EFFECT OF ACIDITY OF THE EQUILBRUIM FLUID ON THE INVARIANT POINT IN THE SYSTEM ALBITE-ORTHOCLASE

Abstract

The paper reports on a study to determine the temperature and the composition at the invariant point in the system Ab-Ort as a function of HCl concentration in the aqueous fluid at Ptot = Pfl = = 1 kb. The procedure resembled that employed earlier and involved modeling of an open system whose components are perfectly mobile because of the sizable excess of fluid (solution/charge ratio 1:2) and the low HCl solubility in melt. The starting materials were natural albite from the Vishnevogorsk deposit (0.3 percent K2O, 0.4 percent CaO) and potassic feldspar (orthoclase) from A.D. Kuznetsov's collection, containing less Na2O than could be detected by microprobe analysis. All tests were run in an exoclave unit (externally controlled pressure device) with externally heated reactors. Two series of runs were made. The first determined the effect of the HCl content of the fluid on the temperature at the invariant point. In the second series of runs the composition of the first melt derived from the studied fluid-silicate system was determined, including the chlorine content of the melt. Study results are discussed.