CaAl_(2)Si_(2)O_(8) polymorphs:Sensitive geothermometers and geospeedometers

Plagioclase is the major rock-forming mineral constituting the Earth's crust,whereas anorthite(CaAl_(2)Si_(2)O_(8))is a common minerals in lunar highlands crust,meteorites,possibly in some comets and on Mercury.Besides anorthite,two high-temperature polymorphs of CaAl_(2)Si_(2)O_(8)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℃))on naturally co-occurring CaAl_(2)Si_(2)O_(8)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(unquenchableβ-dmisteinbergite and quenchable y-dmisteinbergite).It was shown that Ca coordination differs significantly in CaAl_(2)Si_(2)O_(8)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 CaAl_(2)Si_(2)O_(8)polymorphs stability.Our results indicate that CaAl_(2)Si_(2)O_(8)polymorphs could be used for temperature estimations for both Earth and planetary sciences.