Colemanite
Colemanite is CaB3O4(OH)3(H2O), a calcium borate of dry-lake deposits: monoclinic, hardness 4.5, density 2.423, crystals to 30 cm, bright pale yellow under UV.
Colemanite is a hydrated calcium borate, CaB3O4(OH)3(H2O) on the September 2026 IMA list and Ca2B6O11·5H2O on its Handbook sheet: the same atoms, doubled. It is monoclinic, hardness 4.5, density 2.423, with a perfect cleavage, crystals to 30 cm and bright pale yellow fluorescence. Worked from the formula it is 15.78% boron and 21.91% water.
In short
- Two formulas, one composition. The September 2026 IMA-CNMNC Master List lists colemanite as CaB3O4(OH)3(H2O), status G, 1884, USA. The Handbook of Mineralogy colemanite sheet writes Ca2B6O11·5H2O. Counted by script, the Handbook formula is exactly twice the IMA one: 2 calcium, 6 boron, 16 oxygen, 10 hydrogen.
- The CaB3 hydrate ledger. A script finds 5 IMA rows whose formula begins CaB3: fabianite, jarandolite, colemanite, meyerhofferite and inyoite. Written as CaO·1.5B2O3·nH2O they carry 0.5, 1.5, 2.5, 3.5 and 6.5 waters, 5.31% to 42.18% by mass. On their sheets, hardness falls from 6 to 2 and density from 2.77–2.796 to 1.875 as the water rises.
- Where weight fails. Jarandolite, one water fewer, reads 11.97 g in water for 20.00 g against colemanite’s 11.75 g: 0.22 g apart. Hardness separates them, 6 against 4.5. Meyerhofferite, one water more, reads 10.57 g and is 2 hard. Inyoite’s sheet says soluble in water, so it is not weighed.
- Bright and cleavable. The sheet records bright pale yellow fluorescence and pale green phosphorescence, a perfect {010} cleavage, and pyroelectric and piezoelectric behaviour at very low temperature. Worked, colemanite is 15.78% boron, the figure on howlite.
- Where it is found. An important component of sedimentary borate deposits: Death Valley and Boron, California; Nevada; New Brunswick; Sonora; Argentina; Kazakhstan; Serbia; Greece; and Turkey, where the sheet records extensive deposits with about one billion tons of reserves. The Boron comparison with ulexite is on that page.
| Species | IMA formula | IMA status | Waters (n in CaO·1.5B2O3·nH2O) | Water by mass | Boron by mass | Hardness | Density | 20.00 g piece in water | UV on the sheet |
|---|---|---|---|---|---|---|---|---|---|
| Fabianite | CaB3O5(OH) | A, 1967 s.p., Germany | 0.5 | 5.31% | 19.13% | 6 | 2.77–2.796 | Not weighed: slightly soluble (sheet) | Brownish yellow under UV |
| Jarandolite | CaB3O4(OH)3 | A, 1995-020c, Serbia | 1.5 | 14.41% | 17.29% | 6 | 2.49 | 11.97 g | Nothing recorded |
| Colemanite | CaB3O4(OH)3(H2O) | G, 1884, USA | 2.5 | 21.91% | 15.78% | 4.5 | 2.423 | 11.75 g | Bright pale yellow; may phosphoresce pale green |
| Meyerhofferite | CaB3O3(OH)5·H2O | G, 1914, USA | 3.5 | 28.20% | 14.51% | 2 | 2.120 | 10.57 g | Nothing recorded |
| Inyoite | CaB3O3(OH)5(H2O)3·H2O | G, 1914, USA | 6.5 | 42.18% | 11.68% | 2 | 1.875 | Not weighed: soluble (sheet) | Nothing recorded |
What is colemanite? Two formulas, one composition
The row. The September 2026 IMA-CNMNC Master List gives CaB3O4(OH)3(H2O), status G for grandfathered, year 1884, country USA. The country column records where a mineral was first described; it is not a type-locality field.
The sheet’s formula, checked. The Handbook sheet writes Ca2B6O11·5H2O. Counting atoms by script: Ca2, B6, O16, H10, exactly twice CaB3O4(OH)3(H2O). On standard atomic weights the IMA unit is 205.54, and the composition is B2O3 50.81%, CaO 27.28%, H2O 21.91%, the sheet’s ideal column. Its Death Valley analysis reads B2O3 50.70, CaO 27.31, H2O 21.87 and MgO 0.10: 15.75% boron against 15.78% ideal.
On the sheet. Monoclinic. Equant to short prismatic crystals with large {110} and multiple terminating forms, to 30 cm, some pseudorhombohedral or pseudo-octahedral, nearly 50 forms measured; cleavable massive, granular, most commonly nodular. Cleavage {010} perfect, {001} distinct; fracture uneven to subconchoidal. Hardness 4.5; density 2.423 measured. Colourless, milky white, pale yellow or grey; vitreous to adamantine; biaxial (+), α = 1.586, β = 1.592, γ = 1.614.
How much water is in colemanite? The CaB3 hydrate ledger
The CaB3 hydrate ledger: five IMA species share one calcium and three borons and differ only in water, and colemanite is the middle rung. A script over the September 2026 list finds 5 rows whose formula begins CaB3. Written as CaO·1.5B2O3·nH2O, fabianite has n = 0.5, jarandolite 1.5, colemanite 2.5, meyerhofferite 3.5 and inyoite 6.5. Worked on standard atomic weights the water is 5.31%, 14.41%, 21.91%, 28.20% and 42.18% by mass; the sheets print 5.31, 21.91, 28.21 and 42.18 as ideals, and jarandolite’s sheet prints only an analysis, H2O 13.36.
What the sheets show along the ledger. Hardness 6 and 6 for the two driest, 4.5 for colemanite, 2 and 2 for the two wettest. Density falls from 2.77–2.796 through 2.49, 2.423 and 2.120 to 1.875. Boron by mass falls from 19.13% to 11.68%. Fabianite’s sheet says slightly soluble in water and inyoite’s soluble; colemanite’s says neither.
Where the ledger fails as a test. Jarandolite reads 11.97 g in water for a 20.00 g piece, 0.22 g above colemanite’s 11.75 g, so weight cannot separate them; hardness can, 6 against 4.5. Its sheet also prints a calculated density of 2.57 beside the measured 2.49. Jarandolite is known from one place on its sheet, the Jarandol basin, Serbia, and colemanite’s sheet names the same basin, near Bela Stena. Meyerhofferite is 1.18 g lighter than colemanite and 2.5 Mohs steps softer. The method is on how to measure specific gravity at home.
Colemanite, howlite or ulexite?
The three white borates of the same deposits. The colemanite sheet names howlite and ulexite first among its associates. Colemanite is the hardest of the three, 4.5 against howlite’s 3.5 and ulexite’s 2.5, and the only one with crystals recorded to 30 cm. The boron by mass of each is set out on howlite, and the weights in water of the borates of Boron, Kern Co., on ulexite; neither comparison is repeated here.
Not selenite. Colemanite’s sheet also names gypsum as an associate. Gypsum is 1.5 to 2 hard and flexible; the separation of fibrous borate from gypsum is on is ulexite selenite. The hardness method is on how to test mineral hardness properly.
Is colemanite fluorescent? The sheet and the gem society
Yes, on both sources. Read ligature-aware, with hemimorphite and willemite as positive controls, the colemanite sheet records bright pale yellow fluorescence and says it may phosphoresce pale green; it names no wavelength band. The International Gem Society colemanite article (Arem, updated November 2021) gives: may fluoresce and phosphoresce strong yellowish white or greenish white. Of the other four CaB3 sheets, only fabianite records fluorescence, brownish yellow.
Other properties the sources record. The sheet: pyroelectric and piezoelectric at very low temperature. The gem society: very heat sensitive, weak dispersion, and SG 2.42, matching the sheet. Neither source gives a cause for the glow, and this page supplies none. Lamps and the physics are on fluorescent minerals explained.
Where colemanite is found
The sheet’s Distribution field. An important component of sedimentary borate deposits worldwide. In California, Death Valley near Ryan, Furnace Creek district, with fine crystals in the Thompson and Boraxo mines; Borate, about 10 km northeast of Yermo, Calico Hills; and Boron, Kern Co. The Anniversary mine, Muddy Mountains district, Nevada. The Penobsquis and Salt Springs evaporite deposits near Sussex, New Brunswick. Large deposits at El Torreon and La Tinaja del Oso near Magdalena, Sonora. In Argentina, the Salinas Grandes Playa, the Loma Blanca deposit in Jujuy and the Sijes district, Salta. The Inder borate deposit, Kazakhstan; near Bela Stena, Jarandol Basin, Serbia; a lake on West Samos Island, Greece. In Turkey, extensive deposits with about one billion tons of reserves, as in the Bigadiç district with large crystals, Kestelek, Emet and Mustafakemalpasa.
Setting and name. The Occurrence field: a common constituent of borate deposits formed in arid alkaline lake environments, deficient in sodium and carbonate, typically under warm conditions. The name honours William Tell Coleman (1824–1893), pioneer developer of the borax industry in California. The species pages are listed in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.