Ulexite
Ulexite, NaCaB5O6(OH)6·5H2O, is a fibrous borate: triclinic, hardness 2.5, density 1.955. Its parallel fibres act as light pipes, hence the name TV stone.
Ulexite is a hydrated sodium calcium borate, NaCaB5O6(OH)6·5H2O: triclinic, hardness 2.5, density 1.955, usually white silky fibres from dry-lake borate deposits such as Boron, California. The Handbook sheet says parallel fibre masses act as optical light pipes, and the International Gem Society says a slab polished across the fibres transmits an image: hence the name TV stone.
In short
- One composition, two notations. The September 2026 IMA-CNMNC Master List lists ulexite as NaCaB5O6(OH)6(H2O)5, status G, 1850, Chile; the Handbook of Mineralogy sheet writes NaCaB5O6(OH)6·5H2O. Worked on standard atomic weights it is 42.95% B2O3, 13.84% CaO, 7.65% Na2O and 35.57% water, within 0.01 of the sheet’s ideal column. By script the IMA list has no row for TV stone or television stone.
- TV stone, sourced. The sheet says parallel fibrous masses can act as fibre optical light pipes. Ghose, Wan and Clark, American Mineralogist 63 (1978) say parallel bundles transmit an optical image with remarkable clarity and resolution, and report Weichel-Moore and Potter’s explanation: a core along the fibre with index γ near 1.529, and cladding from crystals randomly turned about the fibre axis, an index difference of up to 0.038.
- The Boron ladder. The ulexite, probertite, borax and colemanite sheets all name Boron, Kern Co., California. A 20.00 g piece reads 8.34 g in water as borax, 9.77 g as ulexite, 10.63 to 10.66 g as probertite and 11.75 g as colemanite. Probertite, the other fibrous sodium calcium borate, is 0.86 g heavier in water and a full point harder, 3.5 against 2.5.
- Where the sources disagree. The Handbook gives a perfect cleavage on {010}; Ghose et al. found the good cleavage along the fibres on {-120} instead. The sheet’s γ is 1.519 to 1.520, their Table 1 gives 1.529. The gem society gives density 1.65 to 1.95 and blue-green SW fluorescence, the sheet 1.955 and yellow to white under SW and LW. Each is printed with its source.
- Not gypsum. Satin spar and selenite are names for gypsum. The separation, with its 10.5 g line, is on is ulexite selenite. The sheet lists gypsum among ulexite’s associates, so one specimen can carry both.
| Mineral, IMA formula | Crystal system | Hardness | Density | 20.00 g piece in water | Luminescence on the sheet | Water on the sheet |
|---|---|---|---|---|---|---|
| Borax, Na2B4O5(OH)4(H2O)8 | Monoclinic | 2 to 2.5 | 1.715 | 8.34 g | None printed | Soluble in H2O; rapidly dehydrates in air to tincalconite |
| Ulexite, NaCaB5O6(OH)6(H2O)5 | Triclinic | 2.5 | 1.955 | 9.77 g | May fluoresce yellow, greenish yellow, cream, white, SW and LW | Slightly soluble in H2O |
| Probertite, NaCaB5O7(OH)4(H2O)3 | Monoclinic | 3.5 | 2.135 to 2.141 | 10.63 to 10.66 g | None printed | Decomposes partially in hot H2O |
| Gypsum, Ca(SO4)(H2O)2 (not at Boron on its sheet) | Monoclinic | 1.5 to 2 | 2.317 | 11.37 g | None printed | None printed |
| Colemanite, CaB3O4(OH)3(H2O) | Monoclinic | 4.5 | 2.423 | 11.75 g | Bright pale yellow fluorescence; may phosphoresce pale green | None printed |
| Ulexite against probertite | Triclinic against monoclinic | 1 point apart | Probertite 9.2% denser at the closest | 0.86 g apart | Ulexite’s sheet only | Different wording |
What is ulexite? A sodium calcium borate with water
The name and the formula. The September 2026 IMA-CNMNC Master List gives ulexite as NaCaB5O6(OH)6(H2O)5, status G for grandfathered, with the year 1850 and Chile as the country of first discovery. The Handbook sheet writes NaCaB5O6(OH)6·5H2O and says the name honours George Ludwig Ulex (1811–1883), the German chemist who obtained the first reliable analysis. Ghose, Wan and Clark, American Mineralogist 63 (1978) say, citing Dana’s System, that ulexite has been recognised as a valid species since about 1840; the IMA list’s 1850 is printed beside it, as the two differ.
The composition, worked. On standard atomic weights the formula mass is 405.23, and the mineral is 42.95% B2O3, 13.84% CaO, 7.65% Na2O and 35.57% water: the sheet’s ideal column prints 42.95, 13.84, 7.65 and 35.56. Boron is 13.34% of the mass. The sheet’s Suckow mine analysis, from Boron, California, gives 43.07, 13.92, 7.78 and 35.34.
Why it grows as fibres. Ghose et al. refined the structure on a crystal from the Boron mine, Boron, California, and found chains of sodium octahedra and calcium polyhedra running parallel to c, the elongation direction, which they say cause the fibrous habit essential to the optical fibre bundles. The sheet records crystals elongated to acicular along [001], to 5 cm, as cottonball-like masses, compact parallel fibrous veins and nodules; colourless, white in aggregates, grey with clay; silky or satiny when fibrous; hardness 2.5, brittle.
Why is ulexite called TV stone? The light-pipe effect
Ulexite is called TV stone because a slab cut across its parallel fibres carries an image from one face to the other. The Handbook sheet says parallel fibrous masses can act as fibre optical light pipes. The International Gem Society ulexite article says the fibres in tightly packed seams are transparent along their length and act like an array of parallel glass fibres; polished perpendicular to the fibres, a piece transmits an image from the bottom of the slab to the top, and has been nicknamed TV stone.
The physics, as published. Ghose et al. (1978) write that less frequently the fibres are oriented in parallel bundles that transmit an optical image with remarkable clarity and resolution. They report that Weichel-Moore and Potter explained it as fibre optics: the core is the crystals aligned along the fibre direction c, with index γ approximately 1.529, and the cladding comes from crystals randomly oriented about the fibre axis, giving a core-to-cladding index difference from 0 to a maximum of γ − α = 0.038. Ghose et al. also report spaces about 0.5 micrometre around the crystals, which they presume do not affect the light transmission.
What a collector sees. The effect needs parallel fibres, not cottonball or radiating aggregates, and faces cut across the fibres. The gem society also says the fibrous material cuts cat’s-eye cabochons, and names California as a source of TV stone and cabochon material.
Ulexite or probertite? The Boron ladder, worked
The Boron ladder: four borate sheets name Boron, Kern Co., California, and a 20.00 g piece of each reads a different weight in water: borax 8.34 g, ulexite 9.77 g, probertite 10.63 to 10.66 g, colemanite 11.75 g. The weights are worked as 20 − 20/D from the sheets’ densities, 1.715, 1.955, 2.135 to 2.141 and 2.423; the method is on how to measure specific gravity at home. Of the five sheets in the table, only ulexite’s prints the light-pipe statement, by script.
The look-alike that matters is probertite. Its IMA formula, NaCaB5O7(OH)4(H2O)3, has the same sodium, calcium and five borons as ulexite, with less water: 25.65% against 35.57%, worked. Its sheet describes acicular crystals elongated along [001], to 5 cm, rosettes and fibrous habits, with a lustre that may be satiny. It is monoclinic, hardness 3.5 and density 2.135 to 2.141; at the closest densities it is 9.2% denser than ulexite, so it reads 0.86 g heavier in water for 20.00 g. Its lowest index, α 1.514, is above ulexite’s α of 1.491 to 1.496. Hardness method: how to test mineral hardness properly.
Where the ladder fails. The ladder assumes compact material; a loose cottonball aggregate is not. Borax, the lightest rung, is recorded on its sheet as rapidly dehydrating in air to tincalconite, so a borax specimen may have become tincalconite. Colemanite’s sheet records bright pale yellow fluorescence; ulexite’s says it may fluoresce yellow, greenish yellow, cream or white. A lamp is a pointer, explained on fluorescent minerals explained.
Where the sources disagree on ulexite
Cleavage. The Handbook sheet, read from the rendered page, gives cleavage on {010} perfect, on {1-10} good and on {110} poor. Ghose et al. (1978) re-examined single crystals, report that the cleavages given by Murdoch (1940) do not seem reasonable in terms of the structure, and found a good cleavage along the elongation parallel to {-120}, with no other cleavage and a reproducible fracture surface parallel to {001}. Both readings are printed; neither is ours.
Indices and density. The sheet gives α 1.491 to 1.496, β 1.504 to 1.506, γ 1.519 to 1.520, and density 1.955. Table 1 of Ghose et al. gives α 1.491, β 1.506, γ 1.529, from unpublished data of W. T. Schaller, and observed density 1.955. The International Gem Society gives α 1.496, β 1.505, γ 1.519, birefringence 0.023, hardness 1 to 2.5 and specific gravity 1.65 to 1.95.
Luminescence. The sheet says ulexite may fluoresce yellow, greenish yellow, cream or white under SW and LW UV. The gem society gives blue-green in SW, with some phosphorescence. Two sources, two descriptions: a lamp reading neither confirms nor rules out ulexite.
Where ulexite is found: Boron, the Atacama, Argentina and Turkey
The Occurrence field: typically in playa and salt-marsh deposits in arid regions and bedded sedimentary deposits formed from these, the boron supplied from surrounding hot springs; some occurrences contain upwards of one billion tons of ulexite. Its associates are colemanite, borax, meyerhofferite, hydroboracite, probertite, glauberite, trona, mirabilite, calcite, gypsum and halite.
The Distribution field names, in Chile, near Iquique, a large deposit at the Surire salt lake, southeast of Arica, Tarapacá, and the Salar de Ascotan, Antofagasta; more than 60 deposits in Jujuy, Salta and Catamarca Provinces, Argentina; Laguna Salinas, east of Arequipa, Peru; Columbus Marsh, Rhodes Marsh and Teels Marsh, Esmeralda Co., Nevada; Death Valley and Saline Valley, Inyo Co., and the Kramer borate deposit, Boron, Kern Co., California; Windsor, Nova Scotia, and the Penobsquis evaporite deposit, New Brunswick; Niederellenbach, Hesse, Germany; Bela Stena, Serbia; the Inder boron deposit, Kazakhstan; the Da-Chaidan and Zazangzaka salt lakes, Tibet; and immense deposits in the Bigadiç borate district and the Kirka borate deposit, Turkey.
Boron, California supplied the sheet’s powder pattern (Jenifer mine) and analysis (Suckow mine), and the Ghose et al. crystal; that paper calls the locality apparently unique for its excellent long clear crystals. No British locality is on the sheet, and we add none. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.