Bytownite, a rock-forming mineral belonging to the plagioclase (q.v.) series of the felspars. The name was originally given (1835) by T. Thomson, to a greenish-white felspathic mineral found in a boulder near Bytown (now the city of Ottawa) in Ontario, but this material was later shown on microscopical examination to be a mixture. The name was afterwards applied by G. Tschermak to those plagioclase felspars which lie between labradorite and anorthite; and this has been generally adopted by petrologists. In chemical composition and in optical and other physical characters it is thus much nearer to the anorthite end of the series than to albite. Like labradorite and anorthite, it is a common constituent of basic igneous rocks, such as gabbro and basalt. Isolated crystals of bytownite bounded by well-defined faces are unknown.
(L. J. S.)