Low-temperature plagioclase feldspars in the peristerite range An5±2 to An17±2 unmix on a submicroscopic scale to relieve internal stresses created by substituting A1 for Si in the tetrahedral framework. The unmixed components are An0-6 and An20-35, averaging An3 and An25 respectively. Most peristerites exhibit a bluish schiller as a result of diffusion of light by the submicroscopic domains of different optical polarizabilities. The optical properties of the crystal are weighted averages of those of the domains. Heating at 1000° C. causes disordering of Na and Ca between domains, followed by disordering of Si and A1 within domains, and final homogenization by disordering of Si and A1 between domains. Schiller is retained until the final step of homogenization. Homogenization can not be studied by simple x-ray methods, owing to similarity in unit cell geometry of the high-temperature phases, but can be followed by optic angle measurements. An x-ray technique has been developed to determine the composition of individual unmixed grains, and has been applied to confirm the presence of a break or flexure in the plagioclase refractive index curves near An5-An7, the lower limit of the peristerite region. The upper limit of the peristerite region has not been studied in detail, but deviations from the albite structure sufficient to incorporate the additional Al-content are inferred.