The Early History of Tremolite

Tremolite, Ca2(Mg,Fe)5Si8O22(OH)2, is a calcic amphibole commonly found in greenschist-facies to amphibolite-facies calc-silicate and in low-grade ultrabasic rocks. In the new IMA nomenclature of amphiboles (Leake et al., 1997) tremolite forms part of the tremolite-actinolite-ferro-actinolite series: Whereas usually the dividing line between two species in a continuous solid solution series is set at the 50% point (Nickel and Grice, 1998), tremolite occupies only a narrow range. It is defined as having a Mg/(Mg+Fe 2+ ) ratio ≥ 0.9; actinolite has a ratio of 0.5-0.9 and ferro-actinolite has a ratio of less than 0.5. These limits are designed to satisfy the strong desire, expressed especially (but not solely) by metamorphic petrologists, to retain the distinction of green actinolite from colorless tremolite (Leake et al., 1997). Another reason is that tremolite is an important mineral in the definition of metamorphic zones (see below), and the diagnostic color distinction so useful in the field is entrenched in the literature; a change in the definition of the mineral would have complicated the application of important previously published studies.