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 .