Nano-structure of the Cristobalite and Tridymite Stacking Sequences in the Common Purple Opal from the Gevrekseydi Deposit , Seyitömer-Kütahya , Turkey

The strata that include the fine purple opal formations examined in this paper from the magmagenetic hydrothermal dissolutions. The opals are locally known as Gevrekseydi purple opals. These opal-bearing strata are deposited in volcanic lavas and tuffs in the SeyitömerKütahya region of western Turkey. The purple opals are common-type and attractive gem-quality, and they are sold on the worldwide gem markets. We conducted mineralogical investigations to clarify their silica building components, measure their nano-size, and determine their origins. The opals are an opal-CT (opal-cristobalite/tridymite)type silica polymorph with the variable number of cristobalite and tridymite layers. The cristobalite and tridymite stacking sequences were evaluated in terms of maturation, crystallite size, and genesis of the opaline silica material. These sequences are modelled using X-ray diffraction patterns. The relationship between the crystallite size and full width at half maximum values of the Gevrekseydi purple opals were also examined using X-ray diffraction patterns. The crystallite sizes were found to be L=17 nm for the main opal-CT peak (4.09 Å), L=23 nm for the shoulder opal-CT peak (4.29 Å), and L=27 nm for the opal-CT/C peak (2.51 Å).Given the relationship between the crystallite size and maturation of opal-CT, the Gevrekseydi volcanic common purple opals are most likely in a stage of early to mid maturation


INTRODUCTION
The gemmological classification of opal as precious, common, or hyalite (semi-precious) is is called a common (semiprecious) opal.Precious opals are widely known as either sedimentary opaque-translucent black and white opals or volcanic transparent-translucent fire opals (JONES & SEGNIT 1971, 1972;NAGASE & AKIZUKI 1997).However, the common or hyalite opals have the more varied origins.They occur as sedimentary or volcanic rocks as well as volcaniclastic sediments, and are almost entirely opaque.Their appearance and colors are caused by their absorbance and by the inclusion of some metal transition trace elements (LEECHMAN 1975, FLÖRKE et al. 1991, ELZEA & RICE 1996, FRITSCH et al. 2002).In this study, we are interested in a sample of gem-quality, volcanic common opals with an attractive purple coloration (Fig. 1).However, some of the materials of biogenic origin or the concretions (such as diatomites and geyserites) are described as "opaline silica", instead of as "opal" (JONES & SEGNIT 1971).
There is a significant body of research on common opals, that have attractive colors, because of their geological significance.Some common purple opals are mined in some parts of Mexico, and they are sold under the trade name "Morado" opal on the worldwide gem market, because they can be cut into the beautiful cabochons (ELSASS et al. 2000,, FRITSCH et al. 2004).
Accordingly, the present study is the first investigation of the gemmological and geological properties of an opaque-translucent common opal with attractive purple coloration.The opals described here originate a gem-quality common opal mine deposit in the Seyitömer (Kütahya) region in Turkey.The distinctive color of these opals is a vivid purple with swirls of white (Fig. 1).In addition, many samples are multicoloured with a range of colors from lilac to orange, some with whitish features, in the same deposit.Their structure is compact, without macroscopic inclusions.Therefore, the bulk specimens can be fashioned into cabochons, carvings, or slabs.Because the compact is hard enough to take a polish, the structure makes it a superb lapidary material.The gem's beauty is also notably more natural than the product of heating or irradiation, which adds to its appeal.
Therefore, this paper aims (1) to study the geological settling and petrological features of this new opal mine-field, (2) reveal the chemical, spectroscopic and microscopic mineralogical fingerprint of the purple opal, and (3) survey the often under-appreciated "common" opal and its versatility as a lapidary gem to identify their characteristics and compare them to other common purple opals.

MATERIAL AND METHODS
Gem-quality purple common opals were obtained near the village of Gevrekseydi in the Seyitömer-Kütahya region in the western Turkey.
To verify that the rough opal samples were indeed a pseudo-crystalline (or para-crystalline) silica, gemmological (non-destructive) characterization tests were performed on representative samples.The tests were performed in the DGL-Gemmological Testing Laboratory at Dokuz Eylül University.Mineralogical (destructive) tests were also performed.Polarizing microscope images of thin sections of the purple opals were obtained using an Olympus BX41 binocular polarizing microscope with a high-intensity 6 V, 30 W halogen light source combined with U-CPA and U-OPA optical systems, after thin sections of the samples had been mounted on glass lamellae.For the digital images, the microscopic magnification (MM) was 5X (a combined magnification of 0.5X objective and 10X ocular) under crossed nicols (+N) (active polarizer and analyzer) and parallel nicols.Polarizing microscope investigations were performed in the Optical Mineralogy Laboratory of the Department of Geology at Dumlupinar University.
Chemical analyses of the Gevrekseydi purple opals used X-ray fluorescence (XRF) for major oxides, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for trace elements, and WST-SIM (loss on igneous for whole rock package-XRF) to determine ignition losses.These analyses were performed, and certified with the code number ''IZ11037803'', under contract with the accredited ALS Chemex Laboratory in Canada.
The base silica-building components of the purple opals were detected using X-ray powder diffraction patterns of a Cubi-XRD device with a Cu tube and a graphic monochromator.The samples were analyzed with Cu radiation and a 0.3 mm collimator at atmospheric pressure for 10 minutes each, in the range between 5 and 70 o 2-theta.The d-spacing [Å] diffraction matching using the comparative matching technique is based on the positions of peaks with relative intensities of (I/I o ) % e"1, 2-theta values below 70 degree, and a tolerance range of ±0.01.The FWHM values were calculated using the Broker AXS diffract plus software.X-ray diffraction XRD patterns were taken in the Material Research Laboratory of the Bati Anadolu Cement Factory in Izmir.
The dispersive confocal micro-Raman spectroscopy of the purple opals was performed on a dark background at room temperature using a HORIBA Jobin Yvan Scientific XPLORA dispersive confocal micro-Raman spectrometer (DCµRS) with a high-throughput integrated spectrograph.The spectrometer uses one laser excitation of approximately 532 nm.The operating temperature is between 15 and 28 o C. The operation features are as follows: 799.75 of spectro, 1.17066 of exposition, 1200 T of grating, 5 of accumulation, 500 of hole, 100 of slit, 50% filter, 11.4315 of spectrum width, 50X objective, and 1 binning.Spectral manipulation as baseline adjustment was carried out using the device's software.The Raman record was carried out in the DGL-Gemmological Testing Laboratory at Dokuz Eylül University.

Opaline material features
To verify that the investigated rough opal samples were indeed a pseudocrystalline (or paracrystalline) silica variety, non-destructive gemmological characterization tests were performed on representative samples.First, the average specific gravity (SG) values of the eight representative samples were measured using an electronic balance scale (measurement sensitivity of 0.001) using an SG kit, based on the formula (SG = W air / W air -W water ).The specific gravity values ranged from 2.14 and 2.17 gr/cm 3 .Second, because they are opaque, the optical character, optical sign, and refractive index values of the eight representative opal samples were determined via the spot method, using an Eickhorst SR/XS standard refractometer device with an optical contact liquid of 1.79 RI, and a quartz lamp with a wavelength of 589 nm.The refractive index value of the purple opals was between N=1.428 and 1.436.Third, ultraviolet (UV) luminescence excitation of the purple samples was observed using a System Eickhorst UV 240 shortwave (255 nm) and long wave (366 nm) 4W UV lamp.The samples are inert under UV beams.Thus, when the average specific gravity value 2.15 is considered along with the other verifying results, the investigated opal samples appear to have a typical pseudo-or paracrystalline structure (AREM 1987, ROSSMAN 1994, BACK & MANDARINO 2008).

Provenance, Geology, Petrology, and Genesis of the Opals
The region surrounding the gem-quality common opal mine-deposit near the Gevrekseydi village was examined for its geological and petrological features.Western Turkey is dominated by large E-W grabens and a number of approximately N-S-trending grabens.The Tavsanli-Kütahya zone has been the subject of several previous by geologists (LEBKUCHNER 1957, 1959, NEBERT 1961, KALAFATÇIOGLU 1962, OKAY 1981, VICIL 1982, BAS 1986, SARIYILDIZ 1987, YANIK 1997    The volcanics occur as flowing units with thicknesses of 5 m to 25 m.Petrographically, the lavas mainly consist of plagioclase, quartz, biotite, pyroxene, amphibole, and glassy matrix.They have aphanitic texture with grey color in fresh samples and with light-grey color in weathering (altered/ decomposed) samples.The volcanic tuffs are made of a combination of rock and mineral fragments (e.g.feldspar, quartz) in volcanic ash matrix.The rock fragments in the tuffs are predominantly volcanic and rarely sedimentary in origin.The lavas and tuffs show locally silica and clay minerals alteration zones.Thin sections of the purple opal show mostly negative elongation that are sometimes undulatorydamped, indicating the presence of cristobalite and tridymite like layers (opal-CT and opal-C) (PRETOLA 2001).They are referred to as pseudocrystalline (or para-) textures in this study (Fig. 2.A and 2.B).
The purple opal-bearing zone and its surrounding area were geologically re-mapped, which revealed that the fine purple opal strata become intensely clear on the field at the northeast of the village of Gevrekseydi in the region: such stones are therefore called Gevrekseydi opals.These opals are an attractive gem-quality common opal.The opals are deposited in the large strata of the volcanic tuffs.However, the main silica source of the opals is derived from the underlain rhyodacitic and dacitic rocks, and all of them are called the Gevrekseydi volcanics.A fault zone that developed in the Gevrekseydi volcanics allowed the magmagenetic hydrothermal dissolutions to rise.After the silicate minerals were dissolved by a hydrothermal alteration in the rhyodacite and dacite, a silicic acid (H 4 SiO 4 )-rich fluid was released.Thus, the dissolved silicon ions were transported through the crack zone in the volcanics.These fluids then combined with ground water.However, because the Gevrekseydi volcanics do not have large pores and cavities, the silicic acid-rich hydrothermal fluids continued into the overlain tuffs, forming a colloidal silica gel and ultimately opal in a relatively low-pressure and lowtemperature environment with the requisite pH conditions.The silicic acid-rich solutions coagulated as pseudo-crystalline-structured opals of cryptocrystalline or crystalline structured silica varieties in the relatively cooled cavities and pores.
It is well known that the opals occur at temperatures below 100 o C and lower pressure conditions.
A range of opal colors have been observed in an investigated area.These levels were filled with the major purple and minor lilac, violet, and yellow colored opals.In addition, light, dark brown, white and pink opals are also exposed in the same area.
The ICP-AES analysis suggests that purple coloration of the Gevrekseydi opals is due to the transition metals, i.e., the higher abundance of Fe, Cu, Zn, Mn, and Pb.However, the main determination of color in the opal is not only the iron ion content but also the copper and titanium ion contents.Similar elemental contents for the purplish coloration have been reported for some common purple opals from Mexico, which are sold under the trade name "Morado" opal on the worldwide gem market (GÜBELIN 1986;ELSASS et al. 2000;FRITSCH et al. 2004).The XRD patterns belonging to the Gevrekseydi common purple opals are signed opal-CT (Fig. 3).The main peak (4.09Å) on the representative pattern is relatively intense and has a broad diffraction band with 4.29 Å peak on its shoulders.The main peak at 4.09 Å is evidence of opal-CT.In particular, the higher d-value of the main peak (>4.05 Å) and its shape comply with the definition of opal-CT, as do the peaks at 2.51 Å, 2.03 Å, and 1.62 Å, and the lack of peaks at 3.

Fig. 3: X-ray diffraction numerical data (A) and pattern (B) of the Gevrekseydi common purple opal
The XRD patterns of the common opals formed from magmagenetic hydrothermal dissolutions show the presence of more than one pseudo-crystalline phase: opal-CT, containing disordered ±-cristobalite, and a disordered stacking of sub-equal amounts of cristobalite and tridymitelike layers that were formed in

Crystallinity determination using the comparative matching technique
The observed peaks of the XRD patterns of the purple opal samples were modelled and compared with the XRD patterns reported for pseudo-crystalline opals from different parts of the world using comparative matching (HATIPOGLU 2009(HATIPOGLU , 2010) ) to identify the pseudo-crystalline phases in the Gevrekseydi purple opals.
The phase assignments of the XRD peaks for the pseudo-crystalline silica phases of the Gevrekseydi purple opals are given in Table 2 4. Accordingly, the crystallite sizes were found to be L=17 nm for main opal-CT, L=23 nm for shoulder opal-CT, and L=27 nm for opal-CT/C.These sizes agree with the range of the crystallite sizes of opal-CT and opal-CT/C in other type of opals (e.g., fire opals) that have been previously reported in the literature (KOIVULA & FRYER 1984, LU et al. 1995, FRITSCH et al. 2004 Guthrie and his colleagues reported that the crystallite size increases as opal-CT's mature.Given the relationship between the increasing crystallite size and maturation of opal-CT, volcano-sedimentary Gevrekseydi common purple opals are likely to reflect an early-to mid-level of maturation (GUTHRIE et al. 1995).

Proportions of cristobalite and tridymite stacking using the modelling technique
The opal-CT band (between 19 and 24 o 2¸) is sensitive to the relative proportions of the cristobalite and tridymite-like layers, whereas the amount of tridymite present is calculated by modelling the entire diffraction pattern (GUTHRIE et al. 1995).The amount of tridymite obtained by modelling the Gevrekseydi purple opals is given in Table 3.According to this model, a larger peak suggests the presence of more tridymite, 65

Sharpness and breadth of opal-CT peaks using the modelling technique
The main peak of opal-CT in the XRD  and 7).

Confocal Micro-Raman
The confocal micro-Raman spectrum of Gevrekseydi purple common opal is given in Fig. 8.In addition, its band assignments and comparisons are listed in Table 4.The representative micro-Raman spectrum reveals the presence of opal-CT/ C with the characteristic band at 292 cm -1 and the presence of opal-CT with characteristic bands at 363 and 404 cm  2013).However, some vibration bands in the sample cannot be assigned to the specific molecules.We can state that these bands are due to complex heterogeneous inclusions.The two bands at 65 and 144 cm -1 can be assigned to low quartz found as inclusion in the purple opals.The micro-Raman bands below 1000 cm -1 (such as, at 783, 829, 911, and 961 cm -1 ) indicate the presence of nanocrystalline molecules of Fe, Cu, Ti, Zn, Mn, and Pb oxides, as detected in the purple opals.

1.
Gevrekseydi opals are an attractive gemquality common opal.The opals are deposited in the large strata of the volcanic tuffs.However, the main silica source of the opals is derived from the underlain rhyodacitic and dacitic rocks, and all of them are called the Gevrekseydi volcanic because of their proximity to the village of Gevrekseydi.The purple opal strata in the Seyitömer (Kütahya) region are genetically associated with magmagenetic hydrothermal activity.This formation is a true dissolution process in which the spheres remain intact, but are dispersed into the gel.2.

3.
The variability in chemical composition, physical properties and micro-textures indicate that the purple opals formed from different solutions at variable growth rates.4.
Individual modelling techniques using X-ray diffraction (XRD) patterns are suitable detection method for determining the crystallite size of the opaline building phases, as confirmed by the scanning electron microscope (SEM) images and atomic force microscope (AFM) graphics.5..
The degree of crystallinite of the Gevrekseydi common purple opals was interpreted.The opals have a nano-structure at crystallite sizes between 10 and 50 nm comprising cristobalite and tridymite stacking sequences according to the type of opal (precious, common or hyalite) and its origin (sedimentary or volcanic rocks, or volcaniclastic sediments).The crystallite size of the main opaline building phase (opal-CT) in the Gevrekseydi opals is 17 nm.This value indicates a relatively small crystallite size and porosity that is filled with the molecular and silanol waters in opal texture.Accordingly, we can state that the pores in the "volcano-sedimentary" common opals are smaller and of more uniform size compared with those found in "sedimentary and volcanic" common opals.6.
There is no evidence for the presence of any accessory minerals in the Gevrekseydi purple common opal.7.
The band assignments and comparisons of the Opal-CT and opal-CT/C in the confocal micro-Raman spectrum of the Gevrekseydi purple common opal are presented.The Raman bands reveal many silica inclusions, transition metal oxides and carbonaceous matters in the Gevrekseydi purple opals.8.
Further investigations on the Gevrekseydi opals with various body colors would be desirable FT-IR analyses would be particularly interesting, and could indicate the gneiss of this type of opal formations.
, OZGENC & ILBEYLI 2008, TÜRKMENOGLU & YAVUZ-ISIK 2008, ÖZBURAN & GÜRER 2011, AKKIRAZ et al. 2012).Both Paleozoic metamorphic rocks and upper Cretaceous ophiolitic melange are found on the floor of the Seyitömer-Kütahya basin.Neogene volcanic and sedimentary rocks also occur in the region.However, the Upper Miocene to Lower Pleistocene volcanic rocks where the purple opalbearing in volcanics occurred.These volcanics are

1 Fig. 4 :Fig. 5 :
Fig. 4: Relations between crystallite size (L) and width value of the peak at ~35.9 o 2    , which represents the main peak of opal-C (d-value= ~2.5 Å) of the Gevrekseydi common purple opals [Full-Width Half-Maximum (FWHM) value of at ~35.9 o 2    ], (modified from GUTHRIE et al. 1995) The XRD pattern evaluation, which gives largely qualitative estimates (DE PABLO-GALAN 1997), is the most commonly used method to evaluate crystalline degree, crystallite size, amount of tridymite, interstratifications of cristobalite and tridymite, present crystalline inclusions, relative proportions of cristobalite and tridymite layers, degree of ordered stacking, and site of the H 2 O molecules in the opals (GRAETSCH 1994, GAILOU & MOCQUET 2004).Two main interpretive techniques were used to evaluate the XRD patterns of the opals; the comparative matching technique, and the graphical modelling technique, the latter of which was modified from SANDERS & MURRAY 1978, GRAETSCH et al. 1994, GUTHRIE et al.1995, and ELZEA & RICE 1996.
. The XRD patterns of the opals represent opal-CT.The d-spacing of the broad diffraction band of opal-CT and associated shoulder are a 4.09 Å (both opal-T and opal-C) peak and a 4.29 Å (opal-T) shoulder.The broad diffraction band between 19 and 24 o 2¸, which results from the overlap of the [101] from the cristobalite layers at 4.05 Å and the [404] from the interstratified tridymite-like layers at 4.09 Å, is called

Fig. 6 :Fig. 7 :Fig. 8 :
Fig. 6: Modelling the sharpness of opal-CT by plotting the ratio of the principle peak d-spacing of the shoulder peak d-spacing against the principle d-spacing values (Å) of the opal-CT peaks obtained from XRD patterns of the Gevrekseydi common purple opals.The 4.29 Å peak is the main shoulder of the opal-CT peak (4.09Å) (modified from GRAETSCH et al. 1994 and JONES & SEGNIT 1971).Fig. 7: Modelling the sharpness of opal-CT by plotting the ratio of the principle peak d-spacing (Å) of the shoulder peak d-spacing (Å) against the principle peak width ( o 2    ) [Full-Width Half-Maximum (FWHM)] of the opal-CT peaks obtained from XRD patterns of the Gevrekseydi common purple opals.The ~4.3 Å peak is the main shoulder of the ~4.1 Å peak (modified from ELZEA & RICE 1996; GRAETSCH et al. 1994; JONES & SEGNIT 1971) Finally, the presence of carbonaceous matter is marked by 1055 and 1729 cm bands (KINGMA & HEMLEY 1994, GÖTZE, et al. 1998, POP et al. 2004, DUMANSKA-SLOWIK et al. 2013).