Pertsevite, a new silicatian magnesioborate mineral with an end-member composition Mg_2BO_3F, in kotoite marble from east of Verkhoyansk, Sakha-Yakutia, Russia

摘要

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_2BO_3F, 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 an OHfor F. Arepresentative composition, which is an intermediate member of a solid solution series Mg_2BO_3(F,OH)-Mg_2SiO_4, has the composition (EMP data, OH calculated for charge balance) SiO_2 8.25; Al_2O_3 0.10; B_2O_3 22.44; FeO 3.71; MnO 0.65; MgO 57.39; CaO 0.24; F 7.29; H_2O_(calc) 1.67; total 98.97 wt.%, corresponding to (Mg_(1.88)Fe_(0.07)~(2+)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_2BO_3F, silicatian pertsevite is orthorhombic, space group Pna2_1, with a = 20.490(6), b = 4.571(1), c = 11.8903(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_(Zmeans) = 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 silication 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.