Structural Transformations of Minerals in the Earth’s Mantle: the New High Pressure Polymorphous Modifications of Olivine and Wollastonite

Important results of studies of HP-minerals during 2018–2022 years are reviewed. The emphasis is made on the characterization of HP-polymorphous modifications of olivine (asimowite, poirierite) and wollastonite (breyite and davemaoite) discovered in this period. The structure of poirierite (ε-Mg₂SiO₄) contains a unique silicate chain Si₃O₉ with one tetrahedron in its period. Correspondingly, its structural formula is Mg₂[SiO₃]O, and its intergrowth with wadsleyite/ringwoodite in shocked chondrites supports the idea that olivine is transformed into ringwoodite through poirierite formation. The wollastonite (1T) at 8.9 GPa and its synthetic analogue CaGeO₃ have similar unit cell parameters. The HP/HT data confirm the stability of the wollastonite polymorphs (1T, 2M, pseudowollastonite and synthetic analogue of breyite) within wide upper mantle depth range. The crystal structures and composition of new НР-minerals discovered in impact zones, meteorites, and inclusions in diamonds provide insight into a complex petrology of deep geospheres, which cannot be obtained in laboratory experiments.