An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Abstract

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 μm in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na 0.1 Ca 1.0 Mg 0.6 Fe 3+ 0.3 )(Si 1.8 Al 0.2 )O 6 for the pyroxene and Na 0.7 Ca 1.9 (Mg 2.1 Fe 2+ 1.4 Fe 3+ 0.5 Ti 0.1 Cr 0.1 Al 0.8 )(Si 5.9 Al 2.1 ) O 22 (OH) 2 for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.