Franca Caucia

Determination of site population in olivine: Warnings on X-ray data treatment and refinement

Abstract Leverage analysis enables identification of reflections with the greatest influence on the estimate of each refined variable, and thus may be an important tool to improve standard structure-refinement procedures, especially in the case of minerals with complex composition. In this work, leverage analysis was used to investigate in detail the influence of each reflection in high-resolution X-ray diffraction data collected from olivine, a mineral often used to model order-disorder processes and to calculate temperatures of closure and cooling rates of host rocks. Particular attention was paid to the estimate of the scattering power at the cation sites, that are crucial for the above studies. Various procedures for data correction and refinement were also investigated, and the different possible choices were compared to choose the strategy that provides the best results. The presence of high-leverage weak reflections in olivine strongly suggests that systematic data truncation according to intensity threshold should be avoided. The estimates of the site-scatterings obtained under the different conditions tested are very close (always ≤3 σ); they are often smaller than those which may be obtained from electron-microprobe analysis under different experimental conditions or on inhomogeneous (zoned) crystals. Chemical data should thus not be routinely used to constrain the refinement procedure and/or to optimize final site-populations, provided that appropriate errors are given; on the other hand, they are valuable to appreciate the presence and the amounts of very minor, sometimes unexpected, substituents. Our tests show that precision better than 0.001 in site-occupancy determination (sometimes claimed in the literature) is probably not achieved.

The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al

Abstract Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [REE1-x-y(Th,U)xCay](Al3O)2/3[(B4-zSiz)]O10 with x - y + z = 1/3 (Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) Å, V = 172.6 Å3, Z = 1, space group P6̅ 2m. The crystal structure was solved by Patterson methods and refined to Robs = 1.8% (Rall = 2.2%) for 706 unique reflections in the 2q-range 6-136°. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) Å, V = 170.3 Å3, and the structure was refined to Robs = 2.9% and Rall = 3.0% for 271 unique reflections in the 2θ-range 4-80°. The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite, Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is formed by pairs of apical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming Al3O groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a′ = a √ 3 and a ^ a′ = 30°) of three ordered layers randomly translated by ± a.

XRPD patterns of opals: A brief review and new results from recent studies

A new classification of opals through X-ray powder diffraction (XRPD) methodology, by analysing 75 new samples of opal came from different worldwide areas, is introduced. A brief historical summary of the application of XRPD analysis on opals and the most important XRPD results reported in literature were compared with the newly obtained XRPD data. A simple method for the classification of opals on the basis of their degrees of structural order-disorder calculated from the diffraction data is proposed. In addition, a clear boundary, which has not been previously described by others in literature, related to the presence (or absence) of two-peak characteristic of the cristobalite phase is identified. This boundary allows for a discrimination of opals C from CT.

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15− polyanion

Abstract The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1-xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O1815- polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O33- and NaF32- polyhedra, the occurrence of which is mutually exclusive and statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.

Crystal-chemical reasons for the immiscibility of periclase and wüstite under lithospheric P,T conditions

We have analyzed the implications of [Mg] VI =>[Fe 2+ ] VI isomorphous substitution in the periclase structure (B1) which forms, under lithospheric P, T conditions, only very limited solid solutions with wustite. The crystallographic study (by single crystal X-ray structure refinements and microprobe analysis) supports the key role of the cation-cation repulsive interactions in the crystal-chemical behaviour of these closely-packed phases. The anomalously large octahedral bond lengths in periclase (Mg-O=2.106 A) and in wustite (Fe-O=2.167 A) are the result of otherwise too short M-M distances (= 1.414 · M-O). In particular, the wustite instability below 570 °C and the problematic existence of a stoichiometric iron end-member indicate that the M-M separation in the “ideal” wustite, although largely increased by the anomalously large Fe-O bond length, is still too short to support the B1 structure. Any periclase-wustite solid solution with a wustite component higher than 8.3% necessarily entails the presence of couples of adjacent [Fe 2+ ] VI cations with separations shorter than that of “ideal” wustite; this makes the solid solutions in this system even less stable than the pure wustite. One easy way to reduce the electrostatic [Fe 2+ ] VI -[Fe 2+ ] VI repulsion, and therefore the instability of iron-bearing periclase, is the iron oxidation with the formation of a more stable spinel phase (magnesioferrite), as it has been demonstrated by heat treatment and structure refinement of several natural ferropericlase crystals with 2–5% of wustite component. Magnesioferrite has a unit-cell edge of 8.39 A which is almost twice the unit-cell of periclase (2 · 4.21 = 8.42 A) and this allows the spinel to grow with the same orientation of periclase, being the oxygen arrangement of the two structures virtually identical. Under very high pressure (≥ 90 GPa) the electrostatic [Fe 2+ ] VI -[Fe 2+ ] VI repulsion of FeO can be greatly reduced by a phase transition from B1 to B8 (NiAs) structure. In the B8 phase the Fe-Fe separation becomes 2.57 A; this short value corresponds to a change in the electronic properties of iron which can now form metallic bonds, in contrast to MgO (B1) phase which is supposed to maintain its stability up to at least 230 GPa.

New Occurrence of Fire Opal from Bemia, Madagascar

GEMS & GEMOLOGY SUMMER 2010 O pals are water-bearing microand noncrystalline silica minerals, with the chemical formula SiO2•nH2O (see, e.g., Graetsch et al., 1994; Downing, 2003; O’Donoghue, 2006). One attractive variety is fire opal, which is characterized by a red-orange-yellow bodycolor, with or without play-of-color (O’Donoghue, 2006). This opal variety does not have the typical structure of play-of-color opal; rather, it is composed of random aggregates of hydrated silica nanograins ~20 nm in diameter (Fritsch et al., 2006; Gaillou et al., 2008b). The most famous locality for fire opal, one that has been producing fine material for more than 100 years, is the Querétaro area of Mexico (see, e.g., Koivula et al., 1983; Gübelin, 1986). Other sources include the United States, Turkey, Australia, Indonesia, Ethiopia, Somalia, Kazakhstan, Canada, and Brazil (Ball and Daniel, 1976; Smith, 1988; Bittencourt Rosa, 1990; Holzhey, 1991; Bank et al., 1997; Enseli et al., 2001; O’Donoghue, 2006). Opal, including the fire variety, is also known to come from various regions of Madagascar, particularly the Faratsiho deposit, located near the capital Antananarivo, in the center of the island (Lacroix, 1922). A new source of common opal, including fire opal (e.g., figure 1), was discovered a few years ago in the southeastern part of the island. According to A. and L. Pasqualini (pers. comm., 2010), who visited the deposit in May 2008, the opal is found near the city of Bemia, 70 km from the coast (figures 2 and 3). The opal occurs in Cretaceous rhyodacite volcanic rocks. Local people search for the opal by digging small pits, and ~200–400 kg of mixed-quality rough material has been produced. The opal generally occurs as nodules up to several centimeters in diameter or in veins up to 20–30 cm long, with large variations in quality and color. The rough opal is typically sent to the city of Antsirabe, 450 km north of Bemia, where it is fashioned into cabochons or faceted into fine gemstones that typically weigh up to 15 ct. Many of the various colors are typical of fire opal, but no play-of-color has been seen. Building on the work of Simoni and Caucia (2009, in Italian), the present article describes the standard gemological properties of Bemia opal, as well as the inclusions, powder X-ray diffraction patterns, chemical composition, and spectroscopic features.

Physical and chemical properties of some italian opals

The physical and compositional properties of some opals from different parts of Italy have been investigated through several methodologies like optical analysis, specific gravity, refractive indices, xrpd, ir spectroscopy, la-icp-ms. The opals show different colors: white, white brownish, white yellowish, white yellowish greenish and greyish. Black and metallic inclusions, consisting of todorokite, are sometimes present. Play of color have not been observed but some opals show small iridescence zones; opals are inactive to the long and short wavelength uv radiation (366 - 254 nm) with the exception of one sample and also phosphorescence is absent. Refractive index and specific gravity values are n = 1.43 - 1.44 and G = 2.07 - 2.33 g/cm 3 in agreement with literature. xrpd analyses highlighted Italian opals are A, CT and C types, but most of them can be classified as CT opals. IR spectroscopy data confirmed the opal classification. The most abundant elements are Mg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (70-96 ppm) and Ni (20-70 ppm). Similarly to what observed in other opals worldwide, Fe appears to be the principal factor that determines the white brownish color and of the yellowish shade. Chromophore elements like V, Cr, Cu, Ti, Co and Ni are present in very low contents and do not influence the physical properties of the Italian opals. Mn is clearly detected (42 ppm) only in the sample n. 2 and is related to the presence of dendrites. Ca and Mg (non chromophore elements) are probably related to the matrix. On the whole the investigated Italian opals show a rather homogeneous trace element composition that appear well differentiated from that of other opals worldwide.

Aquamarine from the Masino-Bregaglia Massif, Central Alps, Italy

GEMS & GEMOLOGY FALL 2009 he Masino-Bregaglia Massif (also known as the Bergell Massif) contains numerous granitic pegmatites hosting a remarkable variety of minerals—including aquamarine (figure 1)—that have attracted the interest of mineralogists and collectors since the late 18th century (e.g., Bedogné et al., 1995). Beryl from this area was initially mentioned by Repossi (1916) and Staub (1924). Subsequently, many other beryl occurrences were discovered in the massif. In their listing of the locations of historical beryl-bearing pegmatites, Hügi and Röwe (1970) indicated that the most important Italian deposits occurred in the areas of Val Bregaglia (Bregaglia Valley), Valle Mello, Cima di Zocca, Val Masino, Val Codera, and Alpe Vazzeda. In the 1970s, a limited amount of gem-quality aquamarine was recovered and cut from the Filone Silvana (Silvana dike), located in Val Codera. Masino-Bregaglia aquamarine crystals typically show a prismatic habit and measure several centimeters long, although some crystals attain ~15–20 cm in length. They range from light to dark greenish blue to blue or yellowgreen. Some gemand carving-quality aquamarine has been recovered, although the fact that most of the crystals contain numerous inclusions and fractures makes such material rare (Bedogné et al., 1995). To our knowledge, a gemological characterization of this aquamarine is lacking, except for the recent work of Caucia et al. (2008, in Italian). The present article builds on that work by supplying additional data obtained on a larger number of samples from four pegmatites in this area.

Gemological, physical and chemical properties of prase opals from Hanety Hill (Tanzania)

Chemical, physical and gemological properties of some green “prase” opals from Hanety Hill in Central Tanzania were investigated. The color of the opals ranges from clear green to apple green, diaphaneity from translucent to semitransparent, luster is vitreous, and all result inert to UV lamp radiations. Specific gravity values are between 2.11-2.13, refraction indices between 1.439 and 1.458, comparable with literature data. XRD and FTIR analyses show the opals belong to the CT type, with tridymite higher than cristobalite. SEM observations revealed a lepispheric and mammillary structure formed by spherules with a diameter around 10-12 μm, on their turn composed of amorphous silica microspheres. The opals show a homogeneous chemical composition with very high contents of Ni and lower of Mg, Ca, Fe and transition elements like Zn, Cr and Co. Al and K are nearly absent. The chemical composition of the opals reflects that of serpentine rocks. Because of its abundance, the cromophore element responsible for the green color is Ni. Prase opals may have formed through the process of low temperature metamorphism, which generated the serpentinite from a magmatic ultrafemic rock.

AVASPEC 2048: an innovative spectroscopic methodology to differentiate the natural emeralds from the synthetic ones

New and sophisticated synthetic gems are constantly introduced in the market making the identification of them very difficult, if one uses only the microscope. In this paper we present the results obtained from a new methodology, the AvaSpec-2048 spectrometer, that can help to identify the natural emeralds from the synthetic ones. The AvaSpec-2048 spectrometer is an original instrument that can acquire electromagnetic spectra in a range between 400 nm and 1000 nm (VIS-NIR), and can be used to identify gemological materials allowing an accurate discrimination of natural products from the artificial ones. The physical and optical properties, microscopic features and absorption spectra of natural and synthetic emeralds have been also investigated through others traditional methodologies such as optical analysis, specific gravities, refractive indices and Vis-NIR spectroscopy. The comparison of the absorption spectra obtained with the AvaSpec-248 spectrometer allows identifying and distinguishing the origin of the analyzed gems.