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 degrees C and 1 GPa, using a piston-cylinder apparatus and a weight-loss method. Total potassium molality (m(K)) ranged from 0.0011 to 3.9. At the lowest mK, corundum solubility (m(Al)) was 0.0016+/-0.0004 molal, which is slightly higher than that in pure H(2)O at the same conditions (0.0011 molal). Corundum solubility increased with added KOH to a maximum of m(Al) = 2.66 at the highest m(K). At m(K) >= 0.03, m(Al) increased linearly with m(K), with dm(Al)/dm(K) = 1. The results were combined with previous work to evaluate the stability of the neutral ion-pair KAlO(2,aq) at 700 degrees C and 1 GPa. We obtained an optimal fit to the experimental data with an equilibrium constant (K) for the reaction KOH(aq) = K(+) + OH(-) of 10(-1.206), which was extrapolated from the data of Ho and Palmer [Ho, PC and Palmer, D-A., 1997. Ion association of a dilute aqueous potassium chloride and potassium hydroxide solutions to 600 degrees 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 H(2)O density and K. This gave an equilibrium constant for the reaction KAlO(2,aq) = K(+) + AlO(2)(-) 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 (HAlO(2,aq)) at m(K) < 0.01 (pH < 4.8). whereas AlO(2)(-) predominates to higher m(K) and pH, over most geologically realistic conditions. The KAlO(2,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 degrees C and 1 GPa. Thus, KAlO(2,aq) is not a major reservoir for dissolved aluminum in low-chloride crustal and mantle aqueous fluids. (C) 2009 Elsevier B.V. All rights reserved.