Raudseppite, Ba2Ca7(Si3O9)2(CO3)2Cl2 A New Barium Silicate Mineral from the Gun Occurrence, the Yukon, Canada

Abstract

Raudseppite, Ba2Ca7(Si3O9)2(CO3)2Cl2, is a new mineral from the Gun occurrence in the Yukon, Canada, where it is found associated with walstromite in pellyite-, diopside-, and itsiite-bearing rocks. Raudseppite occurs as anhedral, equant to poikiloblastic grains, commonly with inclusions of clinopyroxene. Physical properties could not be determined due to the size and scarcity of the material. The calculated density is 3.31 g/cm3 based on the empirical formula. The empirical formula calculated on the basis of electron probe microanalysis conducted at Queen’s University (n = 18) is Ba2.02(Ca6.71Sr0.05Fe0.03Na0.02Mn0.02)Σ6.83C2(Si6.07Al0.02)Σ6.09O23.99Cl1.97F0.03 and at the University of British Columbia (n = 11) is Ba2.01(Ca6.82Sr0.04Fe0.03Mn0.02)Σ6.91C2(Si6.03Al0.01)Σ6.04O23.99Cl2.01F0.01, which are in good agreement. Raudseppite is triclinic, P1- (space group 2), a = 7.7818(4) Å, b = 7.8360(4) Å, c = 11.0318(6) Å, α = 100.229(1)°, β = 92.743(1)°, γ = 113.200(1)°, V = 603.29(5) Å3, Z = 1. The crystal structure (R1 = 3.35% for 1975 reflections with Fo > 4 σ Fo and R1 = 5.20% for all 2468 unique reflections) shows raudseppite to be the only three-membered cyclosilicate with CO3 groups or Cl. Raudseppite is named for Honorary Professor Dr. Mati Raudsepp (1947–2024) for his invaluable expertise and support of research at UBC in quantitative X-ray powder diffraction for over three decades.