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The crystal structure of chromian kassite from the Saranovskoye deposit, Northern Urals, Russia

¹ CSIRO Minerals, Box 312 Clayton South, Victoria 3169, Australia
² Geology Department, Moscow State University, Vorobievy Gory 119899, Moscow, Russia

Correspondence: ^* E-mail: ian.grey{at}csiro.au

The crystal structure of kassite, ideally Ca[Ti₂O₄(OH)₂], containing 2 wt% Cr₂O₃, from the Saranovskoye chromite deposit, Perm’ district, Northern Urals, has been determined and refined to R1 = 0.06 using single crystal X-ray diffraction data. The crystals have monoclinic symmetry, P2₁/a, with a = 5.275(1), b = 9.009 (2), c = 9.557 (2) Å, ß = 90.43°. A pronounced sub-structure for the mineral, conforming to space group I2/a, is related to the I2/a structure for lucasite-(Ce), Ce[Ti₂O₅(OH)]. It comprises (001) layers of gibbsite-like fused hexagonal rings of edge-shared Ti(O,OH)₆ octahedra with the Ca atoms sandwiched between pairs of opposing rings and displaced from the center of the rings along [010]. Ordering of the protons in chromian kassite lowers the symmetry to P2₁/a. Kassite, CaTi₂O₄(OH)₂, and cafetite, CaTi₂O₅(H₂O), are identical chemically but significantly different in their crystal structures.