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Bostonite

1911 Encyclopedia Britannica

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in petrology, a fine-grained, pale-coloured, grey or pinkish rock, which consists essentially of alkali-felspar (orthoclase, microperthite, &c.). Some of them contain a small amount of interstitial quartz (quartz-bostonites); others have a small percentage of lime, which occasions the presence of a plagioclase felspar (maenite, gauteite, lime-bostonite). Other minerals, except apatite, zircon and magnetite, are typically absent. They have very much the same composition as the trachytes; and many rocks of this series have been grouped with these or with the orthophyres. Typically they occur as dikes or as thin sills, often in association with nepheline-syenite; and they seem to bear a complementary relationship to certain types of lamprophyre, such as camptonite and monchiquite. Though nowhere very common they have a wide distribution, being known from Scotland, Wales, Massachusetts, Montreal, Portugal, Bohemia, &c. The lindoites and quartz-lindoites of Norway are closely allied to the bostonites.