Pietro Vignola

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.

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.

Karenwebberite, Na(Fe2+,Mn2+)PO4, a new member of the triphylite group from the Malpensata pegmatite, Lecco Province, Italy

Abstract Karenwebberite, Na(Fe2+,Mn2+)PO4, belongs to the triphylite group of minerals and corresponds to the Fe-equivalent of natrophilite or to the Na-equivalent of triphylite. It occurs in the Malpensata pegmatite dike, Colico, Lecco Province, Italy. Karenwebberite is found as late-magmatic-stage exsolution lamellae up to 100 μm thick, hosted by graftonite and associated with Na-bearing ferrisicklerite and with a heterosite-like phase. Lamellae are pale green, with very pale grayish-green streak. The luster is greasy to vitreous, and lamellae are translucent (pale green) to opaque (dark green). Optically, the mineral is anisotropic, biaxial (+), α = 1.701(2), β = 1.708(2), γ = 1.717(2) (for λ = 589 nm), 2Vmeas = 87(4)°, 2Vcalc = 41°, Z = b. Pleochroism is moderate with X = dark gray, Y = brown, and Z = yellow. The mineral is brittle with a Mohs hardness of 4.5; in thin section it displays a perfect cleavage along {001} with an irregular fracture. Karenwebberite is non-fluorescent either under short-wave or long-wave ultraviolet light, and its calculated density is 3.65 g/cm3. The mean chemical composition, determined by the electron microprobe from 16 point analyses (wt%), is: P2O5 41.12, Fe2O3* 7.00, FeO* 25.82, MgO 0.23, ZnO 0.11, MnO 9.31, CaO 0.10, Na2O 14.66, total 98.41 (*: calculated values). The empirical formula, calculated on the basis of 1 P atom per formula unit from, is (Na0.817Ca0.003□0.180)Σ1.000 (Fe2+0.622Mn2+0.228Fe3+0.151Mg0.010Zn0.002)Σ1.013PO4. Karenwebberite is orthorhombic, space group Pbnm, a = 4.882(1), b = 10.387(2), c = 6.091(1) Å, V = 308.9(1) Å3, and Z = 4. The mineral possesses the olivine structure, with the M1 octahedra occupied by Na, and the M2 octahedra occupied by Fe and Mn. The eight strongest lines in the X-ray powder pattern are [d in Å (intensities) (hkl)]: 5.16 (50) (020), 4.44 (90) (110), 3.93 (80) (021), 3.56 (90) (120), 3.04 (80) (002), 2.817 (100) (130), 2.559 (100) (131), and 1.657 (50) (061). The mineral is named in honor of Karen Louise Webber, Assistant Professor Research at the Mineralogy, Petrology and Pegmatology Research Group, Department of Earth and Environmental Sciences, University of New Orleans, Louisiana, U.S.A.

Chukhrovite-(Ca), Ca4.5Al2(SO4)F13·12H2O, a new mineral species from the Val Cavallizza Pb–Zn–(Ag) mine, Cuasso al Monte, Varese province, Italy

Chukhrovite-(Ca), Ca 4.5 Al 2 (SO 4 )F 13 ·12H 2 O, is the Ca-dominant species of the chukhrovite mineral group. It occurs at the Val Cavallizza Pb–Zn–Ag mine, Cuasso al Monte, Varese province, Italy. Chukhrovite-(Ca) is found as low temperature hydrothermal crystallizations covering the surfaces of brittle fractures crosscutting a marcasite and REE-bearing fluorite vein. Associated minerals include marcasite, gypsum, and hydrated Fe oxides. This new calcium aluminofluoride forms sharp octahedra up to 150 μm in diameter. Crystals are translucent to transparent, colourless to white, with a white streak and a vitreous lustre. Chukhrovite-(Ca) is isotropic with n = 1.432(1), and is non-fluorescent either under short-wave (254 nm) or long-wave (366 nm) ultraviolet light. The mineral is brittle with a Mohs hardness of 3.5(5), and without a distinct cleavage or fracture. Its density, calculated from the single-crystal unit-cell parameters and assuming 12 H 2 O molecules per formula unit, is 2.23 g/cm 3 . The empirical formula, based on 3 (Al + S) atoms per formula unit and calculated from an average of five selected point analyses, is: (Ca 4.33 Na 0.11 Fe 0.03 ) ∑4.47 Al 2.10 (S 0.90 O 3.72 ) F 13.10 ·5.98H 2 O, with the water content calculated by difference to 100 %. Chukhrovite-(Ca) is cubic with space group Fd 3; its single-crystal unit-cell parameters are a = 16.749(1) A and V = 4698.6(1) A 3 , for Z = 8. The eight strongest lines in the X-ray powder diffraction pattern are [ d in A ( I / I 0 ) hkl ]: 9.665 (100) 111, 5.921 (31) 022, 5.053 (16) 113, 4.190 (10) 004, 3.226 (15) 333 and 115, 2.556 (10) 533, 2.182 (12) 355 and 137, 1.915 (17) 626. The mineral, which has been approved by the CNMNC under number IMA 2010-081, is named chukhrovite-(Ca), since it corresponds to the Ca-rich equivalent of chukhrovite-(REE) (REE = Ce, Nd, Y), in which REE are replaced by Ca.