Bostonite

Bostonite, in petrology, is a fine-grained, pale-colored, grey or pinkish intrusive rock, which consists essentially of alkali-feldspar (orthoclase, perthite, anorthoclase, and albite). Some samples may contain a small amount of interstitial quartz and others may have a small percentage of calcium present in a sodic plagioclase feldspar. Accessory minerals include apatite, zircon and magnetite, with rare biotite, hornblende or pyroxene. They have compositions very similar to the trachytes and are usually grouped with them.^[1] The texture common in these rocks include clusters of divergent or radiating irregular feldspar laths in a fine grained matrix.

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 dikes. Though nowhere very common they have a wide distribution with occurrences in Scotland, Wales, Massachusetts, Ontario, Portugal, Bohemia, and other places.^[1]

The term was widely used in the geologic literature of the late 19th and early 20th centuries, but is currently being discouraged in petrologic usage.

References

1 2 One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Bostonite". Encyclopædia Britannica. 4 (11th ed.). Cambridge University Press. p. 297.

Dictionary of Mining, Mineral, and Related Terms, U. S. Bureau of Mines, 1996
Williams, Howel; Turner, Francis J.; and Gilbert, Charles M.; 1954; Petrography; W. H. Freeman.

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[EB1911-1] 1 2 One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Bostonite". Encyclopædia Britannica. 4 (11th ed.). Cambridge University Press. p. 297.