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

Abstract

There are reviewed important results obtained in the studies of HP-minerals during 2018–2022 years. The emphasis is made on the characterization of HP-polymorphs modifications of olivine (asimovite, poirierite) and wollastonite (breyite and davemaoite) discovered in this period. The structure of poirierite (ε-Mg2SiO4) contains the unique silicate chain Si3O9 with one tetrahedron in its period. Consequently its structural formula is Mg2[SiO3]O, and its intergrowth with wadsleyite/ringwoodite in shocked chondrites support the idea that transformations of olivine into ringwoodite becomes via forming of poirierite. The close ratio of cell parameters in wollastonite (1T) at 8.9 GPa and its synthetic analogue CaGeO3 exhibits the similarity of their structures. The HP/HT data confirm the stability of the wollastonite polymorph (1T, 2M, pseudowollastonite and synthetic analogue of breyite) at the upper mantle conditions. The crystal structures and composition of new НР-minerals discovered as inclusions in diamonds, in meteorites and in the rocks near terrestrial impact craters provide the richer information on the complicate petrology of deep-seated geospheres which cannot be obtained on the base of laboratory experiments.