English: Chromite band in serpentinite in the Precambrian of Montana, USA.

This rock is at an old chromite mine in southern Montana's Red Lodge Mining District, in the Beartooth Mountains. Active mining took place during the 1940s. Most of the old, small mines occur on the high plateaus or just below the eroded plateau edges.

Mines in the Red Lodge District targeted chromium ores in the form of black-colored chromitite rocks hosted in serpentinite and bastite (= serpentinized pyroxene). In this area, lenses, or pods, or fault-chopped stratiform bodies of serpentinite occur among high-grade metamorphic rocks such as quartzites, gneisses, migmatites, amphibolites, etc.

The rocks in the Beartooth Mountains are very old. They are principally Archean in age (2.5+ billion years old). The chromite-bearing serpentinite bodies of the Red Lodge District were originally chromite-bearing peridotites (dunites) of early Neoarchean age, or older. Granulite-grade to upper amphibolite-grade regional metamorphism at about 2.75 Ga altered the original dunites to serpentinites. The rocks of the area were also altered by greenschist-grade metamorphism between 1.6 to 1.8 billion years ago, during the late Paleoproterozoic.

The greenish portions of the rock are serpentinite, a low- to high-grade metamorphic rock dominated by one or more serpentine-group minerals (Mg3Si2O5(OH)4 - magnesium hydroxy-silicate). Serpentinites range in color from light to dark green to black (sometimes whitish or bluish). They usually have a waxy feel and luster. Serpentinite forms by metamorphism of peridotite in the presence of water. Peridotite is an intrusive igneous rock rich in olivine ((Mg,Fe)2SiO4 - magnesium iron silicate). The dark band is chromitite, a coarsely-crystalline textured rock rich in the mineral chromite (FeCr2O4 - iron chromium oxide).

Locality: old chromite mine, near the eroded edge of the Beartooth Plateau, southeastern side of Rock Creek Canyon, southwest of the town of Red Lodge, Red Lodge Chromite District (Red Lodge Mining District), Beartooth Mountains, southern Montana, USA (45° 02' 04.36" North latitude, 109° 24' 35.11" West longitude)