Composition: Hydrous calcium borate (Ca2B6O115H2O)

Physical Properties:
Color: Colorless, milky white, yellowish white, gray or muddy
Streak: White
Luster: Vitreous to adamantine, brilliant
Hardness: 4.5
Tenacity: Brittle
Specific Gravity: 2.42
Cleavage: Perfect, one direction, distinct in another

System: Monoclinic
Habit: Crystals are equant and short prismatic. Aggregates are massive, cleavable, coarse to fine granular, rounded aggregates, and as geodes.

General Information:
Colemanite is similar in appearance to calcite, but the two can be differentiated by some simple tests. When a small fragment of colemanite is held with tweezers in a gas flame, the fragment decrepitates (tiny fragments are thrown off violently, with a crackling sound), and the flame is colored green. Also, colemanite does not effervesce in acids, as does calcite. Most of the colemanite specimen material for these sets is composed of aggregates of small crystals, and the flat, shiny surfaces are crystal faces rather than cleavage surfaces. Colemanite deposits originated by evaporation of boron-rich water in desert basins. Once crystallized, however, colemanite is not easily dissolved in water, so it is abundant in some of the very old dry-lake sediments of the Mojave Desert region. Before the discovery of borax deposits in California, colemanite was mined and converted into borax (sodium borate) by chemical processes. Now it is mined largely for use in ceramic glazes. Cluster of large colemanite crystals, the largest about 2 inches across. Named in 1883 after William T. Coleman, a founder of the California borax industry and owner of the mine where the mineral was first found.

California Counties in Which this Mineral is Found

Los Angeles
San Bernardino

Information on Specimen Photo:
Specimen Description: Colemanite, 12.9 cm wide, Boron, Kern County. California State Mining and Mineral Museum.
Photographer: Jeff Scovil.