- © 2017 The Mineralogical Society
Sulfhydrylbystrite, Na5K2Ca(Al6Si6O24)(S5)(SH), cell parameters a = 12.9567(6) Å, c = 10.7711(5) Å, space group P31c, is a new mineral belonging to the cancrinite group. It was found at Malaya Bystraya lazurite deposit, Lake Baikal area, Eastern Siberian Region, Russia, associated with lazurite, calcite, diopside, phlogopite and pyrite. The mineral develops at the margins of masses of lazurite, replacing it in some areas with the formation of nonequilibrium lazurite-diopside-sulfhydrylbystrite association. It is translucent, yellow to orange, with vitreous lustre, yellow streak and Mohs hardness of 4.5–5. The empirical formula, based on 12 (Si + Al), is Na5.17K1.87Ca0.99[Al6.01Si5.99O24](SH0.86)Cl0.07, Z = 2. The crystal structure of sulfhydrylbystrite may be described as an ABAC stacking of six-membered rings of SiO4 and AlO4 tetrahedra and extra-framework cations and anions located within structural cages. There are two type of cages, cancrinite and losod, stacked into chains at (0, 0, z) and (⅔, ⅓, z), respectively. The cancrinite cage hosts Ca2+ and (SH)– ions, whereas the (S5)2– polyanion is in the losod cage associated with Na+ and K+ cations. In addition, (SH)– and (S5)2– anions are detected in the structure of a mineral for the first time.
For comparison, a structural and compositional study of a bystrite sample from the same deposit was carried out. Bystrite is confirmed to contain pentasulfide anions in the losod cages, similar to those of sulfhydrylbystrite, in contrast to previous studies. However, bystrite has chloride in cancrinite cages, whereas sulfhydrylbystrite has hydrosulfide in that position. The unit-cell parameters are distinctly different: bystrite has a = 12.8527(6) Å, c = 10.6907(5) Å in the same P31c space group.