Colemanite - Encyclopedia

    Class : Carbonates, Nitrates, Borates
    Subclass : Borates
    Crystal System : Monoclinic
    Chemistry : Ca2B6O11 H2O
    Rarity : Fairly rare


Colemanite is like the vast majority of borates, a mineral formed by evaporation from salt lakes and lagoons in a desert environment. The waters of these lakes contain large amounts of borated brines due to volcanic vapors. It owes its name to William Tell Coleman, founder of the Californian boron industry and owner of the mine where it was discovered (Furnace Creek, California). In its salt lake deposits, colemanite is associated with many salts : borates (ulexite, borax, kernite...), halite, glauberite, thenardite, gypsum... More rarely, it is present in hot spring deposits (Chile). It appears to originate from the hydration of primary borates (borax and ulexite). The crystals occupy large dissolution cavities in these primary borates, which allows them to develop various automorphic facies : short prismatic to isometric (pseudo-octahedral), or lanceolate with sharp edges (pseudorhombohedral). Colemanite also occurs in compact masses, sometimes in spherolitic aggregates. It has a vivid vitreous to adamantine luster, a high density for a borate (2.4) ; it is colorless to milky white, sometimes yellowish-white, gray or brown. In the past, it was the world's main source of boron before being replaced by kernite.

Colemanite from Boron, California USA - © Rock Currier
Colemanite from Furnace Creek Mining District, California, USA
© Rock Currier
Colemanite from Boron, California, USA - © Rob Lavinsky
Colemanite from Mustafakemalpascha Mine, Western Anatolia, Turkey

Colemanite in the World

Magnificent groups of decimetric crystals have been collected in the Californian borate deposits (Valley of Death and Boron), and centimetric in the Turkish deposits of Kirka and Mustafakemalpascha (Anatolia). At Boron, the yellowish color of the superb crystals is due to inclusions of orpiment. The Argentine "salines" (the "salars"), great low evaporitic plains, of Jujuy also sporadically deliver some good specimens.

Colemanite in France

This mineral is not reported in the French underground.

Twinning

No twin report for this mineral species.

Fakes and treatments

No fake reported for this species.



Hardness : 4.5
Density : 2,42
Fracture : Irregular to sub-conchoidal
Trace : White




TP : Translucent to transparent
RI : 1.586 to 1.614
Birefringence : 0.028
Optical character : Biaxial +
Pleochroism : Not visible
Fluorescence : Blue, green, yellow


Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None

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