Common gem opal: An investigation of micro- to nano-structure
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American Mineralogist, Volume 93, pages 1865–1873, 2008
Common gem opal: An investigation of micro- to nano-structure
EloïsE Gaillou,1,2,* EmmanuEl Fritsch,1 BErtha aGuilar-rEyEs,3 BEnjamin rondEau,1
jEFFrEy Post,2 alain BarrEau,1 and mikhail ostroumov3
1
Université de Nantes, Nantes Atlantique Universités, CNRS, Institut des Matériaux Jean Rouxel (I.M.N.), UMR 6502, 2 rue de la Houssinière,
B.P. 32229, Nantes, F-44000 France
2
Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20064, U.S.A.
3
Universidad de Michoacán de San Nicolas de Hidalgo, Ciudad universitaria, Fransisco J. Mujica S/N, Apartado postal 52B, C.P. 58000 Morelia,
Michoacán, Mexico
aBstract
The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide
localities was investigated using scanning electron microscopy (SEM). These opals do not show
play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and
opal-CT are primarily built from nanograins that average ~25 nm in diameter. Only opal-AN has a
texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive
concentric layers, or in some cases, radial structures. Common opal does not diffract light because
its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well
ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from
late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from
none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or
platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally
cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth
or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for
non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins
in the spheres of opal-AG.
Keywords: Opal-A, opal-CT, common opal, structure, SEM, nanograin
introduction the proportion of cristobalite much greater than that of tridymite;
Opals are natural hydrous silica with either amorphous we did not encounter this type in our study). The basic types are
(opal-A) or disordered cristobalite/tridymite structures (opal- typically identified using X-ray diffraction (XRD) (e.g., Jones
CT). Gem opal is best known for the highly prized variety and Segnit 1971; Elzea and Rice 1996), but opal-A can also be
showing diffraction of visible light, called play-of-color opal. distinguished from opal-C and -CT on the basis of Raman scat-
Yet, the most widespread gem varieties, so-called common opals, tering spectroscopy (Ostrooumov et al. 1999).
do not show play-of-color but are valued in the gem trade for Langer and Flörke (1974) subdivided opal-A into two
their attractive body colors. The only detailed studies of common groups on the basis of features observed in small angle X-ray
opals reported to date are for Australian potch opals (Bayliss and neutron-scattering experiments: (1) opal-AN (network),
and Males 1965; Barnes et al. 1992) and biogenic opals (e.g., or “hyalite,” which shows only diffuse scattering of X-rays
Kastner et al. 1977; Botz and Bohrmann 1991; Graetsch 1994; or neutrons at small angles, suggesting that it has a glass-like
Elsass et al. 2000). The picture emerging from the previous structure; and (2) opal-AG (gel), which is the most widespread
studies is that opal-A consists of regular spheres and opal-CT of variety. In small-angle X-ray or neutron patterns, it exhibits obvi-
spherical aggregates of plate-like cristobalite crystallites, called ous intensity maxima superimposed upon the diffuse scattering,
lepispheres. In this study, we characterized a large number of indicating a structure consisting of packed silica spheres. The
common gem opals from a wide variety of geologic settings term opal-A typically is synonymous with opal-AG.
and localities to provide a more complete understanding of the Structure of opal-AG
structure of these materials and to determine if they are consistent
with this model. We do not consider here biogenic opals, which The first scanning electron microscopy (SEM) study of the
are not used as gems. structure of opal was published by Jones et al. (1964) for an
Australian play-of-color opal-AG. They demonstrated that it is
BackGround constructed of a near-perfect 3-D stacking of monodisperse silica
There are three recognized opal varieties: opal-A (amor- spheres, which diffracts visible light if the spheres have diameters
phous); opal-CT (cristobalite-tridymite, which consists of disor- ranging from ~150 to 300 nm (Sanders 1964). Common opal-AG,
dered α-cristobalite with tridymitic stacking), and opal-C (which named potch opal by Australian miners, appears to be made of
the same type of spheres (Darragh and Gaskin 1966; Rau and
* E-mail: gailloue@si.edu Amaral 1969; Sanders and Darragh 1971). The absence of visible
0003-004X/08/1112–1865$05.00/DOI: 10.2138/am.2008.2518 18651866 GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL
light diffraction by these common opals is attributed primarily mentary opal-CT is made of 2–5 µm-sized lepispheres (Flörke et al.
to inhomogeneity in the sphere diameters, which makes regular 1975) that can be arranged into a diffracting network (Fritsch et al.
stacking impossible. Lack of light diffraction may also be due 2002). In addition to the lepispheric structure, opal-CT can exhibit
to irregular sphere shapes (Darragh et al. 1966; Sanders and spherulitic or fibrolitic structures (Heaney et al. 1994 and references
Darragh 1971), spheres that are too large (up to 1000 nm; Cole therein). Fritsch et al. (1999, 2002, 2004, 2006) showed that several
and Monroe 1967), or spheres having the same refractive index varieties of opal-CT are composed of 10–40 nm particles that can
as the cement in-between (Graetsch and Ibel 1997). give rise to granular (especially for fire opals) or fibrous (in some
Cementation and section-orientation effects. The silica common pink opals) structures.
spheres in opal-AG are generally cemented by small particles
(Sanders and Darragh 1971). The cement is sufficiently strong matErials and ExPErimEntal mEthods
that a fracture induced to prepare an SEM sample passes through Materials
the spheres rather than between them. On a fresh break, it is Samples of common opals were chosen to cover the widest range of gem
typically difficult to observe the spherical structure, which is materials found in the trade. They come from mines in Mexico (77 samples),
only revealed by etching. Ethiopia (18), Australia (18), Honduras (13), France (8), Turkey (8), Slovakia (7),
Brazil (6), Mali (6), Tanzania (6), Venezuela (5), Kazakhstan (3), Madagascar
It is not straightforward to measure the diameters of the (3), Peru (3), the U.S.A. (3), Austria (2), Serbia (1), and Tchequia (1). Their body
spheres (Sanders and Darragh 1971), as the fracture might pass colors span almost the entire visible spectrum (blue, green, yellow, orange, red,
anywhere through a sphere, not necessarily through the full pink, white, brown, gray, and black), and range from transparent to opaque. As
diameter. Therefore, only the largest measure might serve as an far as was possible for each locality and type of material, a majority of samples
deemed typical were analyzed along with some unusual ones. Opal-CT samples
estimate of sphere diameter.
come from all the countries cited above, and common opal-AG samples are from
In some play-of-color opals from South Australia, Akizuki Australia, Honduras, Mexico, Slovakia, France, the U.S.A., Austria, and Brazil.
(1970) observed a ripple-like pattern at low magnification on the Samples came from volcanic as well as sedimentary environments. A complete list
SEM (and with an optical microscope). This effect is a manifesta- of sample descriptions is in Appendix 1. Part of our study involved a contract with
tion of a fracture that is oriented obliquely to a plane of packed ECOS to study Mexican opals, which explains the large number of samples from
that country. Opals were collected in the field during four trips to the Mexican high
spheres (Gauthier 1986). plateaus. Two samples from Australia (no. 733 and 824) with slight play-of-color
Internal structure of the spheres. Etched Australian were selected to show the subtle differences at the microscopic scale between
opal spheres commonly show a concentric shell-like structure common and play-of-color opals.
(Darragh and Gaskin 1966). The spheres range from solid to
multishelled, with up to three shells. It has been suggested that Experimental methods
the shell-like structure is the result of the deposition around a Samples were identified as opals based on their gemological properties, includ-
ing a specific gravity between 1.97 and 2.2, and index of refraction between 1.42 and
nucleus (Rau and Amaral 1969). The shells are composed of
1.46. They were further classified as opal-AG or -CT based on the position of the
~50 nm-sized silica particles (Darragh et al. 1966; Sanders and main broad Si-O-Si band in Raman scattering spectra, 423 ± 17 cm–1 for opal-AG
Darragh 1971), although such particles are not always observed and 335 ± 11 cm–1 for opal-CT (Ostrooumov et al. 1999; Rondeau et al. 2004).
(Dódony and Takacs 1980), and are commonly referred to as A refractometer was used to measure the index of refraction with an optical
primary particles (Darragh et al. 1966; Sanders and Darragh contact liquid having n = 1.79. The specific gravity was measured by the hydrostatic
method. The variety of opal (-A or -CT) was determined with a Bruker RFS 100
1971). Similar primary particles were observed in synthetic opal Fourier transform Raman spectrometer using operating conditions described by
(Sanders and Darragh 1971). Smallwood et al. (1997) and Ostrooumov et al. (1999); 1000 scans (Appendix 11)
Darragh et al. (1966) and Sanders and Darragh (1971) showed were accumulated at a power of 350 mW and a resolution of 4 cm–1, using a 1064
that the concentric structure is typical of opals from the Central nm Nd YAG laser for excitation, which eliminates most luminescence.
High-resolution images were obtained with a JEOL 6400 SEM equipped with a
Australian fields, and stated that “American opals or equivalent
field-emission electron gun, using a current of 7 kV and 6 × 10–11 A. Two types of samples
opals” (which may mean opal-CT) do not exhibit such a structure. were imaged for each opal: freshly fractured, and fractured followed by etching in 10
However, Sanders (1976) described a star opal from Idaho in vol% HF for 30 s (standard etch to reveal opal microstructures). Samples were coated
which the spheres have six shells. Dodóny and Takacs (1980), with ~5 nm of a gold-palladium alloy.
on the other hand, reported that silica spheres in opals from
Cervenica, Slovakia showed neither a layered internal structure rEsults
nor any primary particles. Structure of common opal-AN
Most observations described above (which were done primar- Opal-AN is rarely used as a gem. The results from the two
ily in the 1960s or the 1970s) were performed on replicas, not samples that we analyzed (from Bohemia, Czech Republic, and
directly on opal. This approach might introduce artifacts or might Mexico) are consistent with those of Langer and Flörke (1974).
not preserve certain extremely fine details. Relatively few studies Opal-AN is typically botryoidal and colorless. The fracture sur-
have been reported using direct observation on fresh opal surfaces
(e.g., Gauthier 1986; Fritsch et al. 1999, 2002, 2004, 2006). 1
Deposit item AM-08-053, Appendix 1. Deposit items are avail-
Structure of opal-CT able two ways: For a paper copy contact the Business Office of
the Mineralogical Society of America (see inside front cover
Opal-CT is often considered a transitional material between of recent issue) for price information. For an electronic copy
opal-AG and quartz (Flörke et al. 1976), as is the case for visit the MSA web site at http://www.minsocam.org, go to the
biogenic opals (e.g., O’Neil 1987), such as those formed from American Mineralogist Contents, find the table of contents for
diatoms (e.g., Clarke 2003). Most previous structural studies of the specific volume/issue wanted, and then click on the deposit
opal-CT were done on marine biogenic opals. Biogenic and sedi- link there.GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL 1867
faces appear smooth in SEM images, without obvious micro- to
nanostructural features either on freshly broken surfaces or after
HF etch, and are similar to those of Libyan Desert glass (Fig. 1).
Structure of common opal-AG
Arrangements of spheres not giving rise to play-of-color.
We observed several conditions for which opal-AG does not
exhibit play-of-color.
Different size spheres. Our study determined that 48% of
the common (no play-of-color) opal-AG specimens consist of
silica spheres that are not of uniform diameter. This finding is
consistent with results from previous studies concluding that
polydisperse spheres are the primary cause of lack of play-of-
color in opals (Darragh and Gaskin 1966; Rau and Amaral 1969;
FiGurE 1. The smooth texture of the freshly broken surface in (a) Sanders and Darragh 1971). We observed that a variation of only
opal-AN (“hyalite”) from Tchequia (Bohemia, no. 942) compared to that 5% in sphere diameters is sufficient to preclude diffraction. In
of (b) Lybian desert glass (both images 30 000×). one Mexican common opal sample (Fig. 2a), diameters range
F i G u r E 2. SEM
images (30 000×) of
common opal-A samples,
after HF etching (except e
and f). (a) Opaque orange
opal-AG from Mexico
(Mina Iris, Queretaro, no.
759). The silica spheres
vary in diameter from 2
to ~250 nm. Both small
and large spheres show
concentric structures, and
the larger the sphere, the
more numerous the layers.
(b) Opaque orange opal-
AG from France (Saint-
Nectaire, no. 950) with
spheres ~6.5 to 7.5 µm in
diameter. Note the overall
botryoidal appearance.
(c) Gray opal-AG from
Australia (Lightning
Ridge mine, New South
Wales, no. 235C) showing
spheres that are not
spherical, but commonly
elongated, leading to
an imperfect packing,
which cannot diffract
light. (d) Transparent
opal-AG from Honduras
(no. 684C) consisting
of well-ordered spheres
too large (~650 nm) to
diffract light. Concentric
layering is seen in the
center of spheres and a
radial structure in the
rims. (e) Fire opal-AG
from Slovakia (Dubník,
no. 637) consisting of
spheres that are not ordered and are too small (~80 nm in diameter) to diffract light. (f) White opal-AG from Honduras (no. 671) with spheres ~280 nm in
diameter (adequate for a red play-of-color), but do not show a regular arrangement.1868 GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL from ~250 to 2000 nm. Similar, large variations were observed they were soft when they were stretched, or that they originally for ~23% of the samples, especially for opals from Honduras, formed in that shape. France, and Slovakia. Interestingly, our results revealed that Spheres that are too large or too small. We found only one contrary to conclusions from the earlier studies, which were common opal-AG, from Honduras (sample no. 684), in which based on a limited number of samples, most Australian common spheres are in a well-packed arrangement but have diameters opals have monodisperse spheres. In the rare cases where the too large (~650 nm) to diffract light (Fig. 2d). In theory (Sand- silica spheres in Australian common opals are polydisperse, the ers 1964), spheres that are too small (
GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL 1869
revealed only after HF etch and is never visible on fresh breaks, Compaction of the spheres. We observed in many samples
suggesting that the individual layers etch differently. In most the effects of compaction of the silica spheres, although we
samples of opal-AG, the silica spheres show concentric layer- did not find mention of it in the literature on natural opals. The
ing, which is more-or-less pronounced in different samples structure of opal-AG is generally believed to consist of rela-
and deposits. For example, the spheres in opals from Mintabie, tively perfect spheres, such as those in Figures 2a, 2e, 2f, or 3c.
Andamooka, and Quilpie, South Australia, as well as from However, spheres may be distorted through compaction, after
Madagascar, show a small depression about 25 nm in diameter they settle, leading to polygonalization (that is, spheres with
in their centers (indicated by arrows in Fig. 3a). Apparently, the polygonal, commonly hexagonal cross-sections). In Figure 2d,
silica in the centers of the spheres is more easily etched and, the compaction effect is very noticeable, but in Figures 3a and
possibly, is the nucleus from which the spheres grew. 3b, spheres are not well packed and the compaction is less. There
The typical concentric structures we observed are shown in is no one deposit particularly noted for producing opals with
Figure 3b for a common opal-AG from Coober Pedy, South Aus- compaction features, and within a deposit, compaction varies
tralia. The HF etch revealed spaces between layers, and showed from sample to sample.
that most spheres consist of three, and in some cases up to six
layers. In one exceptional opal from Slovakia, we observed that Structure of common opal-CT
each individual layer (Fig. 3c) is made up of spherical grains. The In contrast to opal-AG, opal-CT shows a wide variety of in-
thickness of a layer and the average diameter of the grains within ternal structures, but none containing true silica spheres. As with
is ~25 nm, which also is the size of the central depression seen in opal-AG, the structures are always built from silica nanograins,
Figure 3a. These grains are similar to the nanograins described ~25 nm in diameter. We describe here the various structures we
by Fritsch et al. (2002, 2006) in fire opal-CT, and seem to be the observed in common opal-CT in order of increasing degree of
primary building blocks of the spheres in opal-AG. organization.
Figure 3c shows a sphere with three nuclei, each of which is Random aggregation of individual nanograins. Approxi-
surrounded by several layers of nanograins. This situation is not mately 55% of our common opal-CT samples have randomly
unusual and, in this sample, we observed up to six nuclei within arranged nanograins (Fig. 4a), and this is the case for ~80% of
a single sphere. We also documented several common Australian fire opals (transparent, bright orange opals; Fritsch et al. 2002,
opal-AG specimens (15% of our Australian samples) in which 2006). On a freshly broken surface, the internal structure ap-
some spheres contain two (and rarely three) nuclei. pears compact and vaguely granular. HF etch revealed distinct
In addition to the more usual concentrically layered spheres, individual nanograins that are approximately spherical (Fig.
we also observed spheres having radial structures in transparent 4a). The sizes of the grains range from 10 to 50 nm in diameter,
common opal-AG from Honduras (13 samples). In these opals with an average at ~25 nm, which is consistent with measure-
(Fig. 2d), spheres that are cut through their centers show concen- ments obtained using atomic force microscopy (AFM) without
tric structures in their cores, and radial structures around them. any sample preparation (Fritsch et al. 2006). We observed this
The transition between the concentric and radial structures is type of structure, which does not give rise to diffraction of light,
abrupt. The radial structures are formed by the alignment of ~25 in some samples from Australia, Brazil, Ethiopia, Kazakhstan,
nm silica grains from the centers to the edges of spheres. Mexico, Tanzania, and Turkey. All of these deposits formed in
Cementation and section-orientation effects. After sedi- volcanic environments. We did not observe structural features
mentation of the spheres, a new episode of silica precipitation that are specific to fire opals from different localities.
might occur, which settles in the voids left between spheres, Nanograins arranged in fibers. We observed silica nano-
and acts as a cement. In most opal-AG samples, the voids are grains ordered in one dimension to form fibers in some opaque,
almost entirely filled by silica cement, making it difficult to typically pink, opal-CT specimens from Peru (Fig. 4b) and from
observe individual spheres on fresh breaks (Fig. 3d), presumably Mexico (Mapimi and Uruapan areas, states of Durango and
because both the spheres and cement are similar in composition Michoacán, respectively). These opals are composed of bundles
and structure. A brief HF etch reveals the outline of the silica of subparallel fibers (Fig. 4b). A single fiber typically is ~20–25
spheres because their boundaries with the cement are surfaces nm in diameter, which is the average size of the nanograins, or a
of preferred dissolution. multiple of that value. The fibers are typically more than 4 µm
The familiar picture of opal-AG with non-cemented, perfectly long. The opal fibers possibly form through the accumulation of
formed, individual spheres (e.g., Heaney et al. 1994) depicts a nanograins against lath-like palygorskite crystals (abundant in
situation that is, in fact, not common in nature. We observed these opal varieties) that act as a template (Fritsch et al. 2004).
spheres without cementation in only a few opals from Slovakia Nanograins arranged in platelets. In many white opal-CT
(Fig. 2e) and Honduras (Fig. 2e). These opals are white, chalky, samples, the nanograins form platelets that are ~25 nm thick
and extremely porous, and would be considered gems only (once again, the size of the elementary nanograin) and typically
after color treatment. Opal-AG samples from Australia, Brazil, 300 nm wide (Fig. 4c). The finely crenulated edges of individual
France, Madagascar, Mexico, and the U.S.A. always contain platelets (Fig. 4c) reveal their nanograin structures. In most cases,
some cement. several platelets are stacked to form thicker tablets (25% of our
None of the section-orientation effects described above were common opal-CT samples).
observed on common opal-AG, although such effects have been Figure 4d shows tablets that mostly are bundles of four (~100
previously mentioned for play-of-color opal-A samples (Akizuki nm thick) or more platelets. The bundles commonly intersect
1970; Gauthier 1986). each other, but not at specific angles, as has been reported for1870 GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL
biogenic opal (Flörke et al. 1976). The tablets typically are ran- structures of these opals from different localities are similar,
domly oriented, leaving voids between them that are responsible although we did observe some differences. For example, opals
for light scattering and opacity, typically resulting in a white from Turkey have tablets typically ~300 nm thick, whereas they
color. We documented this structure type in common opal-CT are ~100 nm on average in Mexican opals.
samples from Australia (Norseman), Ethiopia, France, Mada- Nanograins arranged in lepispheres. Platelets described
gascar, Mexico, Turkey, and Venezuela. In general, the tabular above may intersect to produce sphere-like aggregates (Fig. 4e),
FiGurE 4. SEM
images (30 000×; except
h: 500× and h′: 10 000×) of
common opal-CT samples
after HF etching (except c
and e). (a) Fire opal from
Mexico (Olimpia mine,
Queretaro, no. 751). The
structure is composed of
an irregular arrangement of
~25 nm silica nanograins.
(b) Pink opal from Peru
(Acari mine, Arequipa, no.
817) with a structure built
from bundles of fibers.
Each fiber is ~25 nm in
diameter. (c) Fire opal
from Mexico (Cerro Viejo
mine, Queretaro, no. 408)
having a structure made
of randomly arranged
platelets (~25 nm thick
and 300 nm wide).
Elementary nanograins
are visible on the irregular
edges. (d) White opal from
Mexico (Las Crucitas
mine, Jalisco, no. 504B).
Nanograins form platelets
that are stacked to form
tablets that randomly
cross each other. In this
particular sample, as in
most Mexican samples,
tablets generally are
four platelets thick. (e)
White chalky opal from
Mexico (La Carbonera
mine, Queretaro, no. flot)
constructed of platelets
that are assembled into
a spherical object, a
lepisphere. (f) Fire opal
from Mexico (La Lupita
mine, Jalisco, no. 737).
After HF attack, the silica
lepispheres were dissolved
to give holes, which are
surrounded by thick walls
of individual and less
soluble nanograins. (g)
Milky opal from Mexico
(La Fe mine, Queretaro,
no. 734). The holes left by dissolved lepispheres are well ordered, but are too large (~480 nm) to diffract visible light. (h–h′) Milky opal from
Mexico (La Fe mine, Queretaro, no. 734). The alternating light and dark strips are a sectioning effect as described in the text.GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL 1871
called lepispheres (Flörke et al. 1976), that resemble gypsum opal-AG replicas by SEM (Darragh et al. 1966; Sanders and
desert roses. All platelets have approximately the same diameter Darragh 1971), but the concept of primary particles in opals
with a nearly common center, hence the spherical form. We have was not expanded upon until recent publications by Fritsch et al.
observed lepispheres ranging in diameter from ~250 to 1000 nm (1999, 2002, 2004, 2006) who observed similar “nanograins” in
(800 nm in Fig. 4e). Opals constructed of disordered lepispheres opal-CT. Our study indicates, however, that opal-AN does not
are not widespread, and only two chalky white Mexican samples appear to have nanograins, which is consistent with conclusions
display this structure (El Cobano mine, Jalisco; La Carbonera by Langer and Flörke (1974) that this opal type does not have a
mine, Queretaro). distinct internal structure. The absence of nanograins in opal-AN
Individual lepispheres might not be obvious on freshly broken suggests that it can be distinguished from opal-AG using SEM
surfaces. Even when they are not visible, however, the fresh break images, a generally faster, more accessible approach than X-ray
typically exhibits a granular texture. Figure 4f shows the surface small-angle scattering.
of a fire opal from Mexico after etching. The approximately
spherical holes are surrounded by thick walls made of individual Influence of growth rate on opal structures
nanograins (~25 nm). Presumably, lepispheres were dissolved, To understand the difference between the concentric vs. radial
leaving the holes. The lepispheres were not well ordered and structures of the silica spheres in opal-AG, it is useful to draw a
range in size from ~100 to 200 nm. In this situation, we did parallel with the growth of single crystals (even if opals are not
not observe the lepispheres, just their molds. About 15% of our single crystals). Layered crystal growth (which corresponds to
common opal-CT samples consist of lepispheres embedded in the concentric structure) typically occurs in stable environments
a silica nanograin matrix. at low growth rates. By contrast, radial growth is typical of
We also distinguished another variety of common opal-CT in higher growth rates (Sunagawa 2005), suggesting that the radial
which lepispheres are well ordered but are too large to diffract structure is linked to fast deposition. Therefore, most opal-AG
visible light. Figure 4g illustrates a milky opal from Mexico samples with concentric structures forms in environments with
in which the ordered lepispheres are approximately 480 nm in low growth rates, which is consistent with the stable environment
diameter. Similar structures were also observed in some trans-
parent opals, with various body colors, from Honduras, Mexico,
Somalia, and Ethiopia. The largest monodisperse, well-ordered,
dense, lepispheres, ~800 nm in diameter, were observed in an
opal-CT specimen from the Magdalena mine, Jalisco, Mexico.
Such opals might diffract light in the near-infrared region.
Section-orientation effects. Although section effects have
been described for opal-AG (Akizuki 1970; Gauthier 1986),
none have been reported for opal-CT. We observed such ef-
fects in approximately two-thirds of common opal-CT samples
having lepispheres in a matrix. In Figure 4h, for example, one
can see light and dark strips corresponding to steps between the
different layers of lepispheres. The light strip corresponds to the
section passing approximately through the center of lepispheres,
whereas dark strip corresponds to the section passing mostly in
the matrix, between lepispheres.
discussion
Common and play-of-color opal: Where to draw the line?
We have encountered some specimens that are transitional
between common and play-of-color opals, i.e., they are play-
of-color opals of low quality, having only limited domains with
well-ordered spheres (Figs. 5a–5b). As these opals appear mostly
disordered in SEM images, they are common opals if judged by
volume, but play-of-color opals based on their visual appearance.
Such opals make up ~20% of all play-of-color opals from each
locality, including both opal-AG (Fig. 5a) and opal-CT (Fig. 5b).
The differentiation between common and play-of-color opal is
therefore not always straightforward.
The nanograin is the elementary building block of all
FiGurE 5. SEM images of borderline play-of-color opals after HF
opals, except opal-AN
etch showing only small regions with well-ordered spheres; the majority
Our observations indicate that all samples of opal-AG and of the opal is not ordered. (a) Play-of-color opal-AG from Australia
opal-CT are built from primary particles, or nanograins, ~25 (Lightning Ridge, New South Wales, no. 824) (10 000×). (b) Play-of-
nm in diameter. Similar particles were initially observed on color opal-CT from Mexico (La Fe mine, Queretaro, no. 733) (3000×).1872 GAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL
predicted from other observations (Vanders and Kerr 1967; Suna- the play-of-color in opals (Raman and Jayaraman 1955).
gawa 2005). Changes in sphere growth from concentric to radial Indeed, it has been proposed that in some common opals,
likely correspond to increasing growth rate, possibly because of (lepi-)spheres might be well ordered and of the right size but
increasing concentration of silica or decreasing temperature. have the same index of refraction as the cement, and therefore
Similarly, the various structures observed in opal-CT samples diffraction of light cannot occur (Graetsch and Ibel 1997). We
might be the result of different growth rates, again by analogy did not observe such a case either in opal-AG or in opal-CT.
with true crystals (Sunagawa 2005). We propose that random
piling of individual nanograins takes place when the growth Formation environment for gem common opal-AG and
rate is relatively rapid (high concentration), and nanograins do opal-CT
not have time to arrange themselves into larger structures. On Gem opal-AG and opal-CT have long been associated with
the other hand, lepispheres might form when the growth rate is sedimentary and volcanic environments, respectively (e.g.,
comparatively slow. The structures with tablets or platelets would Sanders and Darragh 1971; Sanders 1985; Smallwood et al.
then represent an intermediate stage. However, the individual 1997). Similarly, opal-structure types have been correlated
nanograins exhibit approximately the same degree of crystallin- with formation environment, i.e., opals with structures based
ity whatever the structural variety of opal-CT, as their Raman on lepispheres are associated with volcanic environments, and
scattering spectra and XRD patterns are similar. those with structures consisting of spheres with sedimentary
environments. However, there are exceptions; for example, we
Imperfect conditions lead to an imperfect network, thus to found some opal-AG specimens from volcanic environments
common opal (Honduras, Mexico, or Slovakia), and opal-CT can form in
Since opal was first synthesized, it has been shown that many sedimentary environments, as is the case for biogenic, marine
growth parameters must be controlled to obtain a well-ordered ar- opal (e.g., O’Neil 1987).
ray of monodisperse silica spheres (e.g., Darragh et al. 1966; Stöber In order to form opal-AG and -CT, silica must be dissolved
et al. 1968; Filin et al. 2003). However, Darragh et al. (1966) and by water from siliceous rock, such as sandstone or rhyolite, and
Filin et al. (2003) determined that the silica concentration in the redeposited in nearby pore spaces or veins (Gaillou et al. 2008).
original gel is the predominant factor. Darragh et al. (1966) noted In a sedimentary environment, a tectonic event may be needed
that lower concentrations yielded larger spheres, and that the ideal to open voids in the rock (e.g., Payette 1999), whereas some
solution, one that forms ~150 to 300 nm spheres, is of medium volcanic rocks have cavities resulting from various gas-release
concentration (no value specified). Considering our observation processes. Rondeau et al. (2004) determined that in the volcanic
that the silica spheres, or lepispheres, in natural opals are rarely environment of Dubník, Slovakia, opal-AG was deposited by
smaller than 150 nm, we propose by analogy to the laboratory low-temperature (~45 °C) silica-rich circulating fluids in tectoni-
experiments that the aggregation of nanograins in an isotropic cally formed cavities. These authors concluded that deposition
gel forms monodisperse spheres that settle only when they reach temperature, rather than environment type, was the determining
~150 nm in diameter. This is consistent with our observations factor as to whether opal-AG or -CT forms; opal-CT is depos-
that opal with (lepi-)spheres of a diameter less than 150 nm are ited at ~170 °C and is directly associated with a volcanic event,
rare. On the other hand, Filin et al. (2003) concluded that more and opal-AG forms at significantly lower temperatures. This
highly concentrated gels yield irregularly stacked spheres, likely scenario is consistent with fluid inclusion studies by Spencer et
because in concentrated gels nanograins cannot move freely and al. (1992) that indicated that opal-CT in rhyolite from Mexico
aggregation is more random. Such gels might also produce the formed at ~170 °C, which explains why opal-AG is found in
polydisperse spheres we observed in opal-AG (sometimes with voids of both sedimentary and volcanic rocks when deposited
drastically different diameters) and in opal-CT. (Lepi-)spheres at low temperature.
likely form simultaneously, as we did not observe graded bedding It has been widely assumed by many researchers that all ex-
of spheres (positive or negative) in any opal samples. amples of opal-CT form only by transformation from opal-AG
The synthesis experiments by Darragh et al. (1966) did not and are the intermediate stage for the eventual transformation
yield monodisperse (lepi-)spheres larger than 400 nm, but some to quartz (e.g., Flörke et al. 1975; Leinen 1985; Jansen and Van
Honduras and Mexican opals studied here consist of monodis- der Gaast 1988). In fact, this is the case only for the formation
perse silica spheres as large as 750 and 850 nm, respectively. of biogenic opal-CT (produced by diatoms for example) formed
in deep-sea deposits and subjected to intense diagenesis. It is
Cementation not true for gem opal. For all opal deposits we visited, the evi-
Our SEM images indicate that the space between the spheres dence indicates that the opal-CT formed directly. The opal-CT
and lepispheres in samples of opal-AG and opal-CT, respectively, lepispheric structure is not derived from the opal-AG spherical
typically are filled with an opaline cement, which must have been structure. In contrast, the few occurances of opal-AG found in
deposited after the formation of the (lepi-)spheres. The cement dominantly opal-CT deposits clearly formed as secondary (and
and the (lepi-)spheres do not exhibit the same response to HF low-temperature) formations such as stalactites/stalagmites,
etch, indicating that there must be differences in their crystallini- crusts, or concretions.
ties and/or compositions (e.g., water content) and, consequently,
in their refractive indices. In fact, the difference in the refractive Future work
indices of the silica (lepi-)spheres and the matrix (e.g., cement Results obtained in this extensive study open new avenues for
or air) is necessary for diffraction of light that is responsible for research on opal. For example, an investigation at the nanometricGAILLOU ET AL.: STRUCTURE OF COMMON GEM OPAL 1873
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