Thermoelastic and thermodynamic properties of plagioclase feldspars from thermal expansion measurements

Powder X‑ray diffraction patterns between 90 and 935 K have been collected for nine plagioclase samples, with different compositions and degree of Al-Si order. The refined volumes have been modeled using the Wallace and Suzuki formulations based on the Mie-Grüneisen EOS. No significant difference has been found between the Suzuki and Wallace formulations, and between the Einstein and Debye approximations of lattice energy. A Wallace model with the first derivative of the bulk modulus constrained to the experimentally determined values leads to refined Grüneisen parameters between 0.49 and 0.41, without any definite trend between albite and anorthite; the Einstein temperature in intermediate plagioclase is θE - 650 K, but it is lower in albite [θE = 453(5) K].