Carlhintzeite, Ca2AlF7·H2O, from the Gigante granitic pegmatite, Córdoba province, Argentina: description and crystal structure

Carlhintzeite, Ca2AlF7 center dot H2O, has been found at the Gigante pegmatite, Punilla Department, Cordoba Province, Argentina. It occurs as colourless prismatic crystals up to 0.8 mm long, ubiquitously twinned on {001}. Electron microprobe analyses provided the empirical formula Ca1.98Al1.02F6.24(OH)(0.76)center dot H1.62O. A crystal fragment used for the collection of structure data provided the triclinic, C (1) over bar cell: a = 9.4227(4), b = 6.9670(5), c = 9.2671(7) angstrom, alpha = 90.974(6), beta = 104.802(5), gamma = 90.026(6)degrees, V = 558.08(7) angstrom(3) and Z = 4. The crystal structure, solved by direct methods and refined to R-1 = 0.0322 for 723 F-o > 4 sigma F reflections, is made up of linkages of AlF6 octahedra, CaF8 polyhedra and CaF6(H2O)(2) polyhedra. The AlF6 octahedra are isolated from one another, but share polyhedral elements with Ca polyhedra. Most notably, the Al1 octahedron shares trans faces with two CaF8 polyhedra and the Al2 octahedron shares trans edges with two CaF6(H2O)(2) polyhedra. The linkage of the Ca polyhedra alone can be described as a framework in which edge-sharing chains along b are cross-linked by edge-sharing. Edge-sharing chains of Ca polyhedra along b in the carlhintzeite structure are similar to those along c in the structures of gearksutite, CaAlF4(OH)center dot(H2O), and prosopite, CaAl2F4(OH)(4).