Vanadio-pargasite NaCa2(Mg4V)(Si6Al2)O22(OH)2: a New Mineral of the Amphibole Supergroup

Abstract

A new mineral was discovered in Cr–V-bearing marbles of the Sludyanka Complex from the Pereval marble quarry, Sludyanka district, southern Baikal region, Russia. It was named vanadio-pargasite as vanadium-bearing analog of pargasite according to the amphibole supergroup classification and CNMNC recommendations. Black Cr–V-spinel (magnesiocoulsonite–magnesiochromite), red Cr–V-bearing spinel, calcite, dolomite, Cr–V-bearing diopside and chlorite, phlogopite, and forsterite are associated minerals. Vanadio-pargasite occurs as subhedral long- and short-prismatic crystals 0.10–0.8 × 0.05–0.10 mm in size, with (110) and (010) faces and perfect cleavage by (110). Macroscopically, the new mineral is bright green to emerald green with vitreous luster; in thin sections and powder, it is pale green, without pleochroism. The new mineral is biaxial, positive, 2V = 86° ± 2°, γ = 1.659(2), β = 1.651(2), α = 1.643(2). The Mohs hardness is ~ 6, average VHN50;100 is 795; range 752–824 kg/mm2. The measured and calculated density is 3.05(5) and 3.112 g/cm3, respectively. In a thermogram over the range 654–1081°С, H2O is released with a endothermic effect. Over the range 900–1183°C, the main endothermic effect is caused by water and, possibly, F release, as well as melting of the mineral (1020°C). The absorption bands in the IR spectrum are, cm–1: 3445, 1633, 980, and 469. Vanadio-pargasite is monoclinic, space group 2C/m; the unit cell parameters are: a = 9.914(3), b = 18.003(2), c = 5.300(2) A, β = 105.69(3)°, V = 910.7(5) A3, Z = 2. The strongest reflections in the X-ray diffraction pattern are [d, A (I) (hkl)]: 8.98 (15) (020), 8.43 (40) (110), 3.27 (30) (240), 3.14 (100) (310), 2.82 (35) (330), 2.70 (18) (151), 2.34 (15) ( $$4.10\bar 1$$ ), 1.898 (15) (510), 1.445 (25) (4.101). The average chemical composition (528 point analyses) is, wt %: 42.75 SiO2, 0.14 TiO2, 12.75 A12O3, 0.44 Cr2O3, 5.92 V2O3, 19.15 MgO, 0.03 FeO, 0.01 MnO, 12.52 CaO, 3.45 Na2O, 0.41 K2O, 0.74 F (wet chem.) 1.75 H2O (calc.); the total is 99.91. The simplified formula is K0.1Na0.9Ca2.0Mg4.0V0.7Al0.3(Si6.1Al1.9)8.0O22(OH1.7F0.3)2.0. Holotype material has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (registration nos. 5035/1, 5035/2, and 5035/3).