Stability Relations of Grossularite

Abstract

Exploratory experiments have been conducted to determine the stability range of grossularite garnet, $$Ca_{3}Al_{2}(SiO_{4})_{3}$$ Grossularite probably has a field of stability at atmospheric pressure; although the garnet does not require pressure for its formation, pressure should favor its formation. Grossularite may form by reaction of its components in the solid state; the intermediate products obtained are presumed to be meta-stable. The majority of garnets described as grossularite are probably members of the hydrogrossular series as defined by Hutton. Pressure, in addition to temperature and time, determines the stable member of the hydrogarnet series. It is believed that the dry end member, grossularite, exists but that it cannot exist in the presence of water at elevated temperatures. The range of stability of grossularite is outlined indirectly from field and laboratory data in relation to the progressive metamorphism of a limestone containing clay impurities. Grossularite has, as yet, not been synthesized. The inability of pressure to bring about reconstructive transformations is advanced. A critical amount of energy must be supplied before a given metamorphic reaction can take place. This level need not be reached solely by temperature, since the energy of secondary reactions, stress, hydrostatic pressure, and radioactive bombardment may contribute. A theoretical explanation for the birefringence in some garnets is given.