High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 ± 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded K_T0 = 44(3) GPa and (∂K_T/∂P)₀ = 3.3(3), where K_T0 and (∂ K_T/∂P)₀ are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were β_0a^EoS = 6.1(1) and β_0c^EoS = 5.6(1) 10⁻³ GPa⁻¹. The value of β_0b^EoS was 6.2(8) 10⁻³ GPa⁻¹ fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic (β_0a: β_0b: β_0c = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca- and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that SO₄ volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)–1.66(9) ų; 1.4733–1.48 Å]. The SO₄ tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) Å and the larger increases from 1.4735(9) to 1.51(2) Å. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) ų at ambient P to 24.7(1) ų at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) 10⁻³ GPa⁻¹, about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) Å for OW-H1···O2, and from 2.883(2) to 2.69(3) Å for OW-H2···O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the ν₁ sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa.

Twenty-four apatite (Ap) samples mainly from carbonatite and alkaline rocks were studied by electron microprobe, IR spectroscopy and X-ray powder diffraction. The crystal structures of six were refined using single crystal X-ray diffraction data to R = 1.7-2.5%. The generally high Si content of Ap from carbonatite and alkaline rock has been related to the presence of characteristic Si-O absorptions in IR spectra. Bands, whose intensities change with Si content, were observed at 520, 650, 930 and 1160 cm^-1. The IR absorption features of v₃ CO₃ mode of Ap from carbonatite are different from those of v₃ CO₃ mode of Ap from sedimentary rock. This phenomenon is probably due to the different effects of F and OH on the CO₃ substitution for PO₄. The structural refinements yield more information on the CO₃=PO₄ substitution, which is now supported also by the geometrical evolution of the tetrahedron with increasing CO₃ content: the tetrahedral size decreases and the angle distortion increases with C-content. It is likely that the triangular planar CO₃ group is disordered on the four faces of PO₄^3-tetrahedron. It was observed also that Ap from early-stage carbonatite is OH-dominant with considerable LREE, Si, CO₃ and negligible Mn, Fe, Mg, K, S and C1 contents. They have high Sr/Mn, Si/S and C/S ratios.