Magnesiovesuvianite, Ca19Mg(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group mineral

Abstract

Magnesiovesuvianite (IMA 2015-104), ideally Ca19Mg(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group member was found in an old museum specimen labelled as vesuvianite from the Tuydo combe, near Lojane, Republic of Macedonia. It occurs as radiating aggregates up to 2 cm across consisting of acicular tetragonal crystals (up to 7 mm long and 5–40 μm thick) with distinct fibre-optic effect. Associated minerals are calcite, garnet of the grossular-andradite series and clinochlore. Single crystals of magnesiovesuvianite are transparent, light pink with a silky lustre. Dominant crystal forms are {100}, {110}, {101} and {001}. The Mohs’ hardness is 6. Dmeas and Dcalc are 3.30(3) and 3.35 g/cm3, respectively. Mineral is optically uniaxial, negative, ω = 1.725(2), ε = 1.721(2). The chemical composition (wt.% electron-microprobe data) is: SiO2 36.73, Al2O3 20.21, CaO 36.50, MgO 1.80, MnO 0.18, FeO 0.04, Na2O 0.01, H2O 3.10, total 98.37. The empirical formula based on 19 (Ca + Na) apfu is (Ca18.99Na0.01)Σ19.00(Mg0.60Al0.40)Σ1.00(Al11.05Mg0.70Mn0.07Fe0.02)Σ11.84Si17.84O68.72(OH)9. Absorption bands in the IR spectrum are: 393, 412, 443, 492, 576, 606, 713, 802, 906, 968, 1024, 3200, 3450, 3630 cm-1. The eight strongest lines of the powder X-ray diffraction pattern are (I-d(Å)-hkl): 23-10.96-110, 22-3.46-240, 33-3.038-510, 100-2.740-432, 21-2.583-522, 94-2.365-620, 19-2.192-710, 25-1.6165. Magnesiovesuvianite is tetragonal, space group P4/n, unit-cell parameters refined from the powder data are a 15.5026(3), c 11.7858(5), V 2832.4(2) Å3, Z = 2. The crystal structure has been refined to R1 = 0.027 for 3266 unique observed reflections with |Fo| ≥ 4σF. The structure refinements provide scattering factors of the Y1A,B sites close to 12 e- that support predominant occupancy of these sites by the Mg2+ cations, in perfect agreement with the 27Al MAS NMR data. Magnesiovesuvianite is a member of the vesuvianite group with Mg2+ as a dominant cation at the Y1 site. The name magnesiovesuvianite is given to highlight the species-defining role of Mg.