See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals I: In Honor of Bernard W. Evans
Abstract:
Mgcumm forms large bladed crystals in the olivine-rich assemblages. It also forms acicular overgrowths on tremolite. Similar textural relations are noted in the talc chlorite schists. A fine-grained amphibole in some of the schists is seen to replace Mgcumm and this amphibole is thought to be Antho.
Mgcumm has a lower mg# than coexisting silicate phases. Mgcumm in olivine-rich assemblages has a mg# of 86-91, coexisting Tr varies from mg# 96-98. Amphibole compositions are more Fe-rich in the talc chlorite schists (Mg cumm mg# 81-87, Tr mg# 91-96). Calcium contents of the Mgcumm are low, less than 1 wt. %. Manganese contents of the Mgcumm are higher (0.2 to 0.4 wt. %) than MnO contents of Tr (< 0.1 wt. %). Microprobe compositions of Mgcumm and Antho(?) are indistinguishable.
The Mgcumm is a common phase in the Spruce Pine area, but it is not found in metaultramafic rocks to the south (higher grade) or north (lower grade). The author overlooked the coarse grained Mgcumm in his early (pre-probe work)studies of these rocks. Gentle coaching by Bernard Evans resulted in the eventual recognition of Mgcumm in these rocks.
See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals I: In Honor of Bernard W. Evans