Formula: NaCaB5O6(OH)6.5H2O
Crystal System: Triclinic
Specific gravity: 1.955 measured, 1.955 calculated
Hardness: 1 to 2½
Streak: White
Colour: Colourless, white
Luminescence: Fluorescent and some phosphorescence. Fluorescent yellow, greenish yellow, cream or white under UV (Mindat)
Solubility: Slightly decomposed in cold water and more so in hot water, with the loss of Na to the solution

Evaporite Deposits

Ulexite occurs typically as a secondary mineral in playa and salt-marsh deposits in arid regions, and in bedded sedimentary deposits formed from these, the boron being supplied from surrounding hot springs. Some occurrences contain upwards of one billion tons of ulexite. Associated minerals include colemanite, borax, meyerhofferite, hydroboracite, probertite, glauberite, trona, mirabilite, calcite, gypsum and halite (HOM).


At the Flat Bay Quarry, Newfoundland, Newfoundland and Labrador, Canada, the anhydrite and gypsum beds contain small quantities of ulexite and howlite. The ulexite occurs as irregular patches 1-3 cm in diameter, consisting of white fibrous crystals. The normal amounts of boron in seawater are sufficient to account for the observed quantity of borates (CM 13.370-376).

At the type locality, Iquique Province, Tarapacá, Chile, ulexite occurs in playas in a desert region, associated with pickeringite, nitratine, halite and glauberite (Mindat).

At the Jenifer Mine, U.S. Borax Mine, Kramer Borate deposit, Boron, Kern county, California, USA, strontium-bearing ulexite was observed as veinlets on a specimen of bedded borax and clay. The ulexite veins are up to 5 mm wide and irregularly separated by borax altered on exposed surfaces to tincalconite. Clay and disseminated realgar are associated with both borax and the ulexite. Discrete crystals of ulexite, some of which are doubly terminated, are usually not longer than 0.5 mm (AM 43.169-170).

At the Suckow borax mine, West Baker, Kramer Borate deposit, Boron, Kramer District, Kern county, California, USA, ulexite was collected from the waste dump. It consists of small hand specimens of white or transparent ulexite in a matrix of clay and borax. The ulexite occurs as massive aggregates of crystalline grains, irregularly distributed in the matrix. Some of the crystals are partially hollow, due to the inclusion, and subsequent dissolving away, of small grains of borax. Most of the crystals are only a fraction of a millimeter in length, although some reach a maximum of 6 mm (AM 25.754-762).

At the Lang Mine, Tick Canyon Borate deposit, Tick Canyon, Lang, Los Angeles county, California, USA, fibrous white ulexite occurs in association with colemanite, howlite and calcite (AM 3.35).

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