Summary: The X-ray powder diffraction pattern of synthetic anorthite crystallized from a CaO-Al₂O₃-SiO₂ glass having the 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 crystallization 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 10 mol % gave, after crystallization at temperatures below c. 1150 °C distinctively different powder pattern. Several powder reflections that are normally strong, such as 2¯20, 11¯2, etc., were found to be virtually absent. Moreover, the chemical excess of Al₂O₃ did not appear as a separate alumina-bearing phase. Upon reheating these anorthites to temperatures above 1200–50 °C or upon crystallizing a fresh portion of alumina-rich glass above 1200-50 °C, only the normal anorthite powder X-ray pattern was obtained; the pattern also contained some reflections due to corundum (α-Al₂O₃). Examination of the anomalous low-temperature anorthite by electron diffraction shows that the apparent absence of some powder lines is caused by both albite and Carlsbad twinning, which occur on an intimate scale not exceeding a few unit cell repeats. From the unique manner of its occurrence, the twinning is believed to be associated with the inclusion of an excess of Al³⁺ in the anorthite.