An intergrowth of calcic labradorite in a plagioclase-quartz-biotite gneiss from Broken Hill,

SUMMARY. Compositions of An61 and An69. 5 have been determined for two domains in plagioclase from a gneiss. Optical properties indicate that he same compositions apply respectively foran intergrowth of'matrix ' and lamellae found in zones between the domains. The intergrowth appears tospan the composition range between the Beggild and Huttenlocher intergrowths. THE plagioclase feldspar intergrowth described in this paper was obtained from a plagioclase-quartz-biotite gneiss containing minor amounts of garnet, muscovite, and opaque minerals. This rock belongs to a series ofgneisses outcropping as elongate layers on both the eastern and western sides of the Broken Hill orebody in New South Wales) Descriptions of the intergrowth. Under relatively low magnification the plagioclase shows an irregular, patchy, almost undulatory extinction with no regular zonation (fig. I). Near extinction, in sections normal to I-Ioo], two domains with slightly different birefringence are clearly indicated by an extinction angle difference of 6 ~ The light-grey areas in fig. I are the more calcic and have an extinction angle of 41 ~ and the darker areas are more sodic with an extinction angle of 35 '~'. Under higher power an irregularly distributed, extremely fine system of slightly undulator