page_title: Ulexite
section_title: Optical properties
hierarchical_section_title: Ulexite / Optical properties
caption_attribution_description: English: Ulexite Locality: Boron, Kramer District, Kern County, California, USA (Locality at mindat.org) This is a great example of the transparent and colorless mineral known as "TV rock" because it has optical properties like fiber-optic cable. It is transparent along the perfect vertical axis, while opaque along other axes. When polished, as this is, you can hold it over writing and witness this phenomenon. Is this a fine mineral specimen? No, not really. But it is really "neat" material and I wasn't sure those outside of California had been exposed to it. It does get sold widely to museums and rock shops, again labelled as "tv rock" or "fiber-optic rock". Fun for kids... 4.3 x 3.1 x 1.9 cm Deutsch: Ulexit Fundort: Bor, Kramer District, Kern County, Kalifornien, Vereinigte Staaten (Fundort bei mindat.org)
context_page_description: Ulexite, 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 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.
context_section_description: In 1956, John Marmon observed that fibrous aggregates of ulexite project an image of an object on the opposite surface of the mineral. This optical property is common for synthetic fibers, but not in minerals, giving ulexite the nickname "TV rock".  According to Baur et al. (1957), this optical property is due to the reflections along twinned fibers, the most prominent twinning plane being on (010). The light is internally reflected over and over within each of the fibers that are surrounded by a medium of a lower refractive index (Garlick, 1991). This optical effect is also the result of the large spaces formed by the sodium octahedral chains in the mineral structure. Synthetic fibers used for fiber optics transmit images along a bundle of threadlike crystals the same way naturally occurring ulexite reproduces images due to the existence of different indices of refractions between fibers. Additionally, if the object is colored, all of the colors are reproduced by ulexite. Parallel surfaces of ulexite cut perpendicular to the fibers produce the best image, as distortion in the size of the projected image will occur if the surface is not parallel to the mineral. Curiously, in situ samples of ulexite are capable of producing a decent, rough image. Satin spar gypsum also exhibits this optical effect; however, the fibers are too coarse to transmit a decent image. The thickness of the fibers is proportional to the sharpness of the projected image.
  Ulexite also displays concentric circles of light if held up to a light source, a strange optical property first observed by G. Donald Garlick (1991). This effect can also be produced by shining a laser pointer at a slightly oblique angle through a piece of ulexite. This optical behavior is a consequence of the different refractive indices of ulexite in different directions of polarization. Microscopic analysis of ulexite also yields cones of light that clearly emerge from each grain that is thicker than 0.1 mm under the Bertrand lens.
  Ulexite is colorless and nonpleochroic in thin sections with low relief. Being triclinic, ulexite is optically biaxial. Interference figures yield addition on the concave side of the isogyers, causing ulexite to be biaxial positive. Ulexite has a high 2V that ranges between 73° - 78° and a maximum birefringence of up to 0.0300 (Anthony et al., 2005). According to Moore and Potter (1963), the orientation of the fibers around the c-axis is completely random based on the variations in extinctions viewed under cross polarization. Ulexite displays polysynthetic twinning parallel to the elongation, along {010} and {100} (Murdoch, 1940). In thin sections cut parallel to the fibers, ulexite grains display both length-fast and length-slow orientations in equal quantities because the intermediate axis (y) of the indicatrix is roughly parallel to the elongation of the fibers along the crystallographic c-axis (Moore and Potter, 1963).
caption: A fragment of ulexite displaying characteristic optical property
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