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Ulexite NaCaB5O9·8H2O (Hydrated Sodium Calcium Borate Hydroxide) is a mineral occurring in silky white rounded crystalline masses. It is a hydrous borate of lime and soda. It was named after the 19th century German chemist G. L. Ulex who first discovered it.

Ulexite is a structurally complex mineral, with a basic structure containing chains of sodium, water and hydroxide octahedrons. 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, and are composed of three borate tetrahedrons and two borate triangular groups. Hardness is 2 (softer than a fingernail) and specific gravity is approximately 1.97.

Ulexite is found with the mineral borax and is directly deposited in arid regions from the evaporation of water in intermittent lakes called playas. The precipitated ulexite commonly forms a "cotton ball" tuft of acicular crystals. Ulexite is also found in a vein-like bedding habit composed of closely-packed fibrous crystals, also known as "TV rock" due to its unusual optical characteristics. The fibers of TV rock act as fiber optics, transmitting light along their lengths by internal reflection, and when a piece of TV rock 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. This effect is partially the result of the large spaces in ulexite's sodium octahedral chains.

Ulexite is found principally in California and Nevada, USA; Tarapaca, Chile and Kazakhstan.

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See also: List of minerals