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A blue to dark-gray variety of plagioclase feldspar that contains a higher proportion of calcium than sodium and occurs in alkaline igneous rocks.

[After Bytown, former name of Ottawa, Ontario, near where a sample of the mineral was found.]


a rare plagioclase found in alkaline igneous rocks
References in periodicals archive ?
The main minerals in the pillow-lava are olivine (varies between chrysolite to hyalosiderite), Ti-augite, plagioclase (variable from bytownite to oligoclase) and minoritary biotite, Ti-hornblende, apatite and Fe-Ti oxides (Puga et al.
Quartz, graphite, pyrite, grossular, diopside, bytownite, dolomite and calcite are the most common associations.
The specific surface area in all size fractions increased in order: quartz (no cleavage), olivine (less regular cleavage {001}), albite < oligoclase, bytownite (cleavage of feldspars {00} and {010}) < hornblende, diopside (stark crossing cleavage {110}).