Summary: The angular separations (2ϑ (131)+2ϑ (220)−4ϑ (1¯31)) and (2ϑ (1¯11)−2ϑ (¯201)) for 111 analysed plagioclase felspars have been determined from diffractometer records. From An ₀ to An₇₀ the petrological environment has a strong influence on these separations, and if the An-content is independently known an accurate estimate of the structural state can he made from the first of these angular separations. From An₇₀ to An₁₀₀ there are only small variations in both separations with An-content and structural state. From An₈₅ to An₁₀₀ inconsistencies in the data suggest that other factors can influence any interpretation so seriously that only qualitative or semi-quantitative deductions may be made. Consideration of the angular separations and subsidiary reflections of sodie plagioclases from a wide range of geological environments has led to the novel hypothesis that in this composition region even plutonic specimens are in a metastable state with slightly disordered structures.

The value to the petrographer of the available methods for the estimation of An- content and structural state is critically discussed. Although it is thought that the standard optical curves in present use are in need of some revision, the determination of optical orientation by universal-stage methods is probably adequate for routine studies. For more detailed investigations, this may be supplemented by X-ray powder data and refractive index measurements, whilst a complete description of the felspar requires the addition both of chemical analysis and single-crystal X-ray work. In particular, the efficacy of the various X-ray methods that have been proposed is discussed, and recommendations of the most suitable method for various composition ranges are made.