Ulexite
Ulexite (NaCaB5O6(OH)6·5H2O, hydrated sodium calcium borate hydroxide), sometimes known as TV rock or television stone due to its unusual optical characteristics, is a mineral occurring in silky white rounded crystalline masses or in parallel fibers. Ulexite was named for the German chemist Georg Ludwig Ulex (1811–1883) who first discovered it.
| Ulexite | |
|---|---|
Ulexite from California (size: 6.9 × 5 × 3.1 cm) | |
| General | |
| Category | Nesoborates |
| Formula (repeating unit) | NaCaB5O6(OH)6·5H2O |
| IMA symbol | Ulx |
| Strunz classification | 6.EA.25 |
| Dana classification | 26.05.11.01 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Unit cell | a = 8.816(3) Å, b = 12.87 Å c = 6.678(1) Å; α = 90.25° β = 109.12°, γ = 105.1°; Z = 2 |
| Identification | |
| Color | Colorless to white |
| Crystal habit | Acicular to fibrous |
| Twinning | Polysynthetic on {010} and {100} |
| Cleavage | Perfect on {010} good on {110} poor on {110} |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 2.5 |
| Luster | Vitreous; silky or satiny in fibrous aggregates |
| Streak | White |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 1.95 – 1.96 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.491 – 1.496 nβ = 1.504 – 1.506 nγ = 1.519 – 1.520 |
| Birefringence | δ = 0.028 |
| 2V angle | Measured: 73° to 78° |
| Ultraviolet fluorescence | Depending on fluorescent impurities, ulexite may fluoresce yellow, greenish yellow, cream, white under short waves and long waves UV |
| Solubility | Slightly soluble in water |
| Other characteristics | Parallel fibrous masses can act as fiber optical light pipes |
| References | |
The natural fibers of ulexite act as optical fibers, transmitting light along their long axes by internal reflection. When a piece of ulexite is cut with flat polished faces perpendicular to the orientation of the fibers, a good-quality specimen will display an image of whatever surface is adjacent to its other side. The fiber-optic effect is the result of the polarization of light into slow and fast rays within each fiber, the internal reflection of the slow ray and the refraction of the fast ray into the slow ray of an adjacent fiber. An interesting consequence is the generation of three cones, two of which are polarized, when a laser beam obliquely illuminates the fibers. These cones can be seen when viewing a light source through the mineral.
Ulexite is found in evaporite deposits and the precipitated ulexite commonly forms a "cotton ball" tuft of acicular crystals. Ulexite is frequently found associated with colemanite, borax, meyerhofferite, hydroboracite, probertite, glauberite, trona, mirabilite, calcite, gypsum and halite. It is found principally in California and Nevada, US; Tarapacá Region in Chile, and Kazakhstan. Ulexite is also found in a vein-like bedding habit composed of closely packed fibrous crystals.