Hydrothermal breakdown reactions of grossular and pyrope garnets in MgCl (sub 2) and CaCl (sub 2) solutions at 2 kbar and 550 and 750 degrees C

The hydrothermal breakdown of grossular and pyrope were studied at 2 kbar and 550 and 750 degrees C in MgCl (sub 2) and CaCl (sub 2) solutions (0.2-2.0 molal). The reaction mechanism is of dissolution-precipitation type; the garnets typically show isotropic dissolution textures and the products either crystallize as large crystals (cordierite, forsterite, anorthite) or clusters of minute crystals (chlorite, spinel, clintonite). The dominant factor controlling the final assemblage is the bulk Mg/Ca ratio with the cation fractionation being characterized by strong positioning of Ca (super 2+) into the solution phase. Cordierite + forsterite + spinel is the stable product assemblage at 750 degrees C and high Mg/Ca (>= 1.8), while anorthite + forsterite + spinel is the stable assemblage at lower ratios. At 550 degrees C chlorite is the major product along with anorthite at low Mg/Ca bulk ratios and quartz at higher ratios. The reactions occur an order of magnitude faster at high concentration (1-2 molal) compared to dilute solutions (0.2 molal or in water). Ionic solutions may thus be important in natural breakdown reactions.