Bytownite

Bytownite is a plagioclase feldspar mineral, a calcium-rich member of the plagioclase solid solution series. It lies between labradorite and anorthite in composition, with higher calcium and lower sodium relative to more sodium-rich varieties. The anorthite content is typically appreciable, commonly described as roughly 50–70% anorthite (An). Its general formula is (Ca,Na)Al1-2Si3-2O8, reflecting ongoing substitution between Ca and Na in the crystal structure.

Bytownite forms in igneous rocks of granitic to dioritic composition and can occur in metamorphic rocks that have undergone granulitic conditions. In plutonic rocks, it may appear as euhedral crystals and as constituents of coarse-grained granitoids; in rocks of calc-alkaline and high-temperature facies it can occur as well as accessory grains.

Physically, bytownite is typically colorless to white, gray, or pale; it has a vitreous to pearly luster and a Mohs hardness of about 6–6.5. Like other plagioclases, it exhibits two directions of cleavage at about 90 degrees and often shows characteristic polysynthetic twinning and striations on cleavage surfaces. Specific gravity is approximately 2.65–2.75.

Bytownite was named after Bytown, the former name of Ottawa, Ontario, Canada, where the mineral was first described.