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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 matrix has a poikiloophitic texture formed predominantly by plagioclase (labradorite, [An.sub.60-65], in some grains bytownite, [An.sub.70-75]) enclosed by elongated hypidiomorphic brownish augite crystals of 0.3-0.8 mm length and isometric grains of subordinated olivine and biotite; accessory phases are apatite and magnetite.
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., 1989; Morata et al., 1996).
Plagioclase phenocrysts (bytownite or labradorite; [An.sub.64-72]) have rims that are both similar to, and more sodic than, the calcic cores, with some solid solution variation within the cluster.
Quartz, graphite, pyrite, grossular, diopside, bytownite, dolomite and calcite are the most common associations.