Mechanisms of phase transitions between Al₂SiO₅ polymorphs

Samtsevich A. I., Oganov A. R.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 5. P. 79-91

https://doi.org/10.31857/S0869605521050051

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Language: English

Abstract

In nature Al₂SiO₅ exists as three polymorphs: kyanite, andalusite and sillimanite, often coexisting in the same rock. Here, we have studied in detail the mechanisms of structural transitions between all three phases of Al₂SiO₅ – kyanite-andalusite, andalusite-sillimanite and kyanite-sillimanite at the pressures of 0 and 10 GPa. The phase transition pathways with the lowest energy barriers are found by constructing a number of geometrically likeliest pathways and optimizing them using the variable-cell nudged elastic band method (VCNEB). We have analyzed the structural changes along the obtained lowest-energy pathways. These results have provided insights into the nature of structural relationships between Al₂SiO₅ polymorphs, their coexistence with each other and their transformation pathways. In particular, we confirm that phase transition barriers are very high, which allows these phases to coexist during geological timescales – thus serving as geothermobarometers.

Keywords: Al₂SiO₅ polymorphs, phase transitions mechanisms, density functional theory