Abstract
Comparisons of the structures of the high-pressure and high-temperature phases of both anorthite and clinopyroxene show that they are quite distinct although they share a common space group and site symmetries for all atoms. The low-pressure, low-temperature phases in both systems are formed by zone-boundary transitions from the high-pressure or high-temperature structures, and have structures that are made up of the alternation of portions of the high-pressure and high-temperature structures. This behaviour is analyzed in terms of the order parameter behaviour of the high-symmetry structures, and it is shown that the stability of the low-symmetry phases arises naturally from inhomogeneity in this order parameter.
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Christy, A.G., Angel, R.J. A model for the origin of the cell-doubling phase transitions in clinopyroxene and body-centred anorthite. Phys Chem Minerals 22, 129–135 (1995). https://doi.org/10.1007/BF00202473
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DOI: https://doi.org/10.1007/BF00202473