The crystal structure of alfredstelznerite, Ca4(OH)4[B4O4(OH)8]4 ((H2O)15, a complex hydroxy-hydrated calcium borate mineral.

COOPER, M.; HAWTHORNE, F.; GALLISKI, M.A.; MÁRQUEZ-ZAVALÍA, M.F.

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Toronto, Canada; Año: 2010 vol. 48 p. 129 - 138

0008-4476

The crystal structure of alfredstelznerite, Ca4 (H2O)4 [B4O4(OH)6]4 (H2O)15, orthorhombic, a 12.161(2), b 40.477(8), c 10.1843(17) Å, V 5013(3) Å3, Z = 4, has been solved by direct methods and refined to an R1 value of 14.1% in space group Pca21. There are four Ca sites, each of which is fully occupied by Ca and coordinated by three O atoms, four (OH) groups and one (H2O) group, with <Ca–F> distances (F: unspecified anion) from 2.452 to 2.479 Å. There are sixteen B sites, each of which is occupied by B, forming eight (BF3) triangles and eight (BF4) tetrahedra. There are fifty-nine anion sites, of which sixteen are occupied by O2–, twenty-four are (OH) groups, and nineteen are (H2O) groups. The fundamental building block of the alfredstelznerite structure is the [B4O4(OH)6] polyhedron cluster containing two (BF3) triangles and two (BF4) tetrahedra. One (BF3) triangle and two (BF4) tetrahedra form a three-membered ring that is decorated by an additional (BF3) triangle sharing a vertex with a (BF4) tetrahedron; the algebraic descriptor for this borate cluster is <D2□>D. In alfredstelznerite, there are four symmetrically distinct [B4O4(OH)6] clusters that are strongly pseudosymmetric, both in terms of the stereochemistry of the cluster and in the linkage of Ca atoms to the cluster. The [B4O4(OH)6] clusters link along [100] and [001] via Ca–F bonds to form undulating layers parallel to (010) with a composition Ca(H2O) [B4O4(OH)6]. Hydrogen bonds from (OH) and (H2O) groups provides connectivity within and between the Ca(H2O) [B4O4(OH)6] layers. There are three symmetrically distinct Ca(H2O) [B4O4(OH)6] layers that repeat in the sequence ABCB… along [010]. The three layers A, B and C are chemically equivalent and structurally similar, but differ in their relative stacking. The B and C layers nestle together with the hillocks and valleys of the undulating layers in mutual alignment, and an intervening sandwich of interstitial (H2O) groups. The relative a/2 offset between the A and B layers along [100] results in direct juxtaposition of hillock and valley that gives rise to a greater layer-separation and produces open tunnels that extend along [001] and contain interstitial (H2O) groups. The only other structure containing a borate cluster graphically identical to that in alfredstelznerite is synthetic Mg(H2O)4 [B4O4(OH)6] (H2O)2.