High-pressure behaviour and elastic constants of 1M and 2M1 polytypes of phlogopite KMg3Si3AlO10(OH)2

In the present work, the elastic properties of both 1M and 2M1 phlogopite polytypes, KMg3Si3AlO10(OH)2 (monoclinic crystal system) were investigated from PV equation of state fitting and by analysis of the fourth-rank elastic tensor. The analysis was performed within the Density Functional Theory framework, using all-electron Gaussian-type orbitals basis sets and the B3LYP functional corrected a posteriori to include long-range interactions (B3LYP-D*). In general, the elastic properties of the two polytypes were strongly anisotropic, with the axial moduli ratio M(a) : M(b) : M(c) being close to 4:4:1. The volume-integrated third-order Birch-Murnaghan equation of state fitting parameters at 0 K were K0=57.9(2) GPa, K '=8.29(7) and V0=489.82(3) & Aring;3 for phlogopite-1M, which were very close to those of the 2M1 polytype, i.e., K0=58.3(1) GPa, K '=8.71(8) and V0=978.96(9) & Aring;3. The monoclinic elastic tensors obtained for the two polytypes of phlogopite, which have never been experimentally reported for both minerals so far, were in line with the PV behaviour of the mineral, providing further data related to the directional dependence of the elastic properties and seismic wave propagation. The elastic properties from both PV hydrostatic compression and from the elastic moduli tensor were discussed against the available experimental and theoretical data in the scientific literature, extending the knowledge on this important trioctahedral phyllosilicate.