Margarite pseudomorphs after corundum, Qôrqut area, Godthåbsfjord, West Greenland

Abstract

Margarite (end member CaAI2Si2AI2O1O) is reported as nodular aggregates of fine-grained material partly replacing corundum and as fine dispersed material replacing biotite in an inclusion of migmatitic plagioclase-rich gneiss within the Qôrqut granite. Muscovite replaces biotite in the same sample. A retrograde reaction in the presence of volatiles is suggested in which biotite + plagioclase + corundum go to an assemblage containing margarite + muscovite + epidote. Cell parameters and chemical data are presented. A maximum temperature for the reaction of - 5800 at 5 Kb is suggested.