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Shonkinite
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Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains and buttes in Montana that are remnants of laccoliths and stocks, such as the Highwood mountains.


Composition
Shonkinite is an intrusive . More specifically, it is a foidal ( bearing) , a (completely crystalline) which, , is composed of potassic (in the form of ), with , , , and . 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 - as the pyroxene, and with the addition of (andesine to labradorite). Nepheline in shonkinite from the is largely altered to and .

The close view of the rocks in the Adel mountains show large glossy crystals of augite in a dark grey matrix made up of small crystals of augite and feldspar. This is unusual as augite is usually dull.

(1986). 9780878422029


Formation
In central Montana buttes of shonkinite are capped with white layers of syenite. There appear round globes of syenite at the boundary which suggest that the less dense syenite float up to the top of molten shonkinite as the mass cooled.


Occurrence
  • The Adel Mountains Volcanic Field in north-central Merrill-Maker, Andrea. Montana Almanac. 2d ed. Guilford, Conn.: Globe Pequot, 2005.
  • Mountain Pass, California.
  • India in the Salem Block in the Southern Granulite Terrane and the alkaline complex in India
  • , Canada
  • , Indonesia


Etymology
The rock gets its name from the type locality at in the Highwood Mountains of north-central .Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.

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