Speciation of Al, Si, and K in supercritical solutions: Experimental study and interpretation

Abstract

Corundum and quartz solubilities in KOH-H 2O and NaOH-H 2O solutions were measured at 500–700°C and 2 kbar. The observed solubility increase is interpreted by the formation of neutral alkalialuminum and alkali-silica complexes, with a minor contribution of SiO(OH) 3 − ions; A1(OH) 4 − ions are believed to be practically non-existent at these conditions. The equilibrium constants for these species have been determined, which allows the calculation of their concentration in solutions in the system K 2O-Al 2O 3-SiO 2-H 2O for any 3-mineral buffering association. These calculated concentrations were compared to direct composition measurements for solutions experimentally equilibrated with the considered 3-mineral assemblages. The agreement between calculated and measured concentrations is good for alkaline assemblages where the alkali-aluminum complex seems to be the main Al-bearing species with molalities ranging from 0.002 to 0.02. But for common assemblages such as quartz-mica-feldspar, the (K, Na)Al complex molality is negligible (10 −6 to 10 −4), whereas the experimental Al-concentrations (10 −3 m) suggest the presence of another Al-bearing species involving both silica and alkalies.