FLUORVESUVIANITE, CA19(AL, MG, FE2+ DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Fluorvesuvianite was found in the abandoned Lupikko iron mine, Pitkäranta, Karelia, Russia. It occurs in cavities of chloritized diopside skarn, as radiating aggregates of acicular crystals in calcite. Crystals (up to 1.5 cm long and 5-20 μm across) are elongate along [001]: the dominant forms are (100) or (110). Associated minerals are sphalerite and clinochlore. Single crystals of fluorvesuvianite are colorless and transparent, aggregates are white with silky luster, and the mineral is non-fluorescent. The Mohs hardness is 6. The mineral is brittle, and no cleavage or parting was observed. Dmeas is 3.46(3) g/cm3, and Dcalc is 3.43 g/cm3. In immersion liquids, the mineral is colorless and non-pleochroic. Fluorvesuvianite is uniaxial (-), ω1 1.702(1), ε 1.699(1) for λ = 589 nm. Chemical composition (electron microprobe, H2O by TGA, F by ion-selective electrode, wt.%): CaO 36.1, MgO 1.9, MnO 0.1, FeO 2.8, Al2O3 17.9, SiO2 36.6, H2O 0.5, F 4.6, -O=2F 1.94, Total 98.56. The empirical formula based on 50 cations per formula unit is Ca19.03 (Al10.38Mg1.39Fe2+1.15 Mn2+0.04)Σ12.96 Si18.01O68.00 (F7.16OH1.64 O0.08)Σ9.60, which corresponds to the ideal formula Ca19 (Al,Mg,Fe2+)13 [SiO4]10 [Si2O7]4 O(F,OH)9. The bands in the IR spectrum are: 3625, 3555, 3400, 3170, 1650, 1575, 1080 shoulder, 1021, 983, 905, 870 shoulder, 800, 710 shoulder, 636, 605, 577, 490, 444, 411, 395 cm-1. The strongest eight lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 4.74(20)(202), 3.465(30)(420), 3.040(30)(510), 2.945(35)(004), 2.743(90)(432,440), 2.589(50)(224,522), 2.453(100)(620), and 1.619(30)(526,922). Fluorvesuvianite is tetragonal, space group P4/nnc, unit-cell parameters refined from the powder data: a 15.516(2), c 11.772(3)Å, V2834(1)Å3, Z = 2. The crystal structure has been refined to R1 = 0.043, calculated for 1108 unique observed reflections (| Fo | ≥ 4σ| Fo |). The structure refinement demonstrates that most of the fluorine is at the F(11) position [the refined site-occupancy is F0.72(OH)0.28]. The elongate <Si(1)-O> bond length (1.664 Å) and the Si(1) site-occupancy factor, 0.803(8), suggest substitution according to the scheme (SiO4)4- ↔ 4F-. If Ca19 (Al,Mg,Fe2+)13 [SiO4]10 [Si2O7]4 O(OH)9 is considered the formula of vesuvianite, the end-member composition of fluorvesuvianite is Ca19(Al,Mg,Fe2+ )13 [SiO4]10[Si2O7 ]4O(F)9. In other words, a vesuvianite-group specimen with more than 4.5 F apfu is named fluorvesuvianite.