High-pressure silicates: crystal chemistry and systematics

Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 5. P. 1-78

https://doi.org/10.31857/S0869605521050038

Language: English

Abstract

The crystal chemistry of high-pressure (HP) silicates has been reviewed with special emphasis on their structural topology and Si coordination. The HP silicates are subdivided into eleven major groups according to their chemical compositions: (i) SiO₂ polymorphs; (ii) feldspar polymorphs; (iii) pyroxene and amphibole high-pressure polymorphs; (iv) garnet-type phases with octahedral Si; (v) MSiO₃ high-pressure polymorphs (M = Mg, Fe); (vi) M₂SiO₄ high-pressure polymorphs (M = Mg, Fe); (vii) dense hydrous Mg silicates and related structures; (viii) high-pressure silicates in the Al₂O₃–SiO₂ and Al₂O₃–SiO₂–H₂O systems; (ix) Ca, Sr and Ba high-pressure silicates and aluminosilicates; (x) alkali metal high-pressure silicates and aluminosilicates; (xi) miscellaneous high-pressure silicates. In total, more than 160 HP silicates are considered that crystallize in over 115 different structure types. On the basis of the recent advances in the field, the whole crystal chemistry of inorganic silicates can be systematized on the basis of the coordination numbers (CNs) of Si atoms relative to oxygen into seven groups corresponding to the following combinations of CNs(Si): 4; 4 + 5; 4 + 5 + 6; 4 + 6; 5; 5 + 6; 6. Less than half of all known HP silicates are based upon closest packings of anions. The topological properties of linkage between Si coordination polyhedra include corner (for all CNs(Si)), edge (for CN(Si) = 5 and 6) and face (for CN(Si) = 6) sharing. One oxygen atom may be shared between three or less Si coordination polyhedra at the same time.

Keywords: high pressures, silicates, crystal structure, crystal chemistry, hexacoordinated silicon, pentacoordinated silicon, high-pressure mineralogy, phase transitions, structural complexity, structural topology