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

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• 作者：Joseph S. Boesenberg ; Roger H. Hewins
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2010
• 出版时间：15 March 2010
• 年：2010
• 卷：74
• 期：6
• 页码：1923-1941
• 全文大小：1302 K

文摘

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 4^VIM⁺² + 2^IVSi⁺⁴ ↔ 3^VIM⁺² + 2^IVP⁺⁵ + ^VI[]. Phosphoran olivine compositions that vary from (Mg,Fe)₂SiO₄ to (Mg,Fe)_1.65[]_0.35Si_0.3P_0.7O₄ 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: 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)₈Si₈O₂₄ to (Mg,Fe)₈Si₃P₄[]O₂₄.