Bytownit

Bytownite is a mineral in the plagioclase feldspar group, representing a calcium-rich end-member of the solid solution between anorthite and labradorite. As a plagioclase, it is a framework silicate with the general formula (Ca,Na)(Al,Si)4O8, and its composition trends toward higher calcium content than many commonly encountered plagioclases.

Name and locality: The mineral is named for Bytown, the historic name of Ottawa, Ontario, Canada, where the type material was first described. Bytownite is one of several plagioclase varieties used to illustrate the plagioclase solid-solution series in geology.

Occurrence and formation: Bytownite occurs in igneous rocks, especially granitic rocks, diorites, and pegmatites, as well as in some metamorphic rocks formed under high-temperature conditions. It is commonly associated with other feldspars, quartz, and mica.

Physical properties: Bytownite typically presents as colorless to white, gray, or pale brown crystals or masses, with a vitreous to pearly luster. It crystallizes in the triclinic system and shows two good cleavages at or near 90 degrees. Its hardness on the Mohs scale is about 6–6.5. Under cross-polarized light, it exhibits the polysynthetic twinning characteristic of plagioclase.

Use and significance: In geology, bytownite helps delimit plagioclase solid-solution trends and provides information about magmatic differentiation and crystallization histories. It is mainly of interest to researchers and mineral collectors and is not a major ore mineral.