EXPERIMENTAL-DETERMINATION AND INTERPRETATION OF THE SOLUBILITY OF THE ASSEMBLAGE MICROCLINE, MUSCOVITE, AND QUARTZ IN SUPERCRITICAL H2O

The concentrations of K, Na, Al, and Si in pure H2O in equilibrium wit h natural microcline, muscovite, and quartz have been measured between 400 and 600-degrees-C at 2 kbar with an extraction-quench apparatus. Silicon is the dominant solute in solution, and its concentration is s imilar to those reported for quartz solubility in pure H2O: increasing from 0.05 molal at 400-degrees-C to 0.12 molal at 600-degrees-C. The small amount of Na in the microcline and muscovite was rapidly leached from the mineral surface into solution so that the initial log molali ty of Na equaled -2.7 but decreased as pure H2O was injected into the charge volume. Log molalities of K in equilibrium with the mineral ass emblage are between -3.0 and -2.7. The calculated solution composition s are basic, and Al(OH)4- is determined to be the dominant Al species in solution. These calculations suggest that both Al(OH)30 and Al(OH)4 - contribute significantly to the solubility of corundum in pure H2O a t these temperatures and 2 kbar. The logarithm of the equilibrium cons tant for Al(OH)03 + H2O = Al(OH)4- + H+ is computed to be -4.1, -5.4, and -6.1 at 2 kbar and 400, 500, and 600-degrees-C, respectively. No e vidence exists for the formation of significant concentrations of alka li-Al complexes despite the fact that Al concentrations in pure H2O fo r microcline + muscovite + quartz are an order of magnitude greater th an those reported for corundum solubility at the same pressure and tem perature.