Hydrothermal breakdown reactions of grossular and pyrope garnets in MgCl2 and CaCl2 solutions at 2 kbar and 550 and 750°C
Gavrieli, Ittai; Matthews, Alan; Bar-Matthews, Miryam; Szafranek, David
European Journal of Mineralogy Volume 4 Number 4 (1992), p. 793 - 811
published: Aug 11, 1992
manuscript accepted: Dec 13, 1991
manuscript received: Jan 11, 1991
ArtNo. ESP147050404010, Price: 29.00 €
Abstract The hydrothermal reactions of grossular and pyrope garnet were been studied at 2 kbar and 550°C and 750°C in MgCl2 and CaCb solutions at concentrations of 0.2 to 2.0 molal. The reaction mechanism is of dissolution- precipitation type; reactant garnets typically show isotropic dissolution textures and products either crystallize as large idiomorphic crystals (cordierite, forsterite, anorthite) or clusters of minute crystals (chlorite, spinel, clintonite). 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 breakdown 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). Ionic solutions may therefore have a marked influence on the kinetics of dehydration and hydration reactions of garnet in nature.