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Dominant species
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Ulexite
Ulexite
Chemical
Formula
NaCaB5O6(OH)6?5(H2O)
Species
Borates
Crystal
System
Triclinic
Mohs
Scale
2
Specific
Gravity
1.95-1.96
Color
Colorless to white
Streak
White
Luster
Vitreous, Silky
Refractive
Index
n = 1.491 - 1.496 n = 1.504 - 1.506 n = 1.519 - 1.520
Diaphaneity
Transparent
Cleavage
PerfectPerfect on ; on good; on poor.
Fracture
Irregular/Uneven
Crystal Habit:Acicular to fibrous
Geological Setting:Arid regions. In salt playas and desiccated saline lakes.
Ulexite (NaCaB5O6(OH)6?5(H2O)) (hydrated sodium calcium borate hydroxide), sometimes known as TV rock, is a mineral occurring in silky white rounded crystalline masses or in parallel fibers. The natural fibers of ulexite conduct light along their long axes, by internal reflection. Ulexite was named for the German chemist Georg Ludwig Ulex (1811–1883) who first discovered it.

Ulexite is a structurally complex mineral, with a basic structure containing chains of sodium, water and hydroxide octahedra. The chains are linked together by calcium, water, hydroxide and oxygen polyhedra and massive boron units. The boron units have a formula of B5O6(OH)6 and a charge of ?3. They are composed of three borate tetrahedra and two borate triangular groups.

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, USA; Tarapacá Region in Chile, and Kazakhstan. Ulexite is also found in a vein-like bedding habit composed of closely packed fibrous crystals.

Ulexite is also known as TV rock due to its unusual optical characteristics. The fibers of ulexite act as optical fibers, transmitting light along their lengths 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 (as shown in the photograph).

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 decomposes in hot water.


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