High Pressure Behavior of Mascagnite from Single Crystal Synchrotron X-ray Diffraction Data

High-pressure synchrotron X-ray diffraction was carried out on a single crystal of mascagnite, compressed in a diamond anvil cell. The sample maintained its crystal structure up to similar to 18 GPa. The volume-pressure data were fitted by a third-order Birch-Murnaghan equation of state (BM3-EOS) yielding K-0= 20.4(7) GPa, K-0' = 6.1(2), and V-0 = 499(1) angstrom(3) , as suggested by the F-f plot. The axial compressibilities, calculated with BM3-EOS, were K-0a = 35(3), K-0a' = 7.7(7), K-0b = 10(3), K-0b' = 7(1), K-0c = 25(1), and K-0c' = 4.3(2) The axial moduli measured using a BM2-EOS and fixing K-0' equal to 4, were K-0a = 52(2), K-0b = 20(1), and K-0c = 29.6(4) GPa, and the anisotropic ratio of K-0a:K-0b:K-0c = 1:0.4:0.5. The evolution of crystal lattice and geometrical parameters indicated no phase transition until 17.6 GPa. Sulphate polyhedra were incompressible and the density increase of 30% compared to investigated pressure should be attributed to the reduction of weaker hydrogen bonds. In contrast, some of them, directed along [100], were very short at room temperature, below 2 angstrom, and showed a very low compressibility. This configuration explains the anisotropic compressional behavior and the lowest compressibility of the a axis.

Automated summary

High-pressure synchrotron X-ray diffraction was used to compress a single crystal of mascagnite in a diamond anvil cell, and the results showed that the sample maintained its crystal structure at high pressures. The incompressibility of sulphate polyhedra and the increase in density were attributed to changes in weaker hydrogen bonds.