The structure of priceite, a basic hydrated calcium borate, by ab initio powder-diffraction methods

Abstract

The crystal structure of priceite, data and refined to Rwp = 0.042 and Rp = 0.030 using the Rietveld method. The monoclinic structure was refined in space group P21/c, a 11.623(1), b 6.976(1), c 12.350(1) Å, β 110.70(1)°, V936.72(1) Å3, Z = 4. The structure consists of infinite chains of pairs of borate rings that extend along [100] via corner-sharing; three corner-linked BΦ4 (Φ: OH-, O2-, or H2O) tetrahedra form one ring linked through a common boron atom to the next ring, composed of two BΦ4 tetrahedra and one BΦ3 triangle. Chains of edge-sharing CaΦ8 polyhedra zigzag along [010]. The borate chains are cross-linked by CaΦ8 polyhedra via edge- and corner-sharing. There are three borates in addition to priceite, that contain a <3B>-<3B> chain motif, but priceite is the only one containing a 3□ ring. These chains extend perpendicular to the calcium chains of polyhedra, in contrast to colemanite, which displays a <3B> motif where the strings are parallel.