ULEXITE
Ulexite – NaCa[B5O6(OH)6].5H2O – is a very rare borate, characteristic of evaporitic parageneses in salt lakes. It is one of the three most important ore minerals of B, along with borax and colemanite. It forms “ulexite-stone”; some occurrences contain more than 1 billion tons of ulexite.
It forms pseudomorphs on borax. Under ultraviolet light, both long and short waves, it fluoresces in yellow, greenish-yellow, cream, and white. It dissolves slightly in cold water and quite a lot in hot water, releasing Na into the solution.
1. Characteristics
Crystal system: Triclinic pinacoidal.
Color: Colorless to white. Gray if contaminated with clay minerals.
Habit: Isolated ulexite crystals are very rare, up to 5 cm, always very elongated and with many different shapes. Ulexite typically occurs in very fine, acicular, and fibrous crystals, which can present themselves in three habits:
(i) They can form delicate tufts in cavities, with the fibers arranged radially.
(ii) They generally compose relatively massive nodular or rounded white aggregates where the fibers are arranged chaotically and dispersedly. These aggregates can be spherical, vaguely resembling a brain. In some cases, the fibers are disaggregated, resembling cotton (“cotton ball”).
(iii) Sometimes ulexite fibers occur in layers, with the fibers arranged perpendicularly to the orientation of the layer. In this last case, the optical effect known as “TV stone” occurs.
Cleavage: {010} perfect, {1-10} good, {110} poor.
Tenacity: Brittle.
Twinning: Polysynthetic, common, various laws.
Fracture: Irregular in fiber clusters.
Mohs Hardness: 2.5
Parting: No.
Streak: No information available.
Lustre: Vitreous, silky.
Diaphaneity: Transparent.
Density (g/cm³): 1.955
2. Geology and Deposits
Ulexite is a mineral characteristic of evaporites formed in ephemeral salt lakes (“playas”) and salt marshes, both always located in closed (isolated) basins of mountainous regions with desert climates that were or are the site of volcanism.
B comes from hot springs located in the vicinity or from the leaching of pyroclastic rocks.
3. Mineral Associations
It occurs associated with common minerals of the paragenesis, such as carbonates (calcite, trona), halides (halite) and sulfates (gypsum, anhydrite, glauberite, mirabilite).
It occurs with other borates such as colemanite, borax, kernite, kurnakovite, meyerhofferite, howlite, tunnelite, probertite, inderite, hydroboracite and inyoite.
In rarer situations (Boron, California, USA) it is associated with orpgment, realgar and pararealgar.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.491 – 1.496 nβ: 1.504 – 1.506 nγ: 1.519 – 1.520
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Low.
Cleavage: {010} perfect, {1-10} good e {110} poor.
Habits: Fibrous.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.030, corresponding to colors up to the middle of the 2nd order: gray, yellow, orange, red, blue, and greenish.
Extinction: Probably oblique.
Elongation sign: ES(+) or ES(-). The fibers show positive and negative elongation signs in approximately equal amounts.
Twins: Polysynthetic, common, follow various laws, parallel fiber elongation.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 73 to 78º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying ulexite. There are no opaque minerals associated with ulexite. All minerals in the paragenesis are indistinguishable from each other under reflected light.