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An experimental investigation into the metastable formation of phosphoran olivine and pyroxene |
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| Authors: | Joseph S Boesenberg Roger H Hewins |
| Institution: | a Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA b Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA c Laboratoire de Minéralogie et Cosmochimie du Muséum, UMR 7202, Muséum National d’Histoire Naturelle, 61 rue Buffon, 75005 Paris, France |
| 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)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. |
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