In the solid-solution series high albite (analbite)-high sanidine, it was impossible to select a set of specimens that were free from minor constituents (anorthite, celsian, Sr- and Rb-feldspars, and Fe), totally free of evidences of exsolution, and all of precisely the same structural state. So data were gathered for 32 feldspars whose range of structural states was 0.5 ≤ Σt1 ≤ 0.65 (average 0.60), where Σt1 = 2t1 for monoclinic and Σt1 = (t1o + t1m) for triclinic feldspars, and t is the Al content of the T1, T1,O, and T1m sites. Only specimens determined to be at least 95% homogeneous were used, and corrections to the refractive indices were made for nonbinary constituents. The resulting refractive index curves–not labeled conventionally as nαnβ, and nγ, according to magnitude, but as na, nb, and nc, according to which crystallographic axis the vibration direction is most nearly parallel– were found to change slope at Or∼60. Each was adequately represented by two linear segments with different slopes, meeting at Or60. The segments and inflection point correspond very closely to those representing the plot of density versus mole percent Or. As in the case of low microcline-high sanidine, the 2VX versus mole percent Or curves are sigmoidal because the nb and nc curves cross; 2VX = 0° at Or∼73 for ≤ Σt1 = 0.60. The composition for this crossover point decreases as ≤ Σt1 decreases.

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