Aluminum speciation in aqueous fluids at deep crustal pressure and temperature

Abstract

We investigated aluminum speciation in aqueous fluids in equilibrium with corundum using in situ Raman spectroscopy in hydrothermal diamond anvil cells to 20kbar and 1000°C. We have studied aluminum species in (a) pure H2O, (b) 5.3m KOH solution, and (c) 1m KOH solution. In order to better understand the spectral features of the aqueous fluids, we used ab initio simulations based on density functional theory to calculate and predict the energetics and vibrational spectra for various aluminum species that are likely to be present in aqueous solutions. The Raman spectra of pure water in equilibrium with Al2O3 are devoid of any characteristic spectral features. In contrast, aqueous fluids with 5.3m and 1m KOH solution in equilibrium with Al2O3 show a sharp band at ∼620cm−1 which could be attributed to the [Al(OH)4]1- species. The band grows in intensity with temperature along an isochore. A shoulder on the high-frequency side of this band may be due to a hydrated, charge neutral Al(OH)3·H2O species. In the limited pressure, temperature and density explored in the present study, we do not find any evidence for the polymerization of the [Al(OH)4]1- species to dimers [(OH)2-Al-(OH)2-Al(OH)2] or [(OH)3-Al-O-Al(OH)3]2−. This is likely due to the relatively low concentration of Al in the solutions and does not rule out significant polymerization at higher pressures and temperatures. Upon cooling of Al-bearing solutions to room temperatures, Raman bands indicating the precipitation of diaspore (AlOOH) were observed in some experiments. The Raman spectra of the KOH solutions (with or without dissolved alumina) showed a sharp OH stretching band at ∼3614cm−1 and an in-plane OH bending vibration at ∼1068cm−1, likely related to an OH− ion with the oxygen atom attached to a water molecule by hydrogen bonding. A weak feature at ∼935cm−1 may be related to the out-of-plane bending vibration of the same species or to an OH species with a different environment.