Hydration and phase relations of grossular-spessartine garnets at P H 2O=2 Kb

The phase relations in the system grossular-spessartine-H₂O were investigated at 2.0 Kb aqueous fluid pressure and at subsolidus temperatures down to 420 ° C. Despite metastable persistence of a compositional gap found in some intermediate members, a complete solid solution between grossular and spessartine exists.Linear relations between the unit cell edge, a ₀, and composition were readily observed down to 620 ° C with a ₀=11.849(2) Å and 11.613(2) Å for grossular and spessartine, respectively. Hydrated garnets began to appear at higher temperature for the Ca-rich members. Grossular and spessartine formed at 420 ° C have a ₀=11.901(2) Å and 11.632(2) Å, indicating the presence of 0.6 and 0.2 mol H₂O, respectively. Intermediate members show varying degrees of hydration. Infrared spectra of the more hydrated members show a major and minor absorption bands at 3,620 cm^–1 and 3,660 cm^–1, respectively, in addition to a broad band around 3,430 cm^–1. All the hydrogarnets formed at 420 ° C were proven to be metastable.The rare occurrence of the intermediate grossular-spessartine garnets may be attributed to the lack of appropriate bulk chemistry of the rock rather than to the P-T conditions to which the rock is subjected. There may be a stability field for hydrogrossular below 420 ° C at 2 Kb, but not for hydrospessartine. Any occurrence of hydrogarnet may be used as a temperature indicator setting the maximum of formation for the hydrogarnet-bearing assemblage below 420 ° C at 2 Kb.