COMPRESSIBILITY AND HIGH-PRESSURE BEHAVIOUR OF AB63OR27AN10 ANORTHOCLASE

A natural sample of anorthoclase of composition Na0.63 K0.27Ca0.10 Al1.10 Si2.90O8 (Ab63 Or27An10) was treated at T = 950°C for one week, and then investigated by single-crystal X-ray diffraction in situ at high pressure using a diamond-anvil cell. After the thermal treatment, the unit-cell parameters at room pressure and temperature are α 8.2523(3), b 12.9363(5), c 7.1416(3) Å, α 92.046(5), β 116.294(3), γ 90.206(4)*, V 682.91(5) Å 3, and indicate nearly complete Al-Si disorder. Unit-cell parameters were measured at room temperature as a function of pressure to 8.48 GPa. The P-V data are described by a fourth-order Birch-Murnaghan equation of state with the following coefficients: V0 = 682.92(5) Å3, K10 = 54.4(4) GPa, K1 = 6.4(3), K11 = -1.16(12) GPa-1. The unit-cell compression is anisotropic, as indicated by the unit strain tensors, with 53% of the total compression over the entire range in pressure accommodated along a direction close to a*. The axial linear compressibilities (βα. = 6.92, βb = 2.83 and βc = 3.91 × 103 GPa-1) reflect a compressibility pattern similar to that of other alkali feldspars. The X-ray intensity data were collected at a pressure of 0.0001, 4.7 and 6 GPa. The results of the refinements indicate that the symmetry of this anorthoclase remains triclinic, C1̄, over the entire range in pressure; no phase transitions were observed. The pressure causes a strong deformation of the extra-framework M site, with a contraction of the volume by 16% at P = 6 GPa. The compression is accompanied by a decrease of the M-O distances and a possible increase in the coordination number of the non-tetrahedrally coordinated cations from "5+1" to "5+2", related to the significant decrease of the M-OC(m) distance. The increase of the coordination number is not associated with any phase transition. Together with the deformation of the M site, the inter-tetrahedron tilting of essentially rigid tetrahedra (detectable by the changes in the T-O-T angles) is the main mechanism of compression of the structure of anorthoclase.