The compositions of seven pairs of coexisting hornblende and grunerite and five assemblages of coexisting hornblende, grunerite, and garnet from Archean silicate iron formation of low and medium metamorphic grade have been obtained by electron microprobe analysis. Important factors affecting the composition of the amphiboles are: (1) the Mg/Fe ratio of the iron-formation beds, which controls the gross Mg/Fe ratio of the amphiboles; (2) the alumina content of the beds, which affects the degree of alumina substitution in hornblende thereby altering the distribution of Mg and Fe in the coexisting amphiboles; and (3) the occurrence of iron-rich garnet, which produces higher Mg/Fe ratios in both amphiboles. A fourth potentially important factor, the oxygen fugacity, cannot be satisfactorily assessed with these data, but has not obscured the effects of the other three. Temperature of crystallization of the amphiboles was an important factor mainly insofar as it affected the crystallization of garnet in the alumina-rich rocks.Four coexisting hornblende–cummingtonite pairs from metatuffs show similar control of Mg–Fe fractionation by alumina substitution in hornblende.

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