Formation of Hydrous, Pyroxene-Related Phases from LiAlSiO4 Glass in High-Pressure Hydrothermal Environments

Hydrous Al-bearing pyroxene-related phases were synthesized by subjecting LiAlSiO4 glass to hydrothermal environments at pressures of 5-10 GPa and temperatures of 400-600 degrees C. LiAlSiO3(OH)(2) formed at 5 GPa, whereas at 10 GPa, product mixtures of LiAlSiO3(OH)(2) and Li3Al4(Si2O7)(SiO3)(2)(OH)(5) were obtained. The monoclinic structure of LiAlSiO3(OH)(2) has been previously characterized from single-crystal X-ray diffraction data (Spektor, K.; Fischer, A.; Haussermann, U. Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures-Dense Hydrous Aluminosilicates. Inorg. Chem. 2016, 55 (16), 8048-8058, 10.1021/acs.inorgchem.6b01181). It resembles that of alpha-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edge-sharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. The structure solution of Li3Al4(Si2O7)(SiO3)(2)(OH)(5), as detailed here, was achieved by rotational electron diffraction analysis, and the model was refined against synchrotron powder X-ray diffraction data (space group C2/c, a = 4.921 angstrom, b = 25.849 angstrom, c = 9.170 angstrom, and beta = 99.42 degrees). The crystal structure of Li3Al4(Si2O7)(SiO3)(2)(OH)(5) features chains and pairs of corner-condensed SiO4 tetrahedra, with the Si atoms equally distributed among the two structural units, and thus Li3Al4(Si2O7)(SiO3)(2)(OH)(5) is a rare example of a mixed inosorosilicate. LiAlSiO3(OH)(2) and Li3Al4(Si2O7)(SiO3)(2)(OH)(5) are structurally closely related to recently discovered hydrous magnesium aluminosilicate phases (i.e., HAPY and HySo), which form at conditions similar to the hydrous lithium aluminosilicates. The conjecture is made that hydrothermal environments following chlorite but also lawsonite breakdown generally afford conditions for the formation of hydrous, pyroxene-related, aluminosilicate phases, with compositions of M2(1-m)M1TO(3+n)(OH)(2-o) (0 < m, n, and o < 1). These phases could be transients in breakdown reactions but also stable at cold slab conditions and, thus, may play an important role to water storage and transport to the transition zone.