Oyelite; new mineralogical data, crystal structure model and refined formula Ca (sub 5) BSi (sub 4) O (sub 13) (OH) (sub 3) .4H (sub 2) O

Oyelite from the Bazhenovskoe deposit, Central Urals, Russia, and the N'Chwaning Mines, Kalahari, South Africa, was studied using electron microprobe, X-ray diffraction (XRD) and infrared spectroscopy, and published data on this mineral were revisited. The crystal structure of oyelite was determined for the first time, on sample from Bazhenovskoe (single-crystal XRD, R = 12.01%). The mineral is triclinic, P-1, a = 7.2557(5), b = 10.7390(11), c = 11.2399(8) Aa, alpha = 89.432(7), beta = 89.198(6), gamma = 72.097(8) degrees , V = 833.30(12) A3) and Z = 2. Oyelite represents a unique, novel structure type. Two different kinds of tetrahedral units with different topology, both linear and running along [100], occur in the structure: (I) the borosilicate chain [BSi (sub 2) O (sub 7) (OH) (sub 2) ]infinity consisting of Si (sub 2) O (sub 7) disilicate groups connected via single BO (sub 2) (OH) (sub 2) tetrahedra, and (II) the interrupted chain ("dotted line") formed by Si (sub 2) O (sub 6) (OH) disilicate groups bonded to each other by very strong H-bonds. The tetrahedral units I and II are linked to (010) layers of seven-fold coordinated Ca polyhedra of three different types: CaO (sub 6) (H (sub 2) O), CaO (sub 3) (H (sub 2) O) (sub 4) and CaO (sub 6) OH. The crystal chemical formula of oyelite is Ca (sub 5) [BSi (sub 2) O (sub 7) (OH) (sub 2) ][Si (sub 2) O (sub 6) (OH)].4H (sub 2) O and the refined idealized formula is Ca (sub 5) BSi (sub 4) O (sub 13) (OH) (sub 3) .4H (sub 2) O. Taking into account published chemical data for samples from two Japanese localities showing that oyelite may contain slightly more H (sub 2) O, its general simplified formula could be Ca (sub 5) BSi (sub 4) O (sub 13) (OH) (sub 3) .4-5H (sub 2) O. The powder X-ray diffraction (PXRD) pattern of oyelite has been refined and the reflections re-indexed. Oyelite demonstrates common crystal-chemical features with vistepite in part of the tetrahedral BSiO-chain and with some Ca-rich silicates, first of all with tobermorite-supergroup members, in the structure of the layered motif built of Ca-centred polyhedra and the possible occurrence of OD phenomena.