Redox‐Independent Low Water Solubility in Quartz and Implications for Water Storage in the Crust

Abstract

AbstractQuartz is an important mineral constituent of the continental crust, which commonly contains trace amounts of water as hydroxyl in the lattice. Despite considerable efforts to document the roles of pressure, temperature, and chemistry on water storage in quartz, the effect of redox state, a key thermodynamic factor in Earth sciences, has not been well established. We assessed the redox effect on water solubility in quartz at 2 and 10 kbar and 800°C, by equilibrating two natural gem‐quality crystals with water over a range of redox conditions. Recovered samples show infrared absorption bands at 3,100–3,600 cm−1. Dehydration experiments at 1200°C (room pressure) suggest that the two bands at ∼3,197 and 3,293 cm−1 are linked to Si‐O overtones, with the other bands to hydroxyl. The water solubility, ∼3.7–32.2 ppm wt. H2O, is redox‐independent, differing from other minerals such as feldspar, pyroxenes, garnet, rutile, and olivine in which the water solubility is significantly redox‐dependent. This provides evidence that the water incorporation in quartz is not controlled by polyvalent elements and/or other redox‐linked mechanisms. Considering the roles of Al, Li, and B in incorporating water and its low solubility over remarkable variations in the abundances of those elements in most natural samples, quartz is not a major water carrier compared to other nominally anhydrous minerals (NAMs) in the crust. The partitioning of water between quartz and other crustal NAMs such as feldspar, pyroxenes, rutile, and garnet is redox‐influenced, and is unlikely to be constant in the crust.