CaAl2Si2O8 polymorphs: Sensitive geothermometers and geospeedometers

Abstract

Plagioclase is the major rock-forming mineral constituting the Earth’s crust, whereas anorthite (CaAl₂Si₂O₈) is a common minerals in lunar highlands crust, meteorites, possibly in some comets and on Mercury. Besides anorthite, two high-temperature polymorphs of CaAl₂Si₂O₈ are known: dmisteinbergite and svyatoslavite, which are found in burnt coal dumps, meteorites and pseudotachylytes. Here we present the results of detailed studies (quenching experiments, elemental analysis, Raman spectroscopy and in situ high temperature single crystal X-ray diffraction (up to 1000 °C)) on naturally co-occurring CaAl₂Si₂O₈ polymorphs (anorthite, dmisteinbergite and svyatoslavite) from a burnt coal dump in Kopeisk, Russia. New polymorphs were found in all natural samples and obtained upon heating of dmisteinbergite (unquenchable β-dmisteinbergite and quenchable γ-dmisteinbergite). It was shown that Ca coordination differs significantly in CaAl₂Si₂O₈ polymorphs, resulting in a different capacity to host Ba and possibly other large ion lithophile elements. Combining our data on natural samples with the previously published data on natural and synthetic compounds, we propose a new scheme of CaAl₂Si₂O₈ polymorphs stability. Our results indicate that CaAl₂Si₂O₈ polymorphs could be used for temperature estimations for both Earth and planetary sciences.