Ulexite (NaCaB5O6(OH)6·5H2O, hydrated sodium calcium borate hydroxide), sometimes known as TV rock or television stone, 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]3– 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, US; 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.
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/dissolves in hot water.
Chemical composition
Ulexite is a borate mineral because its formula (NaCaB5O6(OH)6·5H2O) contains boron and oxygen. The isolated borate polyanion [B5O6(OH)6]3− has five boron atoms, therefore placing ulexite in the pentaborate group.
Related minerals
Borate minerals are rare because their main component, boron, makes up less than 10 ppm (10 mg/kg) of Earth's crust. Because boron is a trace element, the majority of borate minerals occur only in one specific geologic environment: geologically active intermontane basins. Borates are formed when boron bearing solutions, caused from the leaching of pyroclastic rocks, flow into isolated basins where evaporation then takes place. Over time, borates deposit and form into stratified layers. Ulexite occurs in salt playas and dry saline lakes in association with large-scale gypsum deposits and Na-Ca borates. There are no known polymorphs of ulexite nor does ulexite form a solid solution series with any other minerals.
According to Stamatakis et al. (2009) Na, Ca, and Na-Ca borates are found in relation to ulexite. These minerals are: borax Na2B4O7·10H2O colemanite Ca2B8O11·5H2O howlite Ca2B5SiO9[OH]5 kernite Na2[B4O6(OH)2·3H2O] meyerhofferite Ca2B6O6(OH)10·2H2O probertite NaCaB5O9·5H2O More common minerals that are not borates, but also form in evaporite deposits are: calcite CaCO3 gypsum CaSO4·2H2O halite NaCl
Morphology
Ulexite commonly forms small, rounded masses resembling cotton balls. Crystals are rare but will form fibrous, elongated crystals either oriented parallel or radial to each other. Crystals may also be acicular, resembling needles (Anthony et al., 2005). The point group of ulexite is 1, which means that the crystals show very little symmetry as there are no rotational axes or mirror planes. Ulexite is greatly elongated along [001]. The most common twinning plane is (010). Ulexite collected from the Flat Bay gypsum quarry in Newfoundland exhibits acicular "cotton balls" of crystals with a nearly square cross-section formed by the equal development of two pinacoids. The crystals are about 1-3 µm thick and 50-80 µm long, arranged in loosely packed, randomly oriented overlapping bundles (Papezik and Fong, 1975). In general, the crystals have six to eight faces with three to six terminal faces (Murdoch, 1940).