1911 Encyclopædia Britannica/Colemanite

From Wikisource
Jump to: navigation, search
[ 665 ]

COLEMANITE, a hydrous calcium borate, Ca2B6O11+5H2O, found in California as brilliant monoclinic crystals. It contains 50.9% of boron trioxide, and is an important source of commercial borates and boracic acid. Beautifully developed crystals, up to 2 or 3 in. in length, encrust cavities in compact, white colemanite; they are colourless and transparent, and the brilliant lustre of their faces is vitreous to adamantine in character. There is a perfect cleavage parallel to the plane of symmetry of the crystals. Hardness 4-412; specific gravity 2.42. The mineral was first discovered in 1882 in Death Valley, Inyo county, California, and in the following year it was found in greater abundance near Daggett in San Bernardino county, forming with other borates and borosilicates a bed in sedimentary strata of sandstones and clays; in more recent years very large masses have been found and worked in these localities, and also in Los Angeles county (see Special Report, 1905, of U.S. Census Bureau on Mines and Quarries; and Mineral Resources of the U.S., 1907).

Priceite and pandermite are hydrous calcium borates with very nearly the same composition as colemanite, and they may really be only impure forms of this species. They are massive white minerals, the former friable and chalk-like, and the latter firm and compact in texture. Priceite occurs near Chetco in Curry county, Oregon, where it forms layers between a bed of slate and one of tough blue steatite; embedded in the steatite are rounded masses of priceite varying in size from that of a pea to masses weighing 200 lb. Pandermite comes from Asia Minor, and is shipped from the port of Panderma on the Sea of Marmora: it occurs as large nodules, up to a ton in weight, beneath a thick bed of gypsum.

Another borate of commercial importance found abundantly in the California deposits is ulexite, also known as boronatrocalcite or "cotton-ball," a hydrous calcium and sodium borate, CaNaB5O9+8H2O, which forms rounded masses consisting of a loose aggregate of fine fibres. It is the principal species in the borate deposits in the Atacama region of South America. (L. J. S.)