Benjamin Rondeau

Common gem opal : An investigation of micro-to nano-structure

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.

Opals from Slovakia (“Hungarian” opals) a re-assessment of the conditions of formation

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.

Identification of the Endangered Pink-to-Red Stylaster Corals by Raman Spectroscopy

RAPID COMMUNICATIONS GEMS & GEMOLOGY SPRING 2009 n June 2007, delegates from 171 countries convened at The Hague to decide which species to include under the CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) agreement. The aim of CITES is to ensure that international trade in plant and animal specimens does not threaten their survival. The species covered by the convention are listed in three appendices, according to the degree of protection they need. Appendix I includes species threatened with extinction, where trade is permitted only in exceptional circumstances. Species in Appendix II are not necessarily threatened with extinction, but their trade must be controlled to avoid use that would threaten their survival. Appendix III contains species that are protected in at least one country that has asked other CITES parties for assistance in controlling the trade. The gemological significance of this triennial meeting is that corals from the Corallium genus, the most important of all gem coral species, were being considered for protection under Appendix II (CITES, 2008a). Ultimately, it was decided not to include them. More recently, on April 8, 2008, China, which now has domestic laws to protect these species, requested that CITES include four Corallium species (C. elatius, C. japonicum, C. konjoi, and C. secundum) under Appendix III (Fish and Wildlife Service, 2008). Meanwhile, the Stylasteridae family, which includes all Stylaster gem corals (e.g., figure 1), remained listed under Appendix II of CITES (as of January 18, 1990), which means a certificate issued by the management authority from the country (or state) of export is required (CITES, 2008b). Pink-to-red corals have been used for ornamental purposes for about 10,000 years (Liverino, 1989). According to Rolandi et al. (2005), there are two classes, Hydrozoa and Anthozoa, within the Cnidaria phylum (i.e., cnidarians) that have skeletons durable enough for use in gem materials and carvings. These two classes each contain a family (Stylasteridae and Coralliidae, respectively) that together yield the majority of pink-to-red coral species used for ornamentation (Pienaar, 1981; Rolandi et al., 2005; Smith et al., 2007). Most corals found in the IDENTIFICATION OF THE ENDANGERED PINK-TO-RED STYLASTER CORALS BY RAMAN SPECTROSCOPY

Relationship between nanostructure and optical absorption in fibrous pink opals from Mexico and Peru

Translucent pink opals from Mexico (states of Mapimi and Michoacan) and Peru (Acari area, near Arequipa) are opal- CT, containing from 10 to 40 % palygorskite, as demonstrated by XRD, infrared absorption and specific gravity measurements. Their nanostructure is unusual, with bunches of fibres 20 to 30 nm in minimum diameter, related to the fibrous nature of paly- gorskite crystals, as demonstrated by electron microscopy. A complex absorption centred at about 500 nm is the cause of the pink colour. It is proposed that the absorption is due to quinone fossil products associated with the phyllosilicate fibres. The Raman spectrum of monoclinic palygorskite is deduced from that of its mixture with opal. The opal-CT-palygorskite-quinone associa- tion is a geological marker of a specific environment, presumably of a fossil lake environment in a volcanic region.

Petrogenesis of mineral micro-inclusions in an uncommon carbonado

A unique Brazilian sample of a carbonado, displaying unusually large amount of diamond clasts merged within a fine-grained diamond matrix, has been studied by Secondary Ion Mass Spectrometry (SIMS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) on Focused Ion Beam (FIB)-extracted foils. We found for the first time in the diamond clasts (stage-1) micrometer to nanometer-sized inclusions of augite, ilmenite and phlogopite (all Fe-rich). Inclusions of metallic phases (Fe, Ti, Cr, Al and alloys of Fe-Cr, Al-Cr, Al-Fe Cr) described in worldwide carbonado occur in the studied sample exclusively within the fine-grained matrix (stage-2). The carbon isotopic composition of the diamond clasts and the fine-grained matrix falls within the range −27 ‰ to −32 ‰, like worldwide carbonado. The iron-rich silicate-oxide assemblage isolated inside clasts points to an initial growth of that diamond from mafic-rock minerals under oxidizing conditions ( f O 2 > IW). On the other hand metallic phases within the fine-grained matrix indicate an oxygen fugacity drop of at least 15 log units. This change in redox conditions is coeval with a deformation event under shearing stress at upper-mantle depth. During this metamorphic event, stage-1 diamonds were broken giving rise to the stage-2 fine-grained matrix, and syngenetic oxide inclusions were reduced to their metallic elements. This unique sample sheds new light on early 1970s hypotheses that interpreted carbonado as a high-pressure product from prograde metamorphism of crustal mafic rocks.

Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil

Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry, with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized. In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis (abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal. Supplementary Material: The totality of the chemical analyses is available at http://www.geolsoc.org.uk/SUP18519.

Hibonite: a new gem mineral

Myanmar, with no additional specifics given. Standard gemological testing and semiquantitative chemical analysis were inconclusive. The smaller crystal underwent further analysis and was identified as hibonite; it was subsequently cut into a 0.39 ct gemstone (figure 2, right). We believe these are the first gem-quality specimens of hibonite ever documented. Hibonite is a hexagonal mineral with the chemical formula (Ca,Ce)(Al,Ti,Mg)12O19. It has a Mohs hardness of 7.5–8 and an SG of 3.84, and it is uniaxial negative with refractive indices of ω=1.807 and e=1.790 (Roberts et al., 1974). Hibonite was discovered in 1955 as opaque to partially translucent black grains in the Esiva alluvial thorianite and phlogopite deposit, located in Toliara (Tulear) Province, Madagascar (Curien et al., 1956). This rather rare mineral was named after Paul Hibon, the French prospector who discovered it (Fleischer, 1957). Hibonite is known to occur in meteorites, but it is most often associated with moderateto high-grade metamorphic calcareous rocks,

Trinepheline and fabriesite: two new mineral species from the jadeite deposit of Tawmaw (Myanmar)

Two new mineral species, trinepheline (NaAlSiO 4 ) and fabriesite (Na 3 Al 3 Si 3 O 12 · 2H 2 O), are described from late-stage metamorphic veins of the jadeite deposit of Tawmaw-Hpakant (Myanmar). Both minerals and their names were approved by the IMA Commission on New Minerals and Mineral Names (IMA 2012–024 and IMA 2012–080). The name trinepheline is known in literature for the polymorphs of synthetic NaAlSiO 4 with a value of the c parameter that is three times that of nepheline. Fabriesite is named in memory of Jacques Fabries (1932–2000), former professor of the “Museum National d’Histoire Naturelle” in Paris (France). Fabriesite and trinepheline occur intimately intergrown together with nepheline, more rarely with albite and other feldspar-group phases such as banalsite and stronalsite; other associated minerals are jadeite and secondary products like natrolite and harmotome. All phases have been identified via electron backscatter diffraction (EBSD) patterns. Both fabriesite and trinepheline are pseudomorph after jadeite and occur as skeletal allotriomorphic crystals up to 15–20 μm long and 5–10 μm wide. They are white to yellowish in hand specimen, colourless in thin section; the streak is white and the lustre appears vitreous to greasy; they are non-fluorescent; Mohs’ hardness is 5–5½. Empirical formulae (EMPA analysis) are very close to the ideal compositions with traces of Ca and K for trinepheline, and of Ca, K, Ba, Mg, Fe, and Mn for fabriesite. Calculated densities are 2.642 g cm −3 for trinepheline (space group P 6 1 , a = 9.995 A, c = 24.797 A) and 2.386 g cm −3 for fabriesite (space group Pna 2 1 , a = 16.426 A, b = 15.014 A, c = 5.223 A), respectively. The strongest five lines in the calculated X-ray powder diffraction patterns [ d (A) ( I )( hkl )] are: 3.163(100)(122), 3.834(81)(023), 4.133(49)(006), 3.272(40)(120) and 2.403(31)(127) for trinepheline; 3.41(100)(240), 4.41(77)(201), 2.97(70)(421), 2.61(40)(002) and 8.21(36)(200) for fabriesite.

Investigation of hidden periodic structures on SEM images of opal-like materials using FFT and IFFT

We have developed a method to use fast Fourier transformation (FFT) and inverse fast Fourier transformation (IFFT) to investigate hidden periodic structures on SEM images. We focused on samples of natural, play-of-color opals that diffract visible light and hence are periodically structured. Conventional sample preparation by hydrofluoric acid etch was not used; untreated, freshly broken surfaces were examined at low magnification relative to the expected period of the structural features, and, the SEM was adjusted to get a very high number of pixels in the images. These SEM images were treated by software to calculate autocorrelation, FFT, and IFFT. We present how we adjusted SEM acquisition parameters for best results. We first applied our procedure on an SEM image on which the structure was obvious. Then, we applied the same procedure on a sample that must contain a periodic structure because it diffracts visible light, but on which no structure was visible on the SEM image. In both cases, we obtained clearly periodic patterns that allowed measurements of structural parameters. We also investigated how the irregularly broken surface interfered with the periodic structure to produce additional periodicity. We tested the limits of our methodology with the help of simulated images.

New typology and origin of tsavorite based on trace-element chemistry

New electron-microprobe analyses of ‘tsavorites’ from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V >Cr >Mn (in atoms per formula unit); type 2, vanadian grossular with V >Mn >Cr; type 3, Mn-bearing vanadian grossular with Mn > V > Cr; type 4, Mn-bearing chromian grossular with Mn > Cr > V; and type 5, Cr- and Mn-bearing grossular with Cr > Mn > V. These types are also characterized by different absorption spectra in the ultraviolet–visible–near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn 2+ , whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr 3+ at 697 and 701 nm. The different absorption spectra also indicate Fe 2+ -Ti 4+ charge transfer. We measured OH − equivalent to 0.08 to 0.38 wt% eq. H 2 O within the structure. Concentrations of vanadium, chromium and manganese are good chemical “fingerprints” for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar.