An experimental investigation into the metastable formation of phosphoran olivine and pyroxene

Abstract

The formation of phosphoran olivine by crystallization from a melt was investigated experimentally using a one atmosphere furnace, using San Carlos olivine [(Mg,Fe) 2SiO 4] mixed with either iron phosphide (FeP) or magnesium pyrophosphate (Mg 2P 2O 7). Both dynamic crystallization and isothermal experiments produced phosphoran olivine as zoned single crystals and as overgrowths surrounding normal, phosphorus-free olivine grains. The crystallization pathways that form phosphoran olivine were traced and confirm that it is a metastable phase that can crystallize from a phosphorus-rich melt over timescales of hours to days. Removal of the P and equilibration of the olivine however requires weeks to months in the presence of silicate melt. Phosphoran olivine with up to 27 wt% P 2O 5 was generated and up to 69% of the Si tetrahedral sites were replaced by P. The substitution of Si by P into olivine was confirmed as 4 VIM +2 + 2 IVSi +4 ↔ 3 VIM +2 + 2 IVP +5 + VI[]. Phosphoran olivine compositions that vary from (Mg,Fe) 2SiO 4 to (Mg,Fe) 1.65[] 0.35Si 0.3P 0.7O 4 have been produced in these experiments. Phosphoran pyroxene was also generated in a few experiments and forms when phosphoran olivine reacts with either tridymite or melt. It has compositions compatible with protopyroxene, orthopyroxene, pigeonite and sub-calcic augite, and can contain up to 31.5 wt% P 2O 5. Like phosphoran olivine, it is also a metastable phase. Phosphorus replaces Si in pyroxene by the following substitution methods: 8 IVSi +4 ↔ 3 IVSi +4 + 4 IVP +5 + IV[] with Al entering the structure by the exchange 2 IVSi +4 ↔ IVAl +3 + IVP +5. Phosphoran pyroxene compositions vary from (Mg,Fe) 8Si 8O 24 to (Mg,Fe) 8Si 3P 4[]O 24.