Fifteen plagioclase concentrates with a range of An60 to An80 from the lower part of the Eastern Bushveld Complex have been analyzed by microprobe techniques and X-ray powder diffractometer studies to establish a revised determinative curve of composition versus the X-ray parameter г(г = 2θ(131)+2θ(220) + 4θ(131) for CuKα radiation) for the ordered- or low-structural state plagioclases. The present curve is shifted to a lower Ancontent and differs from that of Smith and Gay (1958) by 5 percent An at An60 and 2 percent An at An80. Heating of the analyzed samples at 975°C for periods of time ranging from 6½ to 36½ days significantly disordered most of the specimens.

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