Four different assemblages of iron-rich silicates from the silicate iron-formation of the Cape Smith belt in New Quebec have been investigated. These rocks all contain quartz, magnetite, some calcite, and are composed mostly of two or more of the following silicates: stilpnomelane, minnesotaite, iron chlorite, grunerite, almandite, and biotite.From eight chemical analyses and petrographic examination, using a method developed by J. B. Thompson, an isothermal phase diagram of the system Al2O3–Fe2O3–FeO–MgO–SiO2–H2O has been constructed for the upper part of the greenschist facies of metamorphism.Speculation based on this diagram and on available experimental data is offered on the sequence of reactions leading to the assemblages observed, and on the possibility of finding mineral assemblages representing invariant points in metamorphosed iron-formations.

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