Thermodynamics of ordering and mixing in plagioclase feldspars: atomistic modelling in favour of Landau theory

Benoît Dubacq (1)

1 iSTeP - Institut des Sciences de la Terre de Paris

Plagioclase feldspars are key components of terrestrial planets and deriving their phase diagram and activity-composition relations has been the subject of many studies. Three main approaches have been used for thermodynamic modelling of the behaviour of plagioclase feldspars, and all three broadly disagree in terms of the magnitude and relative importance of mixing energies and ordering energies. This study uses Monte-Carlo simulations based on classical atomistic modelling to provide independent constraints over ordering and mixing for anorthite and along its solid solution with albite. Unsurprisingly, Al avoidance plays a key role. The competition between first-neighbour and second-neighbour interactions is highlighted, providing simple explanations to the high degree of short-range order in macroscopically-disordered plagioclase. The mixing properties obtained in this study support strong non-ideality, with negative mixing enthalpy and temperature-dependent mixing entropy. Ordering energies are consistent with studies using Landau theory. Unfortunately the form of the Landau equations hindered parametrisation of a full thermodynamic model from albite to anorthite, even though the thermodynamic formalism remains simple. Reasons, perspectives and implications for the phase diagram of ternary feldspars are discussed.

Mots clés: Plagioclase; Landau theory; atomistic modelling; ordering; Al-avoidance

Domaines: Minéralogie

Benoît Dubacq. Thermodynamics of ordering and mixing in plagioclase feldspars: atomistic modelling in favour of Landau theory. Contributions to Mineralogy and Petrology, 2022, 177 (10), pp.102. ⟨10.1007/s00410-022-01965-y⟩. ⟨hal-03817721⟩