Coexisting Actinolite and Hornblende from West-Central New Hampshire

John B. Brady, Harvard University

Archived as published with permission.


The coexistence of actinolite and hornblende and of cummingtonite and hornblende has been observed in garnet-zone and staurolite-kyanite-zone amphibolites of the Ammonoosuc Volcanics near Hanover, New Hampshire. The amphibole pairs occur both as separate grains and as intergrowths with boundaries that are optically and chemically sharp, but geometrically irregular; no exsolution lamellae were observed.Electron microprobe analyses of the amphibole pairs show that: (1) hornblende is richer in Al, Na, K, Ti, and Fea+,and has a lower Mg/(Me f Fe) ratio than the coexisting actinolite; (2) hornblende is enriched in Al, Na, and Ca, but has a similar Mel(Mg f Fe) ratio to that of the coexisting cummingtonite. Recalculations show that the I site is nearly empty in actinolites and typically one-third to one-half full in coexisting hornblendes. Single-crystal precession photos of actinolite-hornblende crystals show that the lattices of the two phases share a common b-axis, but are rotated relative to one another by two to three degrees.