Compositional characteristics and equilibrium relations in mineral assemblages of a metamorphosed iron formation [Quebec]

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ID: 293148
1960
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Abstract
The Fe formation is located in E.-central Quebec and probably is a regionally metamorphosed equivalent of the Fe-bearing sediments of the Labrador trough. Mineral assemblages are composed of select combinations of the following: quartz, talc, actinolite, Ca-pyroxene, cummingtonite, magnetite, hematite, calcite, and dolomite. Seventy-six spectrographic analyses of minerals give the following average concentration ranges for minor constituents: Al ~2~ O ~3~ , MnO, and Na ~2~ O \< 0.1%; K ~2~ O \< 0.1%; TiO ~2~ , V ~2~ O ~3~ , Cr ~2~ O ~3~ , NiO, CuO, and BaO \< 0.01%. ZrO ~2~ and SrO were not detected. Therefore the rocks are essentially members of the system CaO-MgO-FeO-O-SiO ~2~ -H ~2~ O-CO ~2~ . Close approach to chemical equilibrium during metamorphism is indicated by 1) orderly distribution of Mg, Fe, and Mn among coexisting actinolite, Ca-pyroxene, and cummingtonite; 2) restrictions on the numbers and types of minerals in association with each other; and 3) correlation of the compositions of silicates with the presence and absence of hematite, i.e., Mg/(Mg+Fe) is increased and its range restricted when hematite is present. Distributions of elements among coexisting minerals may be interpreted in terms of thermodynamic theory and in some cases may be represented by functions derived on the assumption of ideal solid solutions. Observed mineral assemblages and the compositional relations are in general consistent with phase rule considerations and may be elucidated by functions based on the coupling of chemical reactions between minerals.
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Authors Robert F. Mueller
Journal american journal of science
Year 1960
DOI
10.2475/ajs.258.7.449
URL
https://ajs.scholasticahq.com/article/58820.pdf
Keywords Keywords not found

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