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T h e crystal s t ruc tu re of fabiani te , CaBa06(0H), f r om sal t deposits near Diepholz, Germany, has been solved b y Pa t t e r son a n d electron-density syntheses using abou t 1300 reflections. Least-squares ref inement reduced the residual t o 0.075. Fabian i te is monoclinic, Ρ2 ι /α , a = 6.593, b = 10.488, c = 6.365 A, β = 113.38", Ζ = 4, densi ty (calc.) 2.788 g/cm . T h e s t ruc ture contains infinite sheets of composit ion [BaOeiOH)]"", similar t o those previously found in the synthet ic or thorhombic dimorph. The sheets a re formed b y cross-linking colemanite-like chains. I n fabiani te Ca is coordinated by two hydroxyl ions a n d six oxygen a t o m s ; t h e polyhedra share edges t o fo rm infinite chains along a t h a t * Publ icat ion author ized b y t h e Director, U . S. Geological Survey. Z. KiistaUogr. Bd. 132, 4-8 16 242 J U D I T H Α . K O N N E B T , J O A N R . CLARK a n d C. L . CHRIST link polyanion sheets together. In synthetic CaB30s(0H), Ca is coordinated bytwo hydroxyl ions and only five oxygen atoms; the polyhedra form chains by corner-sharing only. Hence, although the Ca—Ο bonding holds adjacent sheets together in each structure, the packing is much more compact in the mineral. Both structures have probable hydrogen bonds from a hydroxyl ion of one sheet to an oxygen ion of an adjacent sheet, but the 2.76 A bond in the mineral is obviously stronger than the longer 2.94 Ä bond in the synthetic dimorph.

Crystal structure of fabianite, CaB3Os(OH), and comparison with the structure of its synthetic dimorph