Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature.

Using an in situ method of Raman spectroscopy and resistance-heated diamond anvil cell, the system datolite CaBSiO4(OH) - water has been investigated at simultaneously high pressure and temperature (up to Р ~5 GPa and Т ~250 °С). Two polymorphic transitions have been observed: (1) pressure-induced phase transition or the feature in pressure dependence of Raman band wavenumbers at P = 2 GPа and constant T = 22 °С and (2) heating-induced phase transition at T ~90 °С and P ~5 GPа. The number of Raman bands is retained at the first transition but changed at the second transition. The first transition is mainly distinguished by the changes in the slopes of pressure dependence of Raman peaks at 2 GPa. The second transition is characterized by several strong changes: the wavenumber jumps of major bands, the merging of strong doublets at 378 and 391 cm−1 (values for ambient conditions), the splitting of the intermediate-intensity band at 292 cm−1, and the transformation of some low-wavenumber bands at 160-190 cm−1. No spectral and visual signs of overhydration and amorphization have been observed. No noticeable dissolution of datolite in the water medium occurred at 5 GPa and 250 °С after 3 h, which corresponds to typical conditions of the 'cold' zones of slab subduction. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]