The high-pressure structural evolution of a natural 3T-phengite [(K 0.90 Na 0.05 ) S ¼ 0.95 Á (Al 1.51 Mg 0.32 Fe 0.18 Ti 0.03 ) S ¼ 2.04 (Si 3.40 Al 0.60 )O 10 (OH) 2 , a ¼ b ¼ 5.2279(11) and c ¼ 29.752(7) A, space group: P3 1 12] 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 investigated. The compressional behavior of the same phengite sample was previously studied up to $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. b(c) ) b(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) ¼ 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", a) as a function of P. The value of the ditrigonal rotation angle increases significantly with P: a ( ) ¼ a P0 þ 0.57(2)P (GPa) [R $ 99%]. The volume of the K-polyhedron and the value of ditrigonal rotation parameter (a) are not independent of one another, showing a correlation of about 99%.
The volume and unit-cell parameters of analbite (i.e., NaAlSi 3 O 8 with complete Al,Si disorder) have been determined by single-crystal X-ray diffraction to a maximum pressure of ~8.71 GPa. The volume variation with pressure is described by a fourth-order Birch-Murnaghan equation of state with K 0T = 50.3(5) GPa, K 0 ′ = 8.9(5), and K 0 ″ = -2.4(3) GPa -1 . The value of the room-pressure bulk modulus is ~4% lower than that of low albite, and the onset of volume softening in analbite is at ~6.7 GPa, some 1.7 GPa higher than the onset in albite. The anisotropy of compression of analbite is less than that in albite.Single-crystal structure determinations of analbite to ~9.4 GPa show that there is no significant detectable compression of the T-O bonds within the structure, and the compression of the framework of tetrahedra is therefore accommodated by changes in the T-O-T angles, which result in significant compression of the "crankshaft chains" within the framework. No significant shear of the tetrahedral rings of analbite was detected, in contrast to the structural compression of albite. Overall, the structural changes that occur in analbite from 0.0001 to 9.4 GPa resemble those seen in ordered albite over the pressure range 0.0001-4 GPa. Therefore analbite shows a significantly greater structural rigidity than low albite up to pressures of 9.4 GPa.
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