Structural evolution of a 3T phengite mica up to 10 GPa : an in-situ single-crystal X-ray diffraction study

The high-pressure structural evolution of a natural 3T- phengite [(K0.90Na0.05)Σ=0.95(Al1.51Mg0.32Fe0.18Ti0.03)Σ=2.04 (Si3.40Al0.60)O10(OH)2, a = b =5.2279(11) and c = 29.752(7)Å, space group: P3112] from 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 ~10 GPa. Nine structural refinements were performed at selected pressures within the P-range investi-gated. The compressional behavior of the same phengite sample was previously studied up to ~27 GPa by synchro-tron X-ray powder diffraction, and the corresponding P-V curve was modeled by a third-order Birch–Murnaghan Equation of State (BM-EoS). The significant elastic anisot-ropy of the 3T-phengite (i.e. (c)>> (a)) is mainly con-trolled by the compression of the K-polyhedra. The evolu-tion of the volume of the inter-layer K-polyhedron as a function of P is monotonic, without any evidence of discon-tinuity. Fitting the P-V data with a truncated second-order BM-EoS, we obtain a bulk modulus value of K0(K-polyhedron) = 35(3) GPa. The tetrahedra and octahedra in the 3T-phengite structure are significantly less compressible than the K-polyhedron, and behave similarly to rigid units within the P-range investigated. The main P-induced effect on the tetrahedral sheet consists in a cooperative rotation of the tetrahedra, describable by the evolution of the “tetrahedral rotation angle” (or “ditrigonal rotation angle”, ) as a func-tion of P. The value of the ditrigonal rotation angle in-creases significantly with P: (°) = P0 + 0.57(2)P (GPa) [R~99%]. The volume of the K-polyhedron and the value of ditrigonal rotation parameter () are not independent of one another, showing a correlation of about 99%.