The stability and natural occurrence of margarite

MARGARITE is a calcic dioctahedral mica that has been known since the mid nineteenth century but rarely found as a mineral of petrographic interest. The most widely developed occurrences are in regional metamorphic rocks of unusual composi t ion-bauxite carbonate sediments that have been metamorphosed to varying degrees (Onay, I949) and in contact metamorphism of similar rocks (Williams, 1888). Other types of more isolated occurrences have been reported in association with ore deposits (Aoki and Shimoda, i965) in metamorphosed anorthositic rocks (Bryhni, I966 ) or in vein deposits or metamorphosed vein deposits (Clarke and Chatard, 1884; Niggli, 1955). Identifications in gneissic or pelito-gneissic rocks have been frequently mistaken and can be considered only with great doubt (Niggli, 1955). In sum, it can be remarked that margarite is rare, highly localized in occurrence, and thus probably represents special compositional or physical conditions of formation. Accepting this brief sketch as a valid summary of the geological importance of margarite, the question can be asked why does it not occur more frequently; is the occurrence of margarite restricted by chemical or physical variables that are rarely realized in nature ? The following experimental investigation was designed to answer these questions.