Abstract

The polymorphism of ice has been widely investigated over the last decades, and the results were found to be applicable over a wide range of terrestrial and extraterrestrial environments with recent findings also among the mineral inclusions entrapped in natural diamonds. In this study, we investigated the behavior of solid [SiOx(OH)4-2x]n silicic acid under anhydrous and hydrous conditions at high pressure and temperature using the diamond anvil cell technique combined with both Raman and Fourier transform mid-infrared spectroscopy. Firstly, we report the stability of silicic acid in the form of silica gel under anhydrous conditions up to about 7 GPa and 200 °C. In the presence of water employed as pressure medium, we observe the formation of ice VI and ice VII at lower pressures than previously reported in pure water systems. Each phase transition is documented by the optical images over compression/decompression and heating/cooling runs. Importantly, we report the decomposition of silicic acid to moganite and metastable quartz in the presence of ice polymorphs up to near 10 GPa and 200 °C and also recovered at ambient conditions along with H2Oliq. Main implications from this study span from planetary sciences to Earth’s deep mineralogy. For instance, the interior of ice giants might need to be revisited considering our enhanced stability of ice VII at lower pressures in the presence of tiny amount of free silica while the formation of moganite + quartz resembles the assemblage found in a lunar meteorite and provides a pressure–temperature range applicable to constrain impact conditions. Furthermore, the stability of silicic acid at high pressure(–temperature) can be taken as preliminary evidence that the formation of orthosilicic acid, such as that found around minerals entrapped in diamonds, might result from the dissolution of minerals (or melts) into acidic aqueous fluids.