colemanite - hydrous calcium borate from the Terry borate deposit, Inyo County, California - teaching hand specimen
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Colemanite is an hydrous calcium borate, often interstratified with lakebed deposits of Tertiary age. It occurs in over 50 forms. At the Terry borate deposit the colemanite occurs as spherical aggregates of radiating acicular (needle-shaped) to bladed crystals. The individual spherical aggregates tend to interlock into poorly packed masses, formed during the alteration of ulexite or inyoite into colemanite. The transformation of primary borates to colemanite is a process that occurs with burial at slightly elevated temperatures. This mineral is an important component of sedimentary borate deposits worldwide.
Colemanite is pyroelectric, gaining an electrical charge during changes in temperature. It weakly fluoresces pale white under shortwave ultraviolet. It produces the green flame of boron when held in the flame of a Bunsen burner or a propane torch. Because of the water in the crystals, it pops and crackles a lot, so use eye protection, long sleeves and gloves. If you are going to use this in a classroom, test it yourself before you do. The green flame, in a dark room, is impressive.
The Terry borate deposit is 7 miles west of Death Valley Junction in the Amargosa borate area. A surface outcrop was discovered in 1972. Drilling showed an ore body at a depth of less than 60 feet. Because it was close to the surface and geologically simple, though in a small deposit estimated at 9,000 tons, it was economical to mine. Tenneco Oil Co. began production in an open pit in December 1974. Four months later the deposit was mined out.
Colemanite’s name honors William Tell Coleman, owner of the Harmony Borax Works in Death Valley, where it was first encountered in 1882. Its use is in the manufacture of borax. When finely powdered and boiled in a sodium carbonate solution, it produces a precipitate of calcium carbonate. The remaining mother liquor is rich in sodium metaborate and is converted into borax by having a stream of carbon dioxide passed through it.
We will pack some small chips for flame tests with this specimen. If you are a teacher and doing classroom demonstrations, let us know and we'll send extras.
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