The system diopside-ureyite at 20 Kb

The join diopside (CaMgSi₂O₆) — ureyite (NaCrSi₂O₆) was studied at pressures of 1 atm, 1 kb, 5 kb, and 20 kb using gel mixtures as starting materials. All runs except those at 1 atm were made under hydrous conditions. The data show that the solubility of ureyite in diopside decreases with increasing pressure. At 20 kb the maximum ureyite content of diopside is 13 weight percent (4.6% Cr₂O₃) as compared to 24 weight percent at 1 atm. It is predicted that pyroxenes that have equilibrated at depths >140 km will not contain a ureyite component; rather Cr will enter diopside in the form of a CaCr(CrSi)O₆ component. Pyroxenes containing this component were found as metastable phases at 20 kb.