Exsolution lamellae and optic orientation of clinoamphiboles

Exsolution lamellae are abundant in coexisting hornblende and cummingtonite, and in hornblende coexisting with anthophyllite in Ordovician volcanics metamorphosed in the kyanite and sillimanite zones in central Massachusetts and adjacent New Hampshire. The lamellae have the same orientation relative to the internal structure as the (100) and (001) exsolution lamellae in clinopyroxenes, but are indexed (100) and (101) with the C2/m space group commonly chosen for amphiboles. Specimens from the kyanite zone contain very thin (100) and (101) lamellae. In the sillimanite zone, both (100) and (101) lamellae are thicker and more abundant in iron-rich specimens than they are in magnesian specimens, as might be expected by analogy with pyroxenes from layered mafic intrusions. The (101 lamellae allow correct determination of the relations between the optic vibration directions and the crystallographic axes for two alternatively selected space group C2/m and I2/m. This evidence shows that there has been much confusion concerning these relations.