The xray powder diffraction pattern of synthetic anorthite crystallised from a CaO-Al2O3-SiO2 glass having an anorthite (1:1:2) molar ratio is identical with that reported in the literature and also with that of a natural Japanese anorthite specimen. Increasing the CaO or SiO2 content of the parent glasses used for crystallisation studies has no measurable effect on that portion of the powder pattern attributable to anorthite. However, glasses containing an excess of Al2O3 ranging from 5 to 10mol% gave after crystallisation at temperatures below c.1150oC, a distinctively different powder pattern. Several powder reflectios that arw normally strong such as 220,112 etc. were found to be virtually absent. Moreover, the chemical excess od Al2O3 did not
