Immiscibility of the grossular-spessartine series under a water pressure of 100 MPa

Abstract

Electron microprobe analysis of grossular-spessartine garnets hydrothermally synthesized from glass at 300–700°C shows that the composition is continuous above 490°C. Below this temperature there is a compositional gap between 70 and 96 mol % spessartine at 480°C and between 55 and 97 mol % at 450°C. The gap extends up to 36 mol % spessartine at 400°C and up to 30 mol % at 300°C on the grossular side, but is not clear on the spessartine side because of the fO₂ unbuffered experiments. Hydrothermal treatment of garnet-anorthite-tephroite mixture with bulk composition of Gr_33.3 Sp_66.7 shows that garnet grows above 430°C, and that no change is recognized at 400°C. These facts indicate that garnet of the grossular-spessartine series forms a complete solid solution above 490°C, and shows a certain indication that they exsolve to Ca-rich and Mn-rich garnets at lower temperatures. The additional experiments indicate that below 415°C the tie-line connecting the two garnets is replaced by the one connecting anorthite and tephroite in the system CaO–MnO–Al₂O₃–SiO₂.