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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.[1] 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.[2]

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.[3][4][5][6]


  1. ^ 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.
  2. ^ Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.
  3. ^ Merrill-Maker, Andrea. Montana Almanac. 2d ed. Guilford, Conn.: Globe Pequot, 2005. ISBN 0-7627-3655-0
  4. ^ 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)
  5. ^ 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)
  6. ^ Hyndman, Donald W., and Alt, David. "Radial Dikes, Laccoliths and Gelatin Models." Journal of Geology. (1987)

Further readingEdit

  • Heinrich, E.W.. Microscopic Petrography. New York: McGraw-Hill, 1956.