Distribution of Mg and Fe in cummingtonite-hornblende and cummingtonite-actinolite pairs from metamorphic assemblages

The atomic fractions Mg/(Mg + Fe) and the Mg-Fe distribution coefficient $$K_{{\text{D}}{\text{.Mg - Fe}}}^{{\text{Ca - am - Cum}}} \left( { = \tfrac{{{\text{Mg/Fe]}}_{{\text{Ca - am}}} }}{{{\text{Mg/Fe]}}_{{\text{Cum}}} }}} \right)$$ are calculated for 31 metamorphic cummingtonite-hornblende pairs. Data on 21 pairs are taken from the litterature, and new electron microprobe analyses and structural formulae are presented of nine pairs from Tydal, Sör-Tröndelag, Norway, and of one pair from Cooma, N.S.W., Australia (cf. Kisch, 1969). The electron microprobe methods used are described, particularly the use of mineral standards, and the variation of the mass absorption in substitution series. The hornblendes from the Tydal pairs are markedly pargasitic in composition, and contain minor proportions of the cummingtonite “molecule”. The Mg-Fe distributions in the cummingtonite-hornblende pairs — as plotted on a Mg/(Mg + Fe)]_Ca-am vs. Mg/(Mg + Fe)]_Cum diagram (Fig. 3) — differ significantly from the Mg-Fe distribution curve for cummingtonite-actinolite pairs from Quebec (Mueller, 1961). Whereas the actinolites have markedly higher Mg/Fe ratios than the co-existing cummingtonites (K _D.Mg-Fe ^Ca-am-Cum ≈ 1.5–2.0), the cummingtonite-hornblende pairs diverge towards lower values from the distribution coefficient. In most of the metamorphic cummingtonite-hornblende pairs — including the nine pairs from Tydal — the Mg/Fe ratio of the hornblende is lower than in the co-existing cummingtonite, i.e K _D.Mg-Fe ^Ca-am-Cum <1. A relation appears to exist between the Mg-Fe distribution and the Al content of the calcic amphibole phase. This is believed to be due to the non-random distribution of Al^Y among the octahedral lattice sites: in hornblende Al^VI enters the M ₁+M₃ positions, in which Mg is preferred over Fe, rather than M ₂, in which Fe is preferred (Ghose, 1965). Since the cummingtonites remain Al-poor, the over-all Mg/Fe ratio in the hornblende is reduced relative to the co-existing cummingtonite as a result. The variations of the Mg-Fe distribution in the cummingtonite-hornblende pairs can also be related directly to the presence and composition of the plagioclase and other Al-rich phases in the metamorphic mineral assemblage. In any range of Mg/Fe ratios, the cummingtonite-hornblende pairs associated with oligoclase have lower distribution coefficients (0.61–0.81; 12 pairs) than those associated with calcic plagioclase or plagioclase-free assemblages (0.97 to 1.89; 6 pairs); the pairs associated with andesine have intermediate Mg-Fe distributions (0.74–1.15; 6 pairs).