Bytownite is a calcium rich member of the plagioclase series of feldspar with composition between anorthite. It is usually defined as having between 70 and 90%An (formula: (Ca0.7-0.9,Na0.3-0.1)). 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.