Chromium Members of the Pumpellyite Group: Shuiskite-(Cr), Ca2CrCr2[SiO4][Si2O6(OH)](OH)2O, a New Mineral, and Shuiskite-(Mg), a New Species Name for Shuiskite

Abstract

A new pumpellyite-group mineral shuiskite-(Cr), ideally Ca2CrCr2[SiO4][Si2O6(OH)](OH)2O, was found at the Rudnaya mine, Glavnoe Saranovskoe deposit, Middle Urals, Russia. It occurs on the walls of 0.5 to 1 cm thick fractures in chromitite, filled with calcite, Cr-bearing clinochlore, and uvarovite. Shuiskite-(Cr) forms long prismatic to acicular crystals up to 0.1 × 0.5 × 7 mm elongated along [010] and slightly flattened on [100]. The crystals are commonly combined into radial, sheaf-like aggregates. Most observed crystals are simple twins with a (001) composition plane. Shuiskite-(Cr) is greenish-black under daylight or purplish-black under incandescent light. It is optically biaxial (–), α = 1.757(5), β = 1.788(6), γ = 1.794(6), 2V (meas.) = 45(10)°, 2V (calc.) = 46° (589 nm). The Dcalc is 3.432 g/cm3. The IR spectrum is reported. The chemical composition (wt.%) is CaO 21.33, MgO 3.17, Al2O3 5.41, Cr2O3 28.50, TiO2 0.18, SiO2 33.86, H2O 5.82, total 98.27. The empirical formula calculated based on the sum of eight metal cations and Si atoms per formula unit is Ca2.02Mg0.42Cr3+1.99Al0.56Ti0.01Si3.00O10.57(OH)3.43. The simplified formula is Ca2(Cr,Mg)(Cr,Al)2[SiO4][Si2O6(OH,O)](OH,O)(OH)2. Shuiskite-(Cr) is monoclinic, C2/m, a = 19.2436(6), b = 5.9999(2), c = 8.8316(3) Å, β = 97.833(3)°, V = 1010.17(6) Å3, and Z = 4. The crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0469), is based on a pair of chains of edge-sharing Cr-centred octahedra running along the b axis, linked together via the [SiO4] and [Si2O6(OH)] groups and Ca-centred polyhedra. The mineral species shuiskite, ideally Ca2MgCr2[SiO4][Si2O6(OH)](OH)3, was renamed to shuiskite-(Mg) by the decision of the IMA CNMNC. The shuiskite solid solution series with the general formula Ca2XCr2[SiO4][Si2O6(OH,O)](OH)2(OH,O), which includes shuiskite-(Mg) and shuiskite-(Cr) with X = Mg and Cr3+, respectively, appeared in the pumpellyite group.