COLEMANITE
Colemanite – Ca[B3O4(OH)3].H2O – is a very rare borate, characteristic of evaporitic sequences. It integrates B ores with other B minerals. It was the most important B ore until the discovery of kernite in 1926.
Some colemanites from Turkey exhibit luminous pale yellow or pale green fluorescence due to the presence of As. Other colemanites exhibit white fluorescence, but fluorescence generally does not occur.
At very low temperatures, colemanite is pyroelectric and piezoelectric. It can form pseudomorphs on inyoite. It shows exfoliation (crepitation) upon heating and produces a green flame. It may contain Mg.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Colorless, white, yellowish, gray.
Habit: The common habit of the crystals is equidimensional to short prismatic, more rarely they are pseudo-rhombohedral or pseudo-octahedral. The crystals can reach 30 cm and almost 50 different combined forms have been observed. Colemanite usually occurs in nodules, which may exhibit radial structure and reach 20 cm in diameter. It also occurs massively, in cleavable or compact masses.
Cleavage: {010} perfect, {001} distinct. (perpendicular to each other).
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular to subconchoidal.
Mohs Hardness: 4.5
Parting: No.
Streak: White.
Lustre: Adamantine to vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 2.43
2. Geology and Deposits
Evaporites with borates, in general, are formed in alkaline lacustrine environments, deficient in Na and carbonates, in arid and hot climates. In borate deposits of this type, colemanite is common, forming as a secondary mineral from the alteration of borax and ulexite. There is a rock called “colemanite-stone”.
Borates are very rare: in South America, they occur only in the salt flats of the border region between Chile, Argentina and Bolivia. In Africa, Russia and Australia there are no occurrences. The main occurrences are in Boron (California, USA) and Turkey.
3. Mineral Associations
It is associated with carbonates (calcite) and sulfates (gypsum, celestine).
It evidently occurs with other B minerals such as ulexite, kernite, howlite, searlesite, priceite, veatchite, ginoite, hydroboracite, gowerite, lüneburgite, nobleite, inyoite, and meyerhofferite.
Macroscopically, they are all very similar to each other.
In other parageneses with realgar, pararealgar, and orpiment.
4. Transmitted Light Microscopy
Refraction indices: nα: 1,586 nβ: 1,592 nγ: 1,614
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: {010} perfect and {001} distinct.
Habits: Equidimensional, short prismatic, pseudo-rhombohedral, pseudo-octahedral, massive, cleavable, nodular forms are very common.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.028, corresponding to colors ranging from gray, orange, purple, blue, and greenish up to the middle of the 2nd order.
Extinction: Parallel in sections parallel to (001). Oblique (26º) in sections parallel to (010).
Elongation sign: ES(+).
Twins: Possible.
Zoning: It can show color zoning.
CONVERGENT LIGHT
Character: B(+)
2V angle: 55 a 56º
Alterations: Alters to calcite. Colemanite generally forms as an alteration product of other borates, such as kernite and ulexite.
May be confused with: Other minerals from the evaporite paragenesis. Colemanite has higher refractive indices than many of the other borates with which it occurs in association.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying colemanite. There are no opaque minerals associated with colemanite. All minerals in the paragenesis are indistinguishable from each other under reflected light.