Hydrothermal breakdown reactions of grossular and pyrope garnets in MgCl₂ and CaCl₂ solutions at 2 kbar and 550 and 750°C.

The reaction mechanism is of dissolution-precipitation type; reactant garnets typically show isotropic dissolution textures and products either crystallize as large idiomorphic crystals or clusters of minute crystals. Metastable intermediate assemblages precede the formation of the final (equilibrium) assemblages, in accordance with the Ostwald step rule, provided the latter is regarded in terms of reaction path and metastable assemblages rather than single phases. The dominant factor controlling the final mineral assemblage is the bulk Mg/Ca ratio with the cation fractionation being characterized by a strong partitioning of Ca²⁺ into the solution phase. Cordierite+forsterite+spinel is interpreted to be the stable product assemblage at 750°C and high Mg/Ca (Mg/Ca≥1.8), and anorthite+forsterite+spinel the stable product at lower ratios. At 550°C chlorite is the major run product along with anorthite at low Mg/Ca bulk ratios and quartz at higher ratios. The kinetics of the break-down reaction of the garnets are shown to be strongly dependent on the solution concentration, and occur an order of magnitude faster at high concentrations (1 and 2 molal) compared to reaction in dilute solutions (0.2 molal or in water). -from Authors