Formation of Hydrous, Pyroxene-Related Phases from LiAlSiO 4 Glass in High-Pressure Hydrothermal Environments

Abstract

Hydrous Al-bearing pyroxene-related phases were synthesized by subjecting LiAlSiO 4 glass to hydrothermal environments at pressures of 5-10 GPa and temperatures of 400-600 °C. LiAlSiO 3 (OH) 2 formed at 5 GPa, whereas at 10 GPa, product mixtures of LiAlSiO 3 (OH) 2 and Li 3 Al 4 (Si 2 O 7 )(SiO 3 ) 2 (OH) 5 were obtained. The monoclinic structure of LiAlSiO 3 (OH) 2 has been previously characterized from single-crystal X-ray diffraction data (Spektor, K.; Fischer, A.; Häussermann, U. Crystallization of LiAlSiO 4 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 α-spodumene (LiAlSi 2 O 6 ) and constitutes alternating layers of chains of corner-condensed SiO 4 tetrahedra and chains of edge-sharing AlO 6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO 4 tetrahedron chain layers. The structure solution of Li 3 Al 4 (Si 2 O 7 )(SiO 3 ) 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 Å, b = 25.849 Å, c = 9.170 Å, and β = 99.42°). The crystal structure of Li 3 Al 4 (Si 2 O 7 )(SiO 3 ) 2 (OH) 5 features chains and pairs of corner-condensed SiO 4 tetrahedra, with the Si atoms equally distributed among the two structural units, and thus Li 3 Al 4 (Si 2 O 7 )(SiO 3 ) 2 (OH) 5 is a rare example of a mixed ino-sorosilicate. LiAlSiO 3 (OH) 2 and Li 3 Al 4 (Si 2 O 7 )(SiO 3 ) 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.