What is "bytownite"

n. (context mineral English) A plagioclase feldspar, the fifth member of the Albite-Anorthite solid solution series. Bytownite is rare, and normally occurs in small grains in lime-rich igneous rocks. Clear pale yellow crystals have come from Mexico and Oregon.

Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals. It is usually defined as having "% An" between 70 and 90. Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning and associated fine striations.

The specific gravity of bytownite varies between 2.74 and 2.75. The refractive indices ranges are nα=1.563 – 1.572, nβ=1.568 – 1.578, and nγ=1.573 – 1.583. Precise determination of these two properties with chemical, X-ray diffraction, or petrographic analysis are required for identification.