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. 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.
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.
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