Low-temperature plagioclase feldspars in the peristerite range (2-17 wt. % anorthite) are structurally inhomogeneous mixtures of An (sub 3+ or -2) and An (sub 25+ or -5) in varying proportions. Dark-field and phase-contrast microscopy by transmitted light are used to examine this peristerite unmixing, and these techniques are described in considerable detail to emphasize their present and potential applications in mineralogic investigations. It is determined that the An 25 phase unmixed from its An 3 host may take the form of ellipsoidal blebs less than 0.5 mu in dimension. Frequent occurrence of this exsolved material along twin boundaries suggests that there may be a cause-and-effect relationship of unmixing to twinning. Schiller as a reflection and scattering phenomenon is discussed.

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