Hydrous calcium borate (Ca2B6O115H2O)
Color: Colorless, milky white,
yellowish white, gray or muddy
Luster: Vitreous to adamantine,
Specific Gravity: 2.42
Cleavage: Perfect, one direction,
distinct in another
Habit: Crystals are equant and short
prismatic. Aggregates are massive,
cleavable, coarse to fine granular,
rounded aggregates, and as geodes.
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.
in Which this Mineral is Found
Specimen Description: Colemanite, 12.9
cm wide, Boron, Kern County. California
State Mining and Mineral Museum.
Photographer: Jeff Scovil.