Coexisting Amphiboles from Blueschist Facies Metamorphic Rocks |
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Authors: | HIMMELBERG G R; PAPIKE J J |
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Institution: | U.S. Geological Survey, Menlo Park, California, 94025
U.S. Geological Survey, Washington, D.C. 20242, U.S.A. |
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Abstract: | Four pairs of associated calcic and sodic amphiboles from blueschistfacies metamorphic rocks were analyzed with the electron microprobeand studied by single-crystal X-ray diffraction techniques.Except for ranges in the ratios Mg/(Mg+Fe) and Fe3+/(Fe3++Al+Ti),the sodic amphiboles are similar in chemical composition. Theamount of calcium in the M(4)-site ranges only from 0·18to 0·21 ion per formula unit. The calcic amphiboles,in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio(0·290·48). Three of the calcic amphibolescontain less than 1·5 calcium ions per formula unit,indicating a significant solid solution of sodic amphibole componentsin the calcic amphibole phase. The a and b unit-cell parametersof the calcic amphiboles decrease with increased content ofthe sodic component. |
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