Shonkinite

It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite.

More specifically, it is a mafic foidal (feldspathoid bearing) syenite, a holocrystalline (completely crystalline) intrusive rock which, in the restricted sense[clarify], is composed of potassic feldspar (in the form of sanidine), with 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).

[2] In central Montana buttes of shonkinite are capped with white layers of syenite.

[2] The rock gets its name from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana.