Is ulexite selenite?
Ulexite is not selenite: it is a borate, NaCaB5O6(OH)6·5H2O, density 1.955; selenite is gypsum, 2.317. A 20.00 g piece reads 9.77 g against 11.37 g in water.
No. Ulexite is a sodium calcium borate, NaCaB5O6(OH)6·5H2O; selenite is a name for colourless, transparent crystals of gypsum, CaSO4·2H2O, and satin spar for fibrous gypsum. Weigh it: a 20.00 g piece reads 9.77 g in water as ulexite and 11.30 to 11.42 g as gypsum, so under 10.5 g is not gypsum.
In short
- The criteria first. White, fibrous, silky and soft: ulexite and satin spar share all four, and both can show a cat’s eye. Five tests are left: weight in water, chemistry, refractive index, the image test and the UV lamp. Only the first gives a home answer; the table ranks all five.
- The 10.5 g line. On the Handbook of Mineralogy densities, 1.955 and 2.317, a 20.00 g piece reads 9.77 g in water as ulexite and 11.37 g as gypsum: 1.60 g apart. The International Gem Society’s ranges, 1.65 to 1.95 and 2.30 to 2.33, give 7.88 to 9.74 g and 11.30 to 11.42 g. Under 10.5 g is not gypsum; over it is not ulexite.
- What fails. Hardness overlaps once the gem society’s 1 to 2.5 for ulexite is admitted. The sheets’ indices are 0.001 apart, ulexite’s γ 1.520 against gypsum’s α 1.521, and Ghose et al. (1978) give ulexite a γ of 1.529, inside gypsum’s range. Ulexite may fluoresce yellow on its sheet, and the gem society lists yellow among gypsum’s colours.
- The image test is a pointer. The gem society says ulexite polished across its fibres carries an image from one face to the other, which is why it is called TV stone. No source read for this page says whether satin spar does or does not, so a carried image points to ulexite and a missing one proves nothing.
- Who each name is wrong for. “Selenite” is wrong for any fibrous piece: the gem society defines it as colourless, transparent gypsum crystals and calls the fibrous form satin spar. “Ulexite” is wrong for a piece reading above 10.5 g. The gypsum page’s 12-gram line was set over seven other minerals; ulexite reads under it too.
| Criterion | Ulexite | Selenite or satin spar (gypsum) | Does it choose? |
|---|---|---|---|
| IMA formula and row | NaCaB5O6(OH)6(H2O)5; row, G, 1850 | Ca(SO4)(H2O)2; row, G, no year. No row for selenite or satin spar | Names the species, not the specimen |
| 20.00 g piece in water | 9.77 g (sheet, 1.955); 7.88 to 9.74 g (IGS, 1.65 to 1.95) | 11.37 g (sheet, 2.317); 11.30 to 11.42 g (IGS, 2.30 to 2.33) | Yes: the 10.5 g line, for compact pieces |
| Chemistry, worked | B2O3 42.95%; H2O 35.57%; no sulphur | SO3 46.50%; H2O 20.93%; no boron | Yes, but only by analysis |
| Hardness | 2.5 (sheet); 1 to 2.5 (IGS) | 1.5 to 2, varies with direction (sheet) | No: the ranges meet |
| Refractive indices | 1.491 to 1.520 (sheet); γ 1.529 (Ghose et al.) | 1.521 to 1.530 (sheet) | No: 0.001 apart on the sheets, overlapping on Ghose |
| Image through a slab cut across the fibres | Yes, TV stone (IGS); light pipes (sheet) | Not stated by any source read here | A pointer only |
| Cat’s-eye cabochon | Yes (IGS) | Satin spar, yes (IGS) | No |
| UV on the sources | May fluoresce yellow, greenish yellow, cream, white (sheet); blue-green SW (IGS) | None on the sheet; IGS: may fluoresce, including yellow | No |
| Tenacity on the sheet | Brittle | Flexible, inelastic | A pointer on a crystal; not a test to perform on a specimen |
| Water on the sheet | Slightly soluble in H2O | None printed; see the solubility page | No: neither is a test |
Is ulexite a type of selenite? The criteria
Ulexite is not a type of selenite: it is a different mineral, a borate, while selenite is gypsum, a sulphate. The September 2026 IMA-CNMNC Master List has a row for ulexite, NaCaB5O6(OH)6(H2O)5, and one for gypsum, Ca(SO4)(H2O)2, and, by script, none for selenite, satin spar or TV stone. The International Gem Society gypsum article calls colourless, transparent gypsum crystals selenite and the massive, fibrous variety satin spar. Whether gypsum is a mineral at all is on is gypsum a mineral, and not repeated here.
Why the two get mixed up. Ulexite’s sheet describes compact parallel fibrous veins with a silky or satiny lustre; gypsum’s describes fibrous habits, silky if fibrous. The gem society says fibrous ulexite cuts cat’s-eye cabochons and that some satin spar cabochons show very pronounced chatoyancy. Both are soft. And the ulexite sheet lists gypsum among its associates, so one specimen can carry both.
The tests left. Weight in water, chemistry, refractive index, the image test and the UV lamp. The table ranks them; only the scale gives an answer at home, and only on a compact piece.
The 10.5 g line: weighing ulexite against gypsum
The 10.5 g line: a 20.00 g piece that reads under 10.5 g in water is not gypsum, and one that reads over it is not ulexite. Worked as 20 − 20/D, the Handbook densities give 9.77 g for ulexite (1.955) and 11.37 g for gypsum (2.317), 1.60 g apart. The International Gem Society’s ranges, 1.65 to 1.95 for ulexite and 2.30 to 2.33 for gypsum, give 7.88 to 9.74 g and 11.30 to 11.42 g. The line sits 0.73 g above ulexite’s sheet value and 0.80 g below gypsum’s lowest. The method is on how to measure specific gravity at home.
Against the gypsum page’s 12-gram line. The gypsum species page found gypsum the only one of seven white-to-colourless minerals under 12 g in water for 20.00 g. Ulexite was not one of the seven, and it reads 9.77 g: under 12 g as well. That rule separates gypsum from calcite, baryte and the others in its table; it cannot separate gypsum from ulexite, and this line can.
Where the line fails, documented. Probertite, another fibrous sodium calcium borate from Boron, California, reads 10.63 to 10.66 g, between the two; a reading there is neither ulexite nor gypsum on these figures. The line assumes a compact piece; a loose cottonball aggregate is not one. The borates at Boron are compared on ulexite.
Why hardness, index and UV do not choose
Hardness. The sheets give ulexite 2.5 and gypsum 1.5 to 2, varying with direction, half a step apart. The International Gem Society gives ulexite 1 to 2.5, which takes in gypsum’s whole range. A scratch test also marks the specimen; the general method and its limits are on how to test mineral hardness properly.
Refractive index. On the sheets, ulexite’s highest index, γ 1.519 to 1.520, is 0.001 below gypsum’s lowest, α 1.521. Ghose, Wan and Clark, American Mineralogist 63 (1978) give ulexite γ 1.529 in their Table 1, which is inside gypsum’s 1.521 to 1.530. A refractometer reading does not separate them.
UV. The ulexite sheet says it may fluoresce yellow, greenish yellow, cream or white under SW and LW UV; the gypsum sheet prints no luminescence line, by a ligature-aware search. The gem society says some gypsum may fluoresce, with yellow among the commonest colours, and gives ulexite blue-green under SW. A glow of either colour fits both; the general case is on fluorescent minerals explained.
Does selenite carry an image like TV stone?
The image is ulexite’s documented property, and no source read here records it for gypsum. The International Gem Society ulexite article says that if ulexite is polished perpendicular to the fibre direction, a piece will transmit an image from the bottom of the slab to the top, and that the material has been nicknamed TV stone. The Handbook ulexite sheet says parallel fibrous masses can act as fibre optical light pipes. Ghose et al. explain it as fibre optics: a core along the fibre and a cladding of crystals turned at random about it.
What that does and does not prove. Neither the gypsum sheet nor the gem society’s gypsum article mentions light pipes or a transmitted image, and an absence in two sources is not a test. So a sharp image through a slab cut across the fibres points to ulexite; a blurred or missing one proves nothing, because the effect also needs parallel fibres and cut faces. Weigh it.
The selenite questions already answered. Water is on is selenite water soluble and colour change on why is my selenite turning yellow; the ulexite sheet records only that ulexite is slightly soluble in H2O. This page gives no cleaning, drying or storage method.
Who each answer is wrong for
“Selenite” is wrong for any fibrous white piece, gypsum or not: the gem society defines selenite as colourless, transparent gypsum crystals and calls the fibrous form satin spar. It is also wrong for anything reading under 10.5 g in water for 20.00 g, which is too light for gypsum.
“Ulexite” is wrong for a piece reading above 10.5 g, a transparent crystal tabular on {010} whose cleavage flakes are flexible, as the gypsum sheet describes. “TV stone” is a trade name, not a species: on the gem society’s usage it means ulexite.
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