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Thermodynamics of plagioclases I: Theory of the \(I\bar 1 - P\bar 1\) phase transition in anorthite and Ca-rich plagioclases

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Abstract

Landau theory of the \(P\bar 1 - I\bar 1\) phase transition in Ca-rich plagioclases reveals the sensitivity of the phase transition behaviour to a) Al, Si disorder, b) structural replacement of Ca by Na, and c) inhomogeneities of lattice strains. The following effects are predicted:

  1. 1)

    A tricritical phase transition exists in fully ordered anorthite. Al, Si disorder and Na, Ca exchange lead to second order phase transitions.

  2. 2)

    The transition temperatures depend sensitively on the degree of Al, Si disorder and the chemical composition of the Ca-rich plagioclases. Increasing Na-content decreases the transition temperatures.

  3. 3)

    The thermal evolution of c and d reflections depends on the homogeneity of the crystal and do not necessarily reflect the temperature evolution of the macroscopic lattice strain. A simple quadratic dependence of the X-ray scattering intensity on the order parameter exists only for fully ordered, homogeneous anorthite.

The role of inhomogeneous Al, Si distributions and lattice relaxations are discussed including possible structural modulations.

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  1. Department of Earth Sciences, Downing Street, CB3 2EQ, Cambridge, England

    E. Salje

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  1. E. Salje
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Salje, E. Thermodynamics of plagioclases I: Theory of the \(I\bar 1 - P\bar 1\) phase transition in anorthite and Ca-rich plagioclases. Phys Chem Minerals 14, 181–188 (1987). https://doi.org/10.1007/BF00308223

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  • DOI: https://doi.org/10.1007/BF00308223

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