Colemanite
Colemanite is a hydrous calcium borate mineral that crystallizes in the monoclinic system and served as one of the earliest commercially important sources of borax before more efficient borate minerals were discovered. It typically occurs as glassy, colorless to white crystals or nodular masses within lakebed evaporite deposits. Its formation is closely tied to arid basin environments where boron-rich waters evaporate and concentrate over long periods.
Colemanite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Ca2B6O11·5H2O |
| Mohs hardness | 4–4.5 |
| Crystal system | Monoclinic |
| Luster | Vitreous, adamantine on cleavage faces |
| Streak | White |
| Cleavage | Perfect on {010} |
| Fracture | Conchoidal to uneven, brittle |
| Specific gravity | 2.42 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Gray, Yellowish |
| Rarity | common |
Geology & background
Colemanite forms in playa lake and lacustrine evaporite basins, where boron leached from volcanic ash and hot springs concentrates in shallow, closed drainage systems and precipitates as the water evaporates under arid climate conditions. Death Valley and the Furnace Creek region of California became the historic center of colemanite mining in the late 1800s, supplying borax to industry via the famous twenty-mule-team wagon trains before sodium borate minerals like borax and kernite took over the market. Turkey now hosts some of the world's largest colemanite reserves.
Collectors seek colemanite mainly for well-formed, transparent crystals showing sharp, glassy faces rather than the massive nodular material typical of mine tailings. Because it is only moderately hard and can be somewhat brittle, specimens benefit from padded storage away from heavy handling. Its historical role in the American borax industry, immortalized by the twenty-mule-team marketing image, gives colemanite specimens an appeal connected to a distinctive chapter of Western mining history.
How to identify Colemanite
Colemanite occurs as colorless to white, sometimes yellowish, glassy crystals or as compact nodular masses, showing a vitreous luster on fresh crystal faces. It has a white streak, a hardness of about 4 to 4.5, and distinct cleavage in one direction that produces bright, glassy cleavage flakes. Fracture on other surfaces is uneven to splintery. It is easily confused with other white borate or sulfate minerals like colorless gypsum, but colemanite's greater hardness and glassy crystal faces help distinguish it in hand specimen.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Colemanite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Colemanite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Colemanite once supplied most of the world's borax before being displaced by richer sodium borate deposits discovered later in the same California basins.
- The mineral is named after William T. Coleman, a businessman who financed early borax mining operations in Death Valley.
- The famous twenty-mule-team wagons of Death Valley folklore were built specifically to haul colemanite ore across the desert to the nearest railroad.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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