Shonkinite

Photographed by Michael P. Klimetz

Clark Mountains

San Bernardino County

CALIFORNIA

Photographed by Michael P. Klimetz

Clark Mountains

San Bernardino County

CALIFORNIA

Photographed by Michael P. Klimetz

Clark Mountains

San Bernardino County

CALIFORNIA

Shonkinite is a mafic foidal (feldspathoid-bearing) syenite. It is an intrusive, holocrystalline rock which, in the restricted sense, is composed of potassic feldspar in the form of sanidine accompanied by nepheline, augite, biotite, and olivine. The term shonkinite is also used for mafic nepheline syenite with aegerine-augite and plagioclase ranging from andesine to labradorite. The type locality of shonkinite is Shonkin Sag in the Highwood Mountains of north-central Montana which is associated with a laccolith. The Adel Mountains Volcanic Field in north-central Montana has a significant and abundant exposure of shonkinite. Additional exposures can be found in Ontario Province in Canada, and on the southwest Pacific Ocean island of Timor.