THE CRYSTAL STRUCTURE OF OLSHANSKYITE

Abstract X-ray intensity data for olshanskyite, Ca2[B3O3(OH)6]OH·3H2O, were collected from a crystal from the Fuka mine, Okayama Prefecture, Japan, using a PW1100 Philips single-crystal diffractometer. Olshanskyite is triclinic, a 7.953(4), b 9.873(9), c 7.362 (6) Å, α 111.00(7), β 94.65(7), γ 107.53(7)°, space group P1̅, Z = 2. The crystal structure was solved and refined to R = 0.017 for 3833 observed reflections and to R = 0.022 for all unique reflections (4419). The anionic group in the structure is a three-membered ring of boron–oxygen tetrahedra [B3O3(OH)6]3−. Two independent calcium atoms occur in a distorted square-antiprism coordination with five oxygen atoms from the borate group and three hydroxyl or H2O groups external to the borate cluster. The apparent discrepancies between the unit-cell parameters and the formula unit of olshanskyite resulting from the present research and those previously determined are attributed to two alternative interpretations of the same chemical and X-ray powder data. Analogies and differences between the crystal structure of olshanskyite and that of the analogous borate nifontovite Ca3[B3O3(OH)6]2·2H2O also are discussed. Olshanskyite must be classified as a neso-triborate, and its structure is based on finite borate clusters (FBB = 3□).