Formation of gaseous molecular water in coesite: From OH− to H2O cluster

Water, either as hydroxyl (OH−) or molecular water (H2O), plays a key role in numerous geological processes. The mechanism for water to enter or leave the silicate structures was uncertain. Here, we show how OH− water leaves coesite (Coe) by conducting a systematic in situ high-temperature infrared (IR) spectroscopic study supplemented with transmission electron microscopy (TEM) analysis. The used Coe contained ∼96(32) wt ppm water and showed IR peaks for OH− only. Heating up to ∼1473 K at 1 atm, a new IR band developed at ∼3659 cm−1, and was likely caused by gaseous H2O water nanoclusters. This attribution has been supported by our TEM observation, which exhibited numerous nanobeads of H2O water ubiquitously and evenly distributing in the sample. It is thus clear that OH− exceeding the Coe water solubility transformed into gaseous H2O water and left the Coe structure. This implies that nanoclusters of H2O water and even larger water-rich fluid inclusions in natural Coe and other silicates with unusual H and O isotopes can be intrinsic in origin and not necessarily a result of micro-fractural infiltration of external fluids.