ON THE REACTION OF OLIGOCLASE WITH WATER UNDER CONDITIONS OF HIGH TEMPERATURE AND PRESSURE

Feldspar group minerals reacting with water at depth determine, by their reaction with environment, the character of newly formed minerals. Although plagioclase is the most abundant member of this group experimental data are very limited on the nature of its reactions under high pressure and temperature conditions. The author experimented on acid plagioclase with analytical data and physical constants corresponding to those of oligoclase. The reaction between oligoclase and water was studied under various pressures and temperatures, using a diffusion autoclave (previously described by the author 1]). Silicon, sodium, and aluminum in significant amounts went into solution; and, smaller amounts of calcium and potassium were observed. With increased temperature, larger amounts of material constituting oligoclase went into solution: The maximum occurred at 350 to 400° C; and, a marked decrease, at 500 to 600° C. Quantitative ratios of material in the condensate were found to correspond to albite indices. Increased pressure does not alter this behavior, there is only increased passage of separate components of the mineral into solution. At 350°, newly formed material was not observed microscopically; at higher temperatures, kaolinite and chalcedony appeared at sucessively higher temperatures. Experimentation by Morey and Chen on albite indicate that transfer of the albite component of oligoclase into solution is predominant. By comparison of its stability with that of two other minerals, albite is the most reactive with water. At 350° Centigrade and under pressure of 350 kilograms per square centimeter, the transfer into solution (in milligrams per liter) is for albite, 318; for microcline, 268; and, for oligoclase, 220. Labradorite was studied under analogous conditions; in that the transfer of labradorite predominates over that of the anorthite molecule, its behavior corresponds to albite. It was found that basic plagioclase shows less stability than is evident from microscopic examination of plagioclase in rocks. Data from these and previous experiments indicate possible types of solutions that can form within intragranular spaces in rocks by reaction between acid plagioclase and water. It is evident from the data that leaching of the albite component predominates by variation in composition of the solution throughout a wide range of temperature and pressure conditions. --D. D.Fisher