Crystal structure of sakhaite from the Solongo deposit in connection with the crystallochemical interpretation of the sakhaite-harkerite mineral series

The crystal structure of mineral sakhaite Ca48[Mg13.2(Fe, Mn)2.8](CO3)16{ Al[SiO3.75(OH)0.25]4}(BO3)28[(H2O)3.3(HCl)3.3] from the Solongo deposit, Zabaykalye, is established (R 0.047) by X-ray diffraction analysis (Bruker Smart CCD diffractometer, 2θ: θ scan, λMoKα radiation, graphite monochromator): a = 14.679(2) Å, sp. gr. Fd3m, Z = 1, ρcalcd = 2.99 g/cm3. It is shown that some part of BO3 triangles in the structure of sakhaite from Solongo is replaced with five-member complexes {Al[SiO3(O,OH)]4}, established previously in the harkerite structure. Three schemes of isomorphism, which are characteristic of the sakhaite-harkerite mineral series, are selected and the general formula is proposed: Ca48(Mg,Fe,Mn)16(CO3)16{Al[SiO3(O, OH)]4}y(B3)32−y · n(H2O,HCl) (Z = 1), ymax = 8, nmax = 16 − y. The structural relationship of sakhaite-harkerite minerals with borate-sulfates of the tychite family Na6(Mg,Fe,Mn)2(BO3)4(SO4) is revealed. The correlation between the number of harkerite fragments forming the mineral structure, the structural symmetry, and the degree of imperfection is ascertained.