Some IR features of SiO4 and OH in coesite, and its amorphization and dehydration at ambient pressure

Coesite (Coe) with grain size in the range of 30-80 μm has been synthesized at 5 GPa and 1600 °C for 12 h by using a cubic press. Its unpolarized single-crystal absorption infrared (IR) spectra show 14 IR bands in the range of ∼1200-650 cm−1, five of which have high intensity (at ∼1161, 1109, 1063, 1028 and 994 cm−1) and are preliminarily assigned to the SiO4 asymmetric stretching (ν3). In addition, three sharp but relatively weak bands at ∼838, 814 and 796 cm−1 are tentatively attributed to the SiO4 asymmetric bending (ν4). The IR data also show six OH peaks in the range of 3700-3300 cm−1, with an estimated H2O content of ∼30(4) wt ppm. Following previous studies, we have assigned the peaks at ∼3464 (#7), 3421 (#8), 3406 (#9) and 3377 cm−1 (#10) to the Type-II hydrogarnet substitution, and the peaks at ∼3500 (#6a) and 3534 cm−1 (#6b) to the B-based defects, with the latter aroused by possible B contamination in the synthesizing experiments. Annealing experiments conducted consecutively at ∼200, 400, 600, 800, 1000 and 1200 °C, with every heating step lasting for 24 h, demonstrate that water diffuses quickly out of Coe at T as low as ∼600 °C. The material annealed at 1200 °C is completely dehydrated and amorphous. A quick response of the water content in Coe to the changes of P, T and composition is thus possible, which may be critical to the preservation of natural Coe in relevant geological processes. It further implies that water in Coe, and possibly in other nominally anhydrous minerals (NAMs), may behave distinctively different from the water located in the hydrous phases such as amphibole and mica, and potentially makes significant contribution to the subduction zone-related fluids.