The crystal structure of gearksutite, CaAlF4(OH)center dot H2O

The discovery of unusually well-crystallized samples of gearksutite allowed determination of its crystal structure. The crystals belong to space group P (1) over bar, a = 4.940(1), b = 6.810(1), c = 6.978(1) Angstrom; alpha = 101.12(1), beta = 94.86(1), gamma = 110.07(1)degrees; V = 213.43(6) Angstr om(3); Z = 2. The structure consists of layers of eightfold-coordinated cal cium polyhedra connected through pairs of [Al2F8(OH)(2)] octahedra which ar e oriented identically, similar to those observed in other hydroxyl aluminu m fluorides. A comparison with other minerals of similar composition is mad e. The structure data allowed us to reliably index the powder pattern and t o show that the reflection intensities are strongly affected by preferred o rientation.