Bystrite, Na₇Ca(Al₆Si₆O₂₄)S₅^2–Cl^–: formula redefinition and relationships with other four-layer cancrinite-group minerals

Abstract

Bystrite is redefined as a four-layer cancrinite-group mineral with the four-layer Losod-type framework and the end-member formula Na7Ca(Al6Si6O24)S5 2-Cl -. The mineral is known only at the Malo-Bystrinskoe gem lazurite deposit, Baikal Lake area, Siberia, Russia. The associated minerals are calcite, lazurite, sodalite, fluorapatite, phlogopite, diopside, dolomite, and plagioclase.Bystrite is brittle, with the Mohs hardness of 5 and distinct cleavage on {10-10}. Yellow color of bystrite is due to the presence of S5 2anions occurring in Losod (LOS) cages of the aluminosilicate framework with the ABAC stacking sequence. Measured and calculated density is, respectively, 2.43(1) and 2.412 g cm -3 for the holotype, and 2.42(1) and 2.428 g cm -3 for the cotype sample.Bystrite is uniaxial (+),  = 1.660( 2),  = 1.584(2). The mineral was characterized by infrared and Raman spectra. The empirical formulae of the holotype and cotype samples are Na6.97K0.04Ca0.98(Si6.03Al5.97O24)(S5 2-)0.93[(SO4 2-)0.15Cl0.83] and Na6.75K0.04Ca1.11(Si6.09Al5.91O24)(S5 2-)1.04[(HS -)0.17Cl0.85], respectively. Bystrite is trigonal, space group P31c. The unit-cell parameters are: a = 12.8527(6) Å, c = 10.6907(5) Å, V = 1529.4(1) Å 3 ; Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 4.821 (32) (102), 3.915 (38) (211), 3.712 (100) (300), 3.307 (50) (212), 2.782 (18) (400), 2.692 (22) (401), 2.673 (30) (004), 2.468 (23) (402). Isomorphism and genesis of bystrite-type minerals is discussed. Bystrite and its K,HS-analogue sulfhydrylbystrite, Na5K2Ca(Al6Si6O24)S5 2-(HS) -, are indicators of highly reducing conditions.