Synthesis of Fe‐pumpellyite and its stability relations with epidote

Hydrothermal synthesis of Fe‐pum‐pellyites was conducted using high pressure cold‐seal apparatus and solid oxygen buffering techniques at temperatures between 250°C and 550°C and 2.0–9.1 kbar Pfluid. Fe‐pumpellyites were synthesized from partially crystalline gel mixtures of compositions: 4CaO ‐ 2.1Al2O3_1.5FeO ‐ 0.3MgO ‐ 6SiO2 (II) and 3CaO ‐ 1.5 Al2O3 ‐ 2.7FeO ‐ 0.3MgO ‐ 6SiO2 (III) in the presence of excess H2O at Pfluid of 5–9.1 Kbar, temperatures between 275°C and 325°C, and fO2 defined by the QFM and HM buffers; for both of these compositions (II and III), the condensed synthetic run products included minor 7Å chlorite ± garnet ± Fe‐oxide. The cell dimensions and aggregate refractive index (a= 19.13(2)Å, b= 5.940(4)Å, c= 8.847(5)Å, ±= 97.37(6)±, and n= 1.702(2)) of the pum‐pellyite synthesized from the bulk composition II mix are compatible with those of natural pumpellyites containing similar total Fe contents. Attempts at synthesizing Fe‐pumpellyites from a Mg‐free bulk composition were not successful; these results are consistent with the total absence of natural Mg‐free pumpellyites. The higher temperature, higher oxygen fugacity assemblages of the equivalent bulk compositions (II and III) consist of epidote ± minor amounts of chlorite, garnet, quartz, hematite, and magnetite. The results of these synthesis experiments accord with the mineral parageneses observed in low‐grade metabasites which imply that Fe‐pumpellyites are replaced by epidote with increasing temperature and/or fO2 and that Fe3+ is preferentially partitioned into epidote with respect to coexisting pum‐pellyite. In addition, these synthesis experiments indicate that Fe‐bearing pumpellyites crystallize at and are stable to lower temperatures than more aluminous pumpellyites—a result also consistent with natural systems.