The thermoelastic behaviour of clintonite up to 10 GPa and 1000°C

The thermoelastic behavior of a natural clintonite-1M [with composition: Ca 1.01(Mg 2.29Al 0.59Fe 0.12) Σ3.00(Si 1.20Al 2.80) Σ4.00O 10(OH) 2] has been investigated up to 10 GPa (at room temperature) and up to 960°C (at room pressure) by means of in situ synchrotron single-crystal and powder diffraction, respectively. No evidence of phase transition has been observed within the pressure and temperature range investigated. P-V data fitted with an isothermal third-order Birch-Murnaghan equation of state (BM-EoS) give V 0 = 457.1(2) Å 3, K T0 = 76(3)GPa, and K′ = 10.6(15). The evolution of the "Eulerian finite strain" versus "normalized stress" shows a linear positive trend. The linear regression yields Fe(0) = 76(3) GPa as intercept value, and the slope of the regression line leads to a K′ value of 10.6(8). The evolution of the lattice parameters with pressure is significantly anisotropic [β(a) = 1/3K T0(a) = 0.0023(1) GPa -1; β(b) = 1/3K T0(b) = 0.0018(1) GPa -1; β(c) = 1/K T0(c) = 0.0072(3) GPa -1]. The β-angle increases in response to the applied P, with: β P = β 0 + 0.033(4)P (P in GPa). The structure refinements of clintonite up to 10.1 GPa show that, under hydrostatic pressure, the structure rearranges by compressing mainly isotropically the inter-layer Ca-polyhedron. The bulk modulus of the Ca-polyhedron, described using a second-order BM-EoS, is K T0(Ca-polyhedron) = 41(2) GPa. The compression of the bond distances between calcium and the basal oxygens of the tetrahedral sheet leads, in turn, to an increase in the ditrigonal distortion of the tetrahedral ring, with ∂α/∂P ≈ 0.1°/GPa within the P-range investigated. The Mg-rich octahedra appear to compress in response to the applied pressure, whereas the tetrahedron appears to behave as a rigid unit. The evolution of axial and volume thermal expansion coefficient α with temperature was described by the polynomial α(T) = α 0 + α 1T -1/2. The refined parameters for clintonite are as follows: α 0 = 2.78(4) 10 -5°C -1 and α 1 = -4.4(6) 10 -5°C 1/2 for the unit-cell volume; α 0(a) = 1.01(2) 10 -5°C -1 and α 1(a) = -1.8(3) 10 -5°C 1/2 for the a-axis; α 0(b) = 1.07(1) 10 -5°C -1 and α 1(b) = -2.3(2) 10 -5°C 1/2 for the b-axis; and α 0(c) = 0.64(2) 10 -5°C -1 and α 1(c) = -7.3(30) 10 -6°C 1/2for the c-axis. The β-angle appears to be almost constant within the given T-range. No structure collapsing in response to the T-induced dehydroxylation was found up to 960°C. The HP- and HT-data of this study show that in clintonite, the most and the less expandable directions do not correspond to the most and the less compressible directions, respectively. A comparison between the thermoelastic parameters of clintonite and those of true micas was carried out.