COMPRESSIBILITY AND HIGH-PRESSURE BEHAVIOR OF Ab63Or27An10 ANORTHOCLASE

Abstract

A natural sample of anorthoclase of composition Na 0.63 K 0.27 Ca 0.10 Al 1.10 Si 2.90 O 8 (Ab 63 Or 27 An 10 ) 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 a 8.2523(3), b 12.9363(5), c 7.1416(3) A, α 92.046(5), β 116.294(3), γ 90.206(4)°, V 682.91(5) A 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: V 0 = 682.92(5) A 3 , K T0 = 54.4(4) GPa, K ′ = 6.4(3), K ″ = −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 (β a = 6.92, β b = 2.83 and β c = 3.91 × 10 −3 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, C 1, 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.