Compressibility of 2M1 muscovite-phlogopite series minerals

Muscovite (Ms) and phlogopite (Phl) belong to the 2:1 dioctahedral and trioctahedral layer silicates, respectively, and are the end members of Ms-Phl series minerals. This series was studied in the 2M₁ polytype and modeled by the substitution of three Mg²⁺ cations in the Phl octahedral sites by two Al³⁺ and one vacancy, increasing the substitution up to reach the Ms. The series was computationally examined at DFT level as a function of pressure to 9 GPa. Cell parameters as a function of pressure and composition, and bulk moduli as a function of the composition agrees with the existing experimental results. The mixing Gibbs free energy was calculated as a function of composition. From these data, approximated solvi were calculated at increasing pressure. A gap of solubility is found, decreasing the gap of solubility at high pressure.

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)S = 0.95(Al1.51Mg0.32Fe0.18Ti0.03)S = 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 about 10 GPa. Nine structural refinements were performed at selected pressures within the P-range investigated. The compressional behavior of the same phengite sample was previously studied up to about 27 GPa by synchrotron 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 anisotropy of the 3T-phengite (i.e. β(c) > β(a)) is mainly controlled by the compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P is monotonic, without any evidence of discontinuity. Fitting the P–V data with a truncated second-order BM-EoS, we obtain a bulk modulus value of K 0(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”, α) as a function of P. The value of the ditrigonal rotation angle increases significantly with P: α (°) = α P0 + 0.57(2)P (GPa) [R about 99%]. The volume of the K-polyhedron and the value of ditrigonal rotation parameter (α) are not independent of one another, showing a correlation of about 99%.