File:Shonkinite from Mountain Pass in California.jpg

English: Shonkinite (Mountain Pass Alkaline Suite, Mesoproterozoic, 1.40-1.41 Ga; Mountain Pass Rare Earth Element District, California, USA) 3

Shonkinite from the Precambrian of California, USA. (7.4 centimeters across at its widest)

This is a rare intrusive igneous rock called shonkinite from Mountain Pass, California. Mountain Pass shonkinites are ultrapotassic, porphyritic phlogopite melanosyenites (a.k.a. melasyenites - "melano-" or "mela-" refers to a dark-colored variety of syenite).

Syenite has a chemical composition between granite's felsic chemistry and gabbro's mafic chemistry. Syenite is an example of an intermediate igneous rock, which has 52 to 65% silica (= SiO2 chemistry) (intermediate has also been defined as 55 to 65% silica). Intermediate igneous rocks are sometimes light-colored, sometimes dark-colored, and sometimes have medium colors. Dark-colored syenite is called "melanosyenite".

The mineral content of syenite is dominated by feldspar with little to no quartz. Syenite is alkali feldspar-rich, with little to no plagioclase feldspar. The minor plagioclase feldspar component is often intergrown with the alkali feldspar to form perthite. Mafic minerals may be present - amphibole, pyroxene, and biotite mica.

Mountain Pass shonkinite is dominated by three minerals:

1) pinkish- to purplish-colored microperthitic microcline feldspar

2) mica that ranges from barium-rich & titanium-rich fluor-phlogopite to biotite

3) augite pyroxene or aegirine pyroxene

Geologic unit: Mountain Pass Alkaline Suite, early Mesoproterozoic, 1.40 to 1.41 Ga

Locality: outcrop ~2 air miles southeast of the Mountain Pass Mine, Mountain Pass Rare Earth Element District, Ivanpah Mountains, northeastern San Bernardino County, southeastern California, USA (vicinity of 35° 27' 25.78" North latitude, 115° 30' 51.51" West longitude)