Valeria Diella

Ferritchromite and chromian-chlorite formation in mélange-hosted Kalkan chromitite (Southern Urals, Russia)

Abstract Spinel is often used as a magmatic indicator of crystallization processes, without considering the effects of metamorphic alteration on spinel geochemical features. Serpentinized mélanges in the southern Urals host different kinds of disseminated to massive chromitite mineralization. In mélange environments, intense metamorphic alteration above 300 °C leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chromian-chlorite. Based on textural and chemical analyses, mélange-hosted Kalkan chromitites exhibit a hydration and oxidation reaction that can explain the formation of ferritchromite and chromian-chlorite from chromite and serpentine: Textural analyses fit well with the proposed reaction and show that it usually proceeds very close to completion. The degree of alteration of chromite into ferritchromite is controlled by the initial chromite to serpentine ratio. In chromitites, high ratios prevent complete transformation of chromite into ferritchromite. The most likely environment for such reaction is a prograde metamorphic event post-dating serpentinization of the Kalkan ophiolite, possibly related to emplacement within an accretionary wedge.

Chemical and paragenetic data on gadolinite-group minerals from Baveno and Cuasso al Monte, southern Alps, Italy

Abstract The crystal structure of a rubidian cesian phlogopite-1M from pegmatite exocontacts at Red Cross Lake, Manitoba, monoclinic, a = 5.343(1),6 = 9.247(2),c = 10.397(3) Å, β = 100.04(2)°, V= 505.8(2) Å3, has been refined to an R index of 4.5% based on 519 observed reflections measured with graphite-monochromated MoΚα X-radiation on an automated four-circle diffractometer. The crystal used in the collection of the X-ray intensity data was also analyzed by electron microprobe, giving the unit formula (K0.46Rb0.28Cs0.23)(Mg1.20Fe1.00Al0.38Li0.34Mn0.04Ti0.04)(Si2.91Al1.09)O10[(OH)1.55F0.45]. The interlayer site, X, contains large amounts of Rb and Cs, and cell dimensions and the distance are in accord with data from synthetic Rb and Cs phlogopites and plutonic phlogopites. The interlayer coordination is much more regular in rubidian cesian phologopite than in other trioctahedral micas, defining a trend of increasing regularity with increasing interlayer-cation size.

Micro-Raman spectroscopy as a routine tool for garnet analysis.

A rapid system to obtain molar compositions of minerals belonging to the garnet group by means of Raman spectroscopy is illustrated here. A series of standard garnets, whose composition was determined by means of Wavelength Dispersive System (WDS) electron microprobe measurements, was used to correlate the wavenumbers of the different Raman peaks with chemical composition. A simple software routine was then developed in order to obtain garnet molar composition starting from the Raman spectrum, based on the assumption that in a solid solution belonging to the garnet family the Raman wavenumbers are linear combinations of end member wavenumbers, weighted by their molar fraction. The choice of the Raman bands used for the calculations and their behaviour are also discussed. The method, called MIRAGEM (Micro-Raman Garnets Evaluation Method), was then tested on a second series of garnets with satisfactory results.

P-V equation of State, thermal expansion, and P-T stability of synthetic zincochromite (ZnCr2O4 spinel)

Abstract The elastic properties and thermal behavior of synthetic zincochromite (ZnCr2O4) have been studied by combining room-temperature high-pressure (0.0001.21 GPa) synchrotron radiation powder diffraction data with high-temperature (298.1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: K0 = 183.1(±3.5) GPa, K’ = 7.9(±0.6), K’’ = .0.1278 GPa-1 (implied value), at V0 = 577.8221 Å3 (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with αV = α0 + α1T + α2/T-2, where only the first coefficient was found to be significant [α0 = 23.0(4) 10-6K-1]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of ZnCr2O4 with respect to its oxide constituents (Cr2O3 and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.

Equation of state and compressibility of phlogopite by in-situ high-pressure X-ray powder diffraction

The elastic properties of a natural phlogopite have been studied by in-situ high-pressure X-ray powder diffraction experiments on the ID9 beamline at the ESRF, at room temperature. Several EoS models (Birch-Murnaghan, Vinet, Poirier-Tarantola) have been fitted to the experimental P-V data and the results obtained are presented and discussed. The third-order Birch-Murnaghan EoS, assuming V 0 fixed at its experimental value, yields K 0 = 49.7(±0.5) GPa, K’ 0 =8.59(±0.19), K’ 0 =-0.5953 GPa −1 (implied value). The axial compressibilities at room conditions, determined by the third-order Birch-Murnaghan EoS, result in β EoS a0 =3.48(5), β EoS b0 =3.2(1), β EoS c0 =13.2(1) 10 −3 GPa −1 . Equilibrium thermodynamic calculations have been carried out to show how the new elastic parameters here reported affect the stability field of phlogopite.

The formation of scandium minerals as an example of the role of complexes in the geochemistry of rare earths and HFS elements

The stability constants of the fluoride complexes of scandium are much higher than those of most other elements; in particular, with respect to the REE, these complexes are in general much more stable than it would be expected on the basis of the corresponding ionic radii only. Accordingly, the presence of fluorides should easily lead to differentiation between scandium and other elements, including the REE, at times giving origin to specific minerals; a similar behaviour can be expected for zirconium or other HFS elements, whose complexes with the fluoride ion are also very stable, and in general for other ligands, such as for instance the phosphate and carbonate ions. Therefore, rather than the specific properties of sites in crystal structures, the formation of complexes of different stability in the depositing solutions and their disruption may become instead fundamental in explaining the enrichment of these elements, and the consequent formation of particular minerals of unusual composition. The practical consequences of this possibility are supported by crystal-chemical considerations and by a number of examples observed in Alpine veins and in pegmatites.

Scandium silicates from the Baveno and Cuasso al Monte NYF-granites, Southern Alps (Italy) : Mineralogy and genetic inferences

Abstract A chemical and paragenetic study has been performed on Sc silicates and Sc-bearing beryl occurring in Hercynian NYF-miarolitic pink granite and granophyric leucogranite at Baveno and Cuasso al Monte, Western Southern-Alps, Italy. In the Baveno and Cuasso al Monte plutons, detailed field work allowed the discovery of a significant number of crystals of bazzite, thortveitite, scandiobabingtonite, cascandite, and jervisite representative of all the known morphological and color varieties of these minerals in the two localities. Other studied samples belong to the historic collections of the Natural History Museum of Milan. Except for beryl, which crystallizes relatively early in aplitic granophyre, all the other Sc minerals crystallize as late-stage phases in cavities associated with fluorite. Chemical analyses reveal moderate Sc enrichment at the rim of beryl crystals. Bazzite displays a relatively large chemical variation, from .primitive. compositions enriched in Fe2O3 and Al2O3, to .highly evolved. compositions with values of Sc2O3 up to 17.54 wt%. Scandiobabingtonite shows a perfect inverse correlation between Fe3+ and Sc concentrations, suggesting complete solid solution between babingtonite and its Sc analogues. A wide variety of compositions have been determined for cascandite, significantly extending the compositional range of this mineral. In particular, the MnO content ranges from 0.37 to 4.87 wt%. The jervisite crystals analyzed in this work have rather homogeneous compositions much closer to the end-member if compared with the holotype analysis reported in literature. Thortveitite shows a wide range of compositions with variation in Sc2O3, Y2O3, HREE, and Fe2O3. Significant Fluctuations of the Sc/Yb ratios are in agreement with similar complex variations in the ratios between REE (e.g., Y/Dy) reported in the literature for crystals of gadolinite-group minerals from the same localities. Two different genetic models are discussed to explain the precipitation of Sc silicates as late stage phases in cavities. (1) during the latest stages of magma crystallization, HFSE and Sc were extracted from the silicate liquid and partitioned into fluids due to the complexing effect of F. Indeed, in view of the NYF geochemistry of the granite and the significant abundance of the associated F-bearing minerals, Fluorides (but not other complexing agents such as carbonates and phosphates) played the major role in concentrating HFSE and Sc. In cavities, such elements resulted in a series of rare accessory phases when F was extracted from Fluids because of the precipitation of zinnwaldite and Fluorite. (2) HFSE, Sc, and Y+REE were mainly incorporated by gadolinite-(Y) and siderophyllite crystallizing from residual magma. Many of the accessory phases crystallized in cavities because of the aggressive effect of subcritical hydrous, F-rich Fluids on the previously formed gadolinite-(Y) (liberating REE, Y, B, Be, Fe, Ca), siderophyllite (liberating Fe, Ti, and possibly Nb-Ta, Sc, etc.), and feldspars (liberating Ca, P, Cs, Ba). This second model is consistent with the widespread hydrothermal alteration of the Baveno and Cuasso al Monte granites and granophyres

Green andradite stones: gemmological and mineralogical characterisation

A multi-methodological investigation of natural gem-quality andradite crystals occurring in serpentinites (Val Malenco, Italy; Nizhniy Tagil, Russia; Kerman, Iran, and Kaghan, Pakistan), or in skarn rocks (Erongo, Namibia, and Antetezambato, Madagascar) has been performed by means of gemmological standard testing, electron-microprobe chemical analyses in wavelength-dispersive mode (EMPA-WDS), ultraviolet, visible and near-infrared (UV-Vis-NIR) and mid-infrared (MID-IR) spectroscopy, and single-crystal X-ray diffraction, in order to describe the gemmological properties and the crystal chemistry of these marketable green stones. The EMPA-WDS data show that these green garnets are almost pure andradite [Adr (Ca3Fe2Si3O12)>96.1 mol%]. The Cr-content ranges, as Cr2O3, from <0.01 up to ~1.0 wt%. This chromophore element, if present, influences the garnet’s colour, which is mainly due to Fe3+. The Cr-content does not affect the unit-cell constant, at least within the range found here. The refined electron content at the Fe-site suggests that chromium occupies the octahedral site along with iron, as found in previous investigations. The MID-IR spectra show the presence of hydroxyl groups. The refined site occupancy factors of the Si-site, modelled with the scattering curve of silicon alone, range between 99.2(4) and 99.9(2), which barely suggests potential hydrogarnet substitution [ i.e. , (SiO4)4− ↔(O4H4)4−].

P-V and T-V equations of State of natural biotite : An in-situ high-pressure and high-temperature powder diffraction study, combined with Mossbauer spectroscopy

Abstract The P-V and T-V equations of state of a natural biotite sample (Mg/Fe ratio ≈ 1) have been studied using in-situ high-pressure (0.0001.11 GPa) synchrotron radiation powder diffraction at the European Synchrotron Radiation Facilities (ESRF) in Grenoble, France, and in-situ high-temperature (298.610 K) laboratory X-ray powder diffraction. A third-order Birch-Murnaghan model [V0 = 498.7(1) Å3, measured value] provides the following elastic parameters: K0 = 49(1) GPa, K’ = 8.1(5). The volume thermal expansion is satisfactorily described by a constant value resulting in 37(2) 10.6 K-1. Mössbauer spectroscopy proves that REDOX reactions have occurred upon heating, presumably 2(OH- + Fe2+) → 2O2- + 2Fe3+ + H2 ↑ and/or 4Fe2+ + 2OH- + O2 → 4Fe3+ + 3O2- + H2O. On the basis of the elastic and thermal parameters measured we have modeled the deformation contribution (Gdeform) to the Gibbs energy. The third-order Birch-Murnaghan model with V0 fixed at its experimental value and the model with refined V0 do not significantly differ from one another in terms of Gdeform. A comparison based on Gdeform between biotite and phlogopite shows a better compliance to P of the former, though balanced in mineral reactions by a difference of molar volume, i.e., V0(biotite) > V0(phlogopite).

Demantoid from Val Malenco, Italy: Review and Update

GEMS & GEMOLOGY WINTER 2009 emantoid is the Cr-bearing yellowish green to green variety of andradite [Ca3Fe2(SiO4)3] (O’Donoghue, 2006). Very popular in Russia (where it was first discovered) from about 1875 to the start of the Russian Revolution in 1917, this gem has enjoyed a resurgence in demand since the beginning of the 21st century (Furuya, 2007). One of the most notable localities for demantoid is Val Malenco, located in Sondrio Province in the Lombardy region of northern Italy. Several deposits in this area have produced well-formed rhombic dodecahedral crystals (e.g., figure 1, left) that are coveted by collectors (Bedogne and Pagano, 1972; Amthauer et al., 1974; Bedogne et al., 1993, 1999). A limited quantity (some thousands of carats) of Val Malenco demantoids have been cut, producing gemstones that are attractive but rarely exceed 2 or 3 ct (e.g., figure 1, right). Val Malenco demantoid was first documented by Cossa (1880), who studied a sample recovered by T. Taramelli in 1876. In the next century, Sigismund (1948) and Quareni and De Pieri (1966) described the morphology and some physical and chemical properties of this garnet. Subsequently, the demantoid was investigated by Bedogne and Pagano (1972), Amthauer et al. (1974), Stockton and Manson (1983), and Bedogne et al. (1993, 1999). Because some of these data are more than 20 years old, and some publications are in Italian, we prepared this review and update on the physical, chemical, and gemological properties of demantoid from Val Malenco. Note that demantoid—although commonly used as a trade or variety name—is not approved by the International Mineralogical Association as a mineral name (Nickel and Mandarino, 1987; O’Donoghue, 2006). However, for reasons of brevity and consistent with gemological convention, throughout this article we will use demantoid instead of andradite, variety demantoid.