Solubility of corundum in aqueous KOH solutions at 700 °C and 1 GPa

The solubility of corundum in aqueous KOH solutions was measured at 700 °C and 1 GPa, using a piston-cylinder apparatus and a weight-loss method. Total potassium molality (mK) ranged from 0.0011 to 3.9. At the lowest mK, corundum solubility (mAl) was 0.0016 ± 0.0004 molal, which is slightly higher than that in pure H2O at the same conditions (0.0011 molal). Corundum solubility increased with added KOH to a maximum of mAl = 2.66 at the highest mK. At mK ≥ 0.03, mAl increased linearly with mK, with dmAl / dmK = 1. The results were combined with previous work to evaluate the stability of the neutral ion-pair KAlO2,aq at 700 °C and 1 GPa. We obtained an optimal fit to the experimental data with an equilibrium constant (K) for the reaction KOHaq = K+ + OH− of 10− 1.206, which was extrapolated from the data of Ho and Palmer [Ho, P.C. and Palmer, D.A., 1997. Ion association of a dilute aqueous potassium chloride and potassium hydroxide solutions to 600 °C and 300 MPa determined by electrical conductance measurements. Geochimica et Cosmochimica Acta, 61, 15, 3027–3040.] using linear isothermal correlations between the logarithms of H2O density and K. This gave an equilibrium constant for the reaction KAlO2,aq = K+ +AlO2− of 10− 0.299. The results permit assessment of the dominant aqueous species in K–Al–O–H fluids at high pressure and temperature. We find that the dominant Al-bearing species in such fluids is predicted to be the neutral hydrate (HAlO2,aq) at mK < 0.01 (pH < 4.8), whereas AlO2− predominates to higher mK and pH, over most geologically realistic conditions. The KAlO2,aq ion pair will only be the most abundant Al-bearing species at very high pH (> 8), which corresponds to KOH molality of > 10 at 700 °C and 1 GPa. Thus, KAlO2,aq is not a major reservoir for dissolved aluminum in low-chloride crustal and mantle aqueous fluids.