Pertsevite, a new silicatian magnesioborate mineral with an end-member composition Mg2BO3F,in kotoite marble from east of Verkhoyansk, Sakha-Yakutia, Russia

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Pertsevite is a new Mg borate from a contact-metasomatic kotoite marble from the region east of Verkhoyansk, Sakha-Yakutia, northeastern Siberia. The end member is defined as Mg 2 BO 3 F, which is the composition of a synthetic phase. The natural material consistently contains from 4 to 12 wt.% SiO 2 as well as some OH. This is due to the substitutions Si + O for B + F and OH for F. A representative composition, which is an intermediate member of a solid solution series Mg 2 BO 3 (F,OH) - Mg 2 SiO 4 , has the composition (EMP data, OH calculated for charge balance) SiO 2 8.25; Al 2 O 3 0.10; B 2 O 3 22.44; FeO 3.71; MnO 0.65; MgO 57.39; CaO 0.24; F 7.29; H 2 O calc 1.67; total 98.97 wt.%, corresponding to (Mg 1.88 Fe 2+ 0.07 Mn 0.01 )(B 0.85 Si 0.18 )O 3.21 F 0.54 (OH) 0.24 or simply Mg 2 (B 0.8 Si 0.2 )O 3.2 (F,OH) 0.8 . A general formula of silicatian pertsevite may be given as Mg 2 (B,Si)O 3 (F,OH,O). Like the synthetic end member Mg 2 BO 3 F, silicatian pertsevite is orthorhombic, space group Pna 2 1 , with a = 20.490(6), b = 4.571(1), c = 11.890(3) A, V=1113.61(5) A 3 , Z = 16 and D calc = 3.12 g/cm 3 for the above composition. Its structure is based on a hexagonal close-packed arrangement of anions similarly as for forsterite, but with cations occupying different sets of voids. BO 3 triangles are randomly replaced by SiO 4 tetrahedra with Si lying outside of the planes defined by the BO 3 triangles. The crystals are colorless in thin section, biaxial (+) with (for the above composition) α = 1.609(1), β = 1.620(1), γ = 1.642(1) for 589 nm, 2V Zmeas = 65(1) ° and no visible dispersion. In the one thin section of the kotoite marble available, pertsevite occurs as anhedral crystals interspersed with calcite, kotoite, forsterite, clinohumite, spinel, ludwigite and a new member of the hulsite group. Based on analytical data, but also on structural considerations, there is a large miscibility gap between silicatian pertsevite and practically boron-free forsterite. Pertsevite is probably more widespread than this single example suggests; its being overlooked may be explained by its close resemblance to forsterite and humite group minerals in thin section.