Chemical analyses for the following amphibole pairs are presented: anthophyllite—tremolite (or actinolite, or hornblende), cummingtonite (or grunerite)–actinolite (or hornblende), cummingtonite (or grunerite)–anthophyllite (or gedrite), and manganoan cummingtonitemagnesioriebeckite.

Nineteen analyses of such pairs are quoted from the literature, and thirty-seven additional pairs have been newly analyzed by electron probe techniques. Quantitative microprobe determinations of Si, Al, Fe, Mn, Mg, Ca, and Na were made on polished thin-sections, using naturally occurring, analyzed, homogeneous amphiboles as standards. The literature analyses and the electron probe analyses for metamorphic, two-amphibole assemblages are given for amphiboles in physical contact, which show no textural evidence of one amphibole being a reaction or alteration product of the other. The chemical data for some of the volcanic, two—amphibole assemblages were obtained from occurrences that probably do not represent equilibrium pairs.

The chemical data are used to determine the extent of the miscibility gaps between the various amphibole series and the fractionation of the major elements between the two amphiboles of a pair. Anthophyllite and members of the cummingtonite-grunerite series generally have a larger Fe(total)/Mg ratio than the coexisting calcic amphibole. The maximum CaO, Al₂O₃and Na₂O contents of cummingtonite in metamorphic cummingtonite—hornblende pairs are 1·9 and 3·2, 0·2 weight per cent, respectively. The maximum CaO, A1₂O₃, and Na₂O contents of cummingtonite in metamorphic cummingtonite-hornblende pairs are 1·9, 3·2, and 0·2 weight per cent, respectively. Larger CaO and Al₂O₃ values reported in the literature were found to be too high because of admixture of actinolite or hornblende in the analyzed separates. Small amounts of MnO tend to concentrate preferentially in anthophyllite or cummingtonite of anthophyllite-hornblende and cummingtonite-hornblende pairs. Anthophyllite-cummingtonite pairs may show very similar Fe(total)/Mg ratios and differ slightly in Al₂O₃ content only.