Structural evolution of a 2M 1 phengite mica up to 11 GPa: an in situ single-crystal X-ray diffraction study

The structural evolution at high pressure of a natural 2M (1)-phengite [(K0.98Na0.02)(I =1.00)(Al1.55Mg0.24Fe0.21Ti0.02)(I pound =2.01)(Si3.38Al0.62)O-10(OH)(2); pound a = 5.228(2), b = 9.057(3), c = 19.971(6), beta = 95.76(2)A degrees; space group: C2/c] from the metamorphic complex of Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to similar to 11 GPa. A series of 12 structure refinements were performed at selected pressures within the P range investigated. The compressional behaviour of the same phengite sample was previously studied up to similar to 25 GPa by synchrotron X-ray powder diffraction, showing an irreversible transformation with a drastic decrease of the crystallinity at P > 15-17 GPa. The elastic behaviour between 0.0001 and 17 GPa was modelled by a third-order Birch-Murnaghan Equation of State (BM-EoS), yielding to K (T0) = 57.3(10) GPa and K' = a,K (T0)/a,P = 6.97(24). The single-crystal structure refinements showed that the significant elastic anisotropy of the 2M (1)-phengite (with beta(a):beta(b):beta(c) = 1:1.17:4.60) is mainly controlled by the anisotropic compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P shows a negative slope, Fitting the P-V(K-polyhedron) data with a truncated second-order BM-EoS we obtain a bulk modulus value of K (T0)(K-polyhedron) = 26(1) GPa. Tetrahedra and octahedra are significantly stiffer than the K-polyhedron. Tetrahedra behave as quasi-rigid units within the P range investigated. In contrast, a monotonic decrease is observed for the octahedron volume, with K (T0) = 120(10) GPa derived by a BM-EoS. The anisotropic response to pressure of the K-polyhedron affects the P-induced deformation mechanism on the tetrahedral sheet, consisting in a cooperative rotation of the tetrahedra and producing a significant ditrigonalization of the six-membered rings. The volume of the K-polyhedron and the value of the ditrigonal rotation parameter (alpha) show a high negative correlation (about 93%), though a slight discontinuity is observed at P > 8 GPa. alpha increases linearly with P up to 7-8 GPa (with a,alpha/a,P a parts per thousand 0.7A degrees/GPa), whereas at higher Ps a "saturation plateau" is visible. A comparison between the main deformation mechanisms as a function of pressure observed in 2M (1)- and 3T-phengite is discussed.