The crystal structure of gearksutite, CaAlF4(OH)•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) Angstrom(3); Z = 2. The structure consists of layers of eightfold-coordinated calcium polyhedra connected through pairs of [Al2F8(OH)(2)] octahedra which are oriented identically, similar to those observed in other hydroxyl aluminum fluorides. A comparison with other minerals of similar composition is made. The structure data allowed us to reliably index the powder pattern and to show that the reflection intensities are strongly affected by preferred orientation.