Crystal-structure analysis of four mineral samples of anhydrite, CaSO_4, using synchrotron high-resolution powder X-ray diffraction data

摘要

The crystal structures of four samples of anhydrite, CaSO_4, were obtained by Rietveld refinements using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and space group Amma. As an example, for one sample of anhydrite from Hants County, Nova Scotia, the unit-cell parameters are a = 7.00032(2), b = 6.99234(1), c = 6.24097(1) A, and V= 305.487(1) A~3 with a> b. The eight-coordinated Ca atom has an average distance of 2.4667(4) A. The tetra-hedral SO_4 group has two independent S-O distances of 1.484(1) to Ol and 1.478(1) A to 02 and an average distance of 1.4810(5) A. The three independent O-S-0 angles [108.99(8) × 1, 110.38(3) × 4, 106.34(9)° x 1; average [6] = 109.47(2)°] and S-O distances indicate that the geometry of the SO_4 group is quite distorted in anhydrite. The four anhydrite samples have structural trends where the a, b, and c unit-cell parameters increase linearly with increasing unit-cell volume, V, and their average and distances are nearly constant. The grand mean = 2.4660(2) A, and grand mean = 1.4848(3) A, the latter is longer than 1.480(1) A in celestite, SrSO_4, as expected.