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. 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 ==