). All Slovakian opals display XRD pattern typical
for amorphous materials, with a main, broad peak at about 4 Å. (a)
and (b) differ by the width and shape of the main peak. The full
width at half maximum (FWHM) is indicated by a straight line on
each diagram, and reported in Table 3.
Opal-A: d max (Å) d min (Å) FWHM (Å)
OP280 4.44 2.65 1.79
OP281 4.44 2.77 1.67
OP282 4.55 3.19 1.36
OP283 4.44 2.75 1.69
OP284 4.49 3.19 1.30
OP338 4.44 3.11 1.33
O.18 4.44 2.81 1.62
2.828 4.55 3.31 1.24
2.828 4.38 3.34 1.04
52.636 4.49 3.11 1.39
61.275 4.44 3.31 1.13
61.275 4.38 2.79 1.59
86.2 4.44 2.81 1.62
86.2 4.49 3.28 1.21
86.2 4.55 3.31 1.24
98.427 4.61 3.28 1.33
203.1
6 4.44 3.19 1.25
Mean: 4.47 3.07 1.4
Standard deviation: (2σ) 0.13 0.49 0.47
Opal-CT: 203.21 4.27 3.85 0.42
Table 3. X-ray diffraction data: FWHM of the main peak around
4.1 Å, high and low position of the peak at mid-height.
Three measurements in a single sample (86.2) give different results.

Slovakian opals. The amorphous structure of the Slovakian
opal is consistent with "sedimentary"-type opals such as
Australian opals (Jones & Segnit, 1971) and Brazilian opals
from Piaui (Bartoli et al., 1990).
The shape of the main peak around 4.1 Å is not constant.
The full width at half maximum (FWHM) of this peak has
been measured, as drawn with a black line on Fig. 6, as well
as positions of minimum and maximum of the peak at mid
height. These three values are reported in Table 3. For
Slovakian opals, the FWHM of the peak varies from 1.04 to
1.79 Å (see Table 3), the maximum position is about constant
(4.47 ± 0.13 Å, 2σ) and the minimum varies from 2.65 to 3.34
Å (3.07 ± 0.49 Å, 2σ). This peak tends to be larger in some
white, opaque opals than in transparent opals (i.e. samples
OP280 or 86.2), but no co-variation between the peak shape
and the macroscopic aspect of the material has been found.
These variations indicate different degrees of geometric
arrangement of silica molecules within opal-A category.
4.4 Raman scattering spectroscopy
Raman spectra of forty five opals from Slovakia and nine-
teen from other localities have been acquired in order to estab-
lish trends in Slovakian opals with respect to other localities.
Raman spectroscopy has recently been established as a non
destructive method to characterize opal structure (Smallwood
et al., 1997; Ostrooumov et al., 1999).
A general pattern is common to all samples: a broad band
around 300-400 cm
-1
and a series of smaller bands around
780, 990 and 1080 cm
-1
. All these bands correspond to Si-O or
Si-OH vibrations, as detailed in Ostrooumov et al. (1999) and
Zarubin (2001). The "water"-related band around 3200 cm
-1
can be difficult to observe although water is a non negligible
component of opal.
Using this method, two families have been distinguished
according to earlier works of Smallwood et al. (1997) and
Ostrooumov et al. (1999). The two groups differ by the posi-
tion and shape of the main Si-O broad band around 400 cm
-1
and, to a lesser extent, by the shape of the band around 800 cm
-1
.
Figure 7 shows six typical Raman spectra. All samples
of a single locality show similar spectra, and thus only one
of each is shown on Fig. 7 However in Slovakia, the very
rare fire opals differ significantly from colourless opals.
The sixty-four spectra have been indexed in order to define
the differences between the two groups. For clarity, several
parameters have been defined as illustrated on the bottom
of Fig. 7: P
400
and P
800
are the two main Raman peaks
centred around 400 and 800 cm
-1
respectively; the Raman
shift position of the high shift side of the band around 400
cm
-1
at half height is called hereafter S
1
parameter (S for
Slope), and the Raman shift position of the low and high
shift sides of the band around 800 cm
-1
at half height are
called respectively S
2
and S
3
parameters. These parameters
are listed in Table 4. The most discriminating parameters are
the peak maximum around 400 cm
-1
and the S
1
parameter.
The first group of spectra is composed of all Australian
but Tintenbar, Brazilian and most Slovakian opals, white to
colourless. The main Si-O band is dissymmetrical with an
abrupt slope on the high frequencies side. P
400
peak posi-
tion ranges from 390 to 460 cm
-1
(mean value: 423 ±
17 cm
-1
), and the S
1
parameter is much more constant at
494 ± 2 cm
-1
(see end of Table 4). P
800
shows a convex
shape on its high frequency side.
The second group is composed of Mexican opals, the
opals from the Tintenbar volcanic deposit, and some rare
fire opal from Slovakia. P
400
is about symmetrical and
centred around 335 ± 11 cm
-1
in that group. The S
1
param-
eter varies from 422 to 459 cm
-1
(mean value: 439 ±
10 cm
-1
) and is not more significant than the maximum of
the peak. This has to be linked to the global shape of the
peak. P
800
shows a concave shape on its high frequency
side. These values have been found in fire opals from other
deposits in the world (Fritsch et al., 2002).
4.5 Oxygen isotope composition
Oxygen isotopic compositions have been measured in
order to constrain the temperature of formation of the
Slovakian opals.
Formation conditions of Slovakian opals
795
Fig. 7. Six typical Raman spectra in the range 100-1500 cm
-1
of opals
from different deposits: Tintenbar, Australia (OP388), Jalisco,
Mexico (JAL-7), Slovakia (fire opal OP299 and white opal 2.828),
Lightning Ridge, Australia (OP 246), and Piaui, Brazil (OP251). Note
that the spectrum from Lightning Ridge is typical for all Australian
opals except those from Tintenbar. Two groups of spectra can be char-
acterized by the shape of their two major bands: "sedimentary-type"
spectra (Piaui, Lightning Ridge, white opal from Slovakia) show a
dissymmetrical main peak around 423 cm
-1
with an abrupt slope
maximum around 494 cm
-1
, and a convex, broad secondary peak
around 790 cm
-1
. "Volcanic-type" spectra (Tintenbar, Jalisco, fire
opal from Slovakia) show a more symmetrical main peak around 335
cm
-1
with a less abrupt slope maximum in the range 430-450 cm
-1
.
The bottom of the figure shows the position of S
1
, S
2
, S
3
, P
400
and
P
800
parameters, as used in Table 4 for Raman spectra indexation.

B. Rondeau, E. Fritsch, M. Guiraud, C. Renac
Oxygen is present in opal in many molecular (or "crystal-
lographic") sites (Knauth & Epstein, 1982): oxygen linked to
the silica lattice is indeed the most abundant although several
sites and bonding are recognized (Zarubin, 2001), there is no
information on different isotopic composition between these
different sites; however oxygen is also linked with hydrogen,
which can occur in opal in as much as five different sites, each
one showing different oxygen isotopic composition (Knauth &
Epstein,
1982). This feature is due to easy exchanges between
silica and external water (meteoric water as well as laboratory
wet air). This process is likely to affect measurements on silica-
related oxygen. Therefore we decided to heat our samples to
250°C under vacuum in order to extract most of the water (e.g.
Brandriss et al., 1998) and to measure the silica-related oxygen
composition with minimum uncertainties
.
We consider that this measurement gives significant
information on geological formation conditions because:
1- Gem-quality opal was preserved despite its fragility,
meaning that it has not been exposed to drastic alteration
conditions (such as high temperatures) since it formed;
2- As opal did not re-crystallize, its silica-related oxygen
composition was little affected by the contact with water of
different oxygen isotopic composition at temperature lower
than 100°C (O'Neil, 1987).
Six samples have been analyzed: four from Slovakia
(OP547, OP282, 86.2-translucent opal, 86.2-opaque opal),
one from Australia (OP-Aus) and one from Mexico (203.21).
All Slovakian opals are white, transparent to opaque samples
displaying sedimentary-type properties; it could have been
interesting to measure also the isotopic composition of a
Slovakian fire opal, which shows volcanic-type physical
properties: unfortunately, our fire opal samples were histori-
cal polished gems forbidden for such a destructive analysis.
Results are shown in Table 5 and expressed in δ
18
O relative
to SMOW, standard mean ocean water. Slovakian and
Australian opals show similar oxygen compositions with
δ
18
O
SMOW
around 31 ‰ whereas the sample from Mexico
shows a significantly lower δ
18
O
SMOW
at 13 ‰.
Calculating the temperature of formation requires to
know the oxygen isotopic composition of the initial water
from which opal formed, as it has been done for other sili-
ca deposition (Harris, 1989); in our case, this initial value
is not known. However, we consider that the initial water
was in all cases of meteoric origin, even in the case of vol-
canic-type opals, as the amount of water needed to form
opal is too high to be originating from the incoming
magma. The highest value of δ
18
O of water is considered at
0 ‰ (sea water) and gives the maximum temperature of
opal formation: 45°C for Slovakian and Australian opals,
and 190°C for Mexican opals.
The low temperature of formation of Slovakian and
Australian opals is consistent with low temperature forma-
tion (down to 50°C) calculated for agates with a similar
high δ
18
O > 30 ‰ (Götze et al., 2001).
796
Table 4. Indexation of Raman spectra of Slovakian opals and additional localities for comparison.
S
1
S
2
S
3
Peak around Peak around
Sample Deposit P
400
peak parameter P
800
peak parameter parameter 950 cm
-1
1070 cm
-1
H7152* Slovakia 431 495 804 777 836 955 n.a.
H7154* Slovakia 438 497 809 775 834 957 1070
H7155* Slovakia 444 494 805 775 832 970 n.a.
H7159* Slovakia 438 492 803 775 836 972 1068
H7160* Slovakia 430 491 797 775 803 972 1072
H7160 Slovakia 424 492 800 773 831 976 1066
H7161* Slovakia 446 495 800 777 833 972 1077
H7161 Slovakia 434 492 787 777 835 976 1066
0.18* Slovakia 455 494 804 n.a. n.a. 972 n.a.
2.828 Slovakia 415 492 794 776 833 972 1069
2.828 Slovakia 420 492 791 774 831 976 1068
2.828 Slovakia 403 492 791 775 831 970 1070
61.275* Slovakia 450 497 797 774 830 962 1066
86.2 Slovakia 420 496 791 775 837 968 1066
86.2 Slovakia 420 494 794 775 833 974 1066
86.2 Slovakia 424 492 791 773 833 974 1066
86.2 Slovakia 407 494 791 773 823 970 n.a.
86.2 Slovakia 432 494 793 773 833 974 1072
87.34c Slovakia 432 492 798 775 833 968 1074
87.34e* Slovakia 445 495 802 774 833 977 1078
87.34f* Slovakia 426 493 797 775 834 975 1068
87.34g* Slovakia 450 494 801 778 832 973 1068
87.34h* Slovakia 458 496 799 776 839 975 1077
87.34i* Slovakia 453 494 797 776 836 973 1066
87.44* Slovakia 445 496 797 775 834 972 1065
98.427 Slovakia 397 492 789 775 830 972 1086
203.16* Slovakia 440 495 791 775 830 963 1077
203.16 Slovakia 415 492 797 775 837 964 1066
OP281 Slovakia 403 495 798 778 833 974 1061
OP282 Slovakia 390 492 793 775 821 970 1088
OP282 Slovakia 417 495 797 774 836 976 1069
OP283 Slovakia 403 492 798 772 837 962 1082
OP284 Slovakia 414 492 797 771 828 964 1062
OP294 Slovakia 415 492 795 775 833 974 1070

Mexican opals have been generated at higher tempera-
ture, possibly up to 190°C: this is consistent with a tem-
perature of about 160°C that has been calculated from fluid
inclusions homogenization temperature in volcanic opal
from Mexico (Spencer et al., 1992).
5. Discussion and conclusion
In general, two groups of gem opal deposits are distin-
guished on the basis of physical properties of opal
(Smallwood et al., 1997; Ostrooumov et al., 1999; Fritsch
et al., 1999; Smallwood, 2000):
- “Sedimentary-type opals” is the term used in the liter-
ature for opal deposits in sedimentary rocks, such as opals
from Australia (Jones et al., 1964; Keller, 1990) and Piaui
in Brazil (Cassedanne, 1968; Bittencourt-Rosa, 1988;
Bartoli et al., 1990). They typically show an amorphous
XRD pattern, a main Raman peak (Si-O-Si) around 423 ±
17 cm
-1
, a S
1
parameter at 494 ± 2 cm
-1
, and silica spheres
ranging from 125 to 300 nm large.
- “Volcanic-type opals” are opals typically found in rhy-
olitic tuffs such as opals from Mexico (Koivula et al., 1983;
Gübelin, 1986), Honduras (Fritsch et al., 1999), Ethiopia
(Gauthier et al., 2004), Kazakhstan (Holzhey, 1991),
Madagascar (Lacroix, 1922), Tintenbar in Australia and a
rare fire opal from Brazil. They all show an XRD pattern
typical for opal-CT to opal-C, a main Raman peak
Formation conditions of Slovakian opals
797
Means and standard deviations are indicated at the bottom of the table for the two groups of spectra as distin-
guished on Fig. 7. P
400
, as well as the S
1
parameter (and to a lesser extent, the S
2
parameter) are found to be
discriminating parameters for the two groups. (F): three fire opals from Slovakia; (T): Australian opals from
Tintenbar; (*): spectra acquired using the T64000 dispersive spectrometer, the others being measured using the
Bruker RFS100 FTspectrometer.
Table 4. continue
S
1
S
2
S
3
Peak around Peak around
Sample Deposit P
400
peak parameter P
800
peak parameter parameter 950 cm
-1
1070 cm
-1
OP295 Slovakia 422 496 793 775 831 974 1066
OP296 Slovakia 422 493 793 775 833 970 1068
OP297 Slovakia 422 494 791 775 831 970 1074
OP298 Slovakia 422 496 793 777 835 975 1076
OP338 Slovakia 418 494 804 779 833 968 n.a.
OP347 Slovakia 415 496 798 775 832 980 1066
OP547 Slovakia 418 496 794 775 831 960 1066
OP548 Slovakia 424 494 791 773 830 970 1066
OP299 (F) Slovakia 350 422 783 769 804 972 1072
OP300 (F) Slovakia 355 438 783 769 803 972 1078
OP346 (F) Slovakia 324 442 781 768 812 962 n.a.
OP250 Brazil 407 494 796 774 836 962 1073
OP251 Brazil 404 493 792 775 834 966 1071
OpalBREF Brazil 421 492 790 775 826 962 1066
OpalBRF1 Brazil 407 493 794 774 831 963 1063
OP AUS* Australia 435 493 801 774 838 960 1068
OP236 Australia 395 494 791 774 831 964 1072
OP243 Australia 423 494 799 775 836 956 1071
OP243 Australia 377 494 791 771 n.a. n.a. n.a.
OP246 Australia 408 492 792 776 831 956 1070
OP246 Australia 411 494 790 774 819 958 1066
OP387 (T) Australia 334 434 781 769 800 972 1076
OP388 (T) Australia 337 432 779 769 799 962 1076
OP398 (T) Australia 330 438 781 770 804 968 1076
OP422 (T) Australia 338 444 779 771 797 962 1076
JAL-10 Mexico 318 429 780 768 827 n.a. 1079
JAL-7 Mexico 324 448 781 770 813 972 1080
203.21* Mexico 337 n.a. 780 772 830 968 1087
OP355 Mexico 341 459 779 769 804 974 1072
OP362 Mexico 334 447 783 766 835 982 1090
Group 1 423 494 796 775 832 969 1070
St. deviation 18 2 5 2 6 6 6
Group 2 335 439 781 769 811 970 1078
St. deviation 11 10 2 2 13 6 6
Table 5. Oxygen isotopic composition (δ
18
O) for six samples dried
at 250°C.
Sample Yield δ
18
O
(SMOW)
(µmol of O
2
/mg) (‰)
OP547 16.5 31.5
OP282 16.9 29.3
86.2OP 16.5 31.5
86.2Hy 16.7 31.4
OPAust 15.9 32.3
203.21 16.5 13.0
MQ 16.8 10.1
Slovakian and Australian opals display a very high δ
18
O at
about 31 ‰ (29.3 to 31.3 ‰), whereas that of Mexican opal
is significantly lower at 13 ‰. MI: milky, translucent opal;
WH: white, opaque opal; MQ: Standard quartz.

B. Rondeau, E. Fritsch, M. Guiraud, C. Renac
(Si-O-Si) around 335 ± 11 cm
-1
, a variable S
1
parameter at
439 ± 10 cm
-1
and silica spheres 20 to 50 nm large.
Following this division, it was first tempting to link the
opal formation process to the volcanic activity, as is the
case for Mexican or Ethiopian opal deposits: however, we
have seen in this study that Slovakian opals surprisingly
show all physical properties typical for a "sedimentary-
type" deposit, although they are found in a volcanic host
rock. Moreover the geological setting indicates that
Slovakian opal deposition is related to a low temperature
tectonic event, responsible for breccia formation, rather
than to a high temperature volcanic event. This setting is
close to the tectonic-controlled model proposed for the for-
mation of Australian opal (e.g. Payette, 1999).
Furthermore, by comparison to all opal deposits, for-
mation temperature appears as the most discriminating cri-
terion between the two types of opal defined above.
Temperatures for the formation of Slovakian and Australian
opals (which show a similar δ
18
O at about 31 ‰) are lower
than 45°C, contrasting with Mexican opal (with
δ
18
O = 13 ‰) formed at a temperature possibly as high as
190°C. Although biogenic opal-CT is known to form at
very low temperature (Botz & Bohrmann, 1991), we
believe that biologic activity is largely more important in
see water sedimentation processes than in the gem-opal
formation processes (low or high temperature types) in a
continental environment. Australian gem opal genesis, for
example, is easily explained by physical and chemical phe-
nomena only (Harder, 1995), even if a biogenic origin has
been also proposed for them (Payette, 1999).
From these data, we propose to revisit the gem opal clas-
sification based on the geological environment: the terms
"sedimentary" and "volcanic"-type appear misleading in the
case of Slovakian opals. The terms "low temperature" and
"high temperature" are more appropriate to describe the
genesis of gem opal-A and opal-CT respectively.
These two types could co-exist in the Slovakian deposit:
by comparison with the Australian opals, most Slovakian
opals precipitated during a tectonically-controlled low tem-
perature stage, whereas the formation of the rare fire opals,
which display every high temperature-type physical prop-
erty, is possibly related to a higher temperature stage.
Whether the occurrence of the two types is related to one
single event (from high to low temperature crossing a
threshold temperature) or to two separate events (with the
high temperature possibly being liked to the andesitic
stage) cannot be known from the present data. Moreover, as
the oxygen isotopic composition of the Slovakian fire opals
has not been undertaken, the question of their temperature
of formation remains unanswered, until new samples from
this locality are unearthed either from mineral collections
or from new field exploration.
Acknowledgments: Pr. Rudolf
Ďud’a, Mineralogy
Museum of Ko
šice, Slovakia, is thanked for providing pub-
lications on geology, mineralogy and historical aspects of
Dubník opal deposit; the Royal Ontario Museum and
Jurgen Schutz (Emil Weis Opals, Kirschweiler, Germany)
for the loan of several rough and facetted Slovakian opals;
Jaroslav Hyrsl (consultant in Prague, Czech Republic) for
providing most of the CRG rough opal samples from
Slovakia; Franck Notari (Gemtechlab, Genève,
Switzerland), Jean-Marc Fourcault (MNHN), MM. J.-C.
Samama, M. Tremonti and Don Cabrera for providing other
samples. Pr. Jean-Pierre Gauthier, Lyon University, is
thanked for constructive comments on opal structure.
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Received 25 November 2003
Modified version received 3 March 2004
Accepted 17 May 2004
Formation conditions of Slovakian opals
799