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
igneous rock. More specifically, it is a
mafic foidal (
feldspathoid bearing)
syenite, a
holocrystalline (completely crystalline)
intrusive rock which, , is composed of potassic
feldspar (in the form of
sanidine), with
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 is largely altered to
natrolite and
stilbite.
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.
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.
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Occurrence
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The Adel Mountains Volcanic Field in north-central Montana
[Merrill-Maker, Andrea. Montana Almanac. 2d ed. Guilford, Conn.: Globe Pequot, 2005. ]
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Mountain Pass, California.
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India in the Salem Block in the Southern Granulite Terrane
and the Elchuru alkaline complex in India
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Ontario, Canada
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Timor, Indonesia
Etymology
The rock gets its name from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana.[Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.]