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shonkinite
Shonkinite is a mafic foidal (feldspathoid bearing) syenite, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar in the form of sanidine, nepheline, augite, biotite, and olivine.〔(Weed, Walter H. and Pirsson, Louis V. ''Geology of the Little Belt mountains, Montana, With Note on the Mineral Deposits of the Neihart, Barker, Yogo, and Other Districts.'' Washington, D.C.: Government Printing Office, 1900 ), p. 319.〕 Shonkinite is also used for mafic nepheline syenite with aegerine-augite as the pyroxene, and with the addition of plagioclase (andesine to labradorite). Nepheline in shonkinite from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana, from which the rock gets its name, is largely altered to natrolite and stilbite. The Shonkin Sag laccolith is a significant source of shonkinite.〔Iddings, Joseph Paxon. ''Igneous Rocks: Composition, Texture and Classification, Description and Occurrence.'' New York: J. Wiley & Sons, 1909, p. 402.〕
The Adel Mountains Volcanic Field in north-central Montana is a significant and abundant source of shonkinite, which is found primarily in Montana in the United States, Ontario in Canada, and the southwest Pacific Ocean island of Timor.〔Merrill-Maker, Andrea. ''Montana Almanac.'' 2d ed. Guilford, Conn.: Globe Pequot, 2005. ISBN 0-7627-3655-0〕〔 Gunderson, Jay A. and Sheriff, Steven D. "A New Late Cretaceous Paleomagnetic Pole from the Adel Mountains, West Central Montana." ''Journal of Geophysical Research.'' 96:B1 (January 10, 1991)〕〔Roberts, Eric M. and Hendrix, Marc S. "Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics." ''Palaios.'' 15:5 (October 2000)〕〔 Hyndman, Donald W., and Alt, David. "Radial Dikes, Laccoliths and Gelatin Models." ''Journal of Geology.'' (1987)〕
==References==
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