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

The formation of phosphoran olivine by crystallization from a melt was investigated experimentally using a one atmosphere furnace, using San Carlos olivine [(Mg,Fe)2SiO4] mixed with either iron phosphide (FeP) or magnesium pyrophosphate (Mg2P2O7). 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% P2O5 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 4VIM+2 + 2IVSi+4 ↔ 3VIM+2 + 2IVP+5 + VI[]. Phosphoran olivine compositions that vary from (Mg,Fe)2SiO4 to (Mg,Fe)1.65[]0.35Si0.3P0.7O4 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% P2O5. Like phosphoran olivine, it is also a metastable phase. Phosphorus replaces Si in pyroxene by the following substitution methods: 8IVSi+4 ↔ 3IVSi+4 + 4IVP+5 + IV[] with Al entering the structure by the exchange 2IVSi+4 ↔ IVAl+3 + IVP+5. Phosphoran pyroxene compositions vary from (Mg,Fe)8Si8O24 to (Mg,Fe)8Si3P4[]O24.