Is selenite water soluble
Selenite is gypsum, and a crystal dissolves slightly: 0.24 g per 100 ml of water at 25 °C. That is about 171 times calcite's figure, 150 times less than halite.
Is selenite water soluble? Slightly. Selenite is gypsum, CaSO4·2H2O, hardness 1.5 to 2, and the ILO-WHO chemical safety card for gypsum gives 0.24 g per 100 ml at 25 °C: 2.4 g per litre. That is about 171 times calcium carbonate's figure and 150 times less than salt's: enough to matter to a wetted specimen, nowhere near salt.
In short
- Yes, slightly: 0.24 g per 100 ml at 25 °C. That is the ILO-WHO International Chemical Safety Card for gypsum (mineral). Selenite is the International Gem Society's name for colourless, transparent gypsum, and the same article calls gypsum water soluble.
- Between calcite and salt, a long way from both. Calcium carbonate is 14 mg per litre on its ILO-WHO card, so gypsum's 2.4 g per litre is about 171 times more. Sodium chloride is 36.0 g per 100 g of water in PubChem, 150 times gypsum's figure.
- The worked example: one litre, taken to saturation, carries off 2.4 g. Spread evenly over a 15 x 2 x 2 cm wand, 128 square centimetres of surface, that is 81 micrometres from every face, or 1.73% of the wand's 139 g. It is a ceiling per litre of standing water, not a rate.
- The Handbook sheet does not say soluble. Six of the 176 sheets in this site's reference set record solubility in water: borax, chalcanthite, epsomite, halite, portlandite and sylvite. Gypsum's sheet instead records it forming in soils by evaporation and as efflorescences in mines.
- Heat is a different reaction. Gypsum's water is part of the crystal, 20.93% of the mass, calculated. The ILO-WHO card gives an apparent melting point of 100 to 150 °C caused by losing that water; that is dehydration, not dissolving.
| Mineral (compound) | Formula | Solubility in water, as published | Source | Per litre, approximately | Against gypsum | Hardness on the sheet |
|---|---|---|---|---|---|---|
| Halite (sodium chloride) | NaCl | 36.0 g per 100 g water at 25 °C | PubChem | 360 g per kg of water | 150 times more | 2 to 2.5 |
| Gypsum, including selenite and satin spar | CaSO4·2H2O | 0.24 g per 100 ml at 25 °C | ILO-WHO card 1215 | 2.4 g | 1 | 1.5 to 2 |
| Calcium sulfate dihydrate, the reagent | CaSO4·2H2O | 0.2 g per 100 ml at 20 °C | ILO-WHO card 1734 | 2 g | Similar, at 20 °C | Not a mineral sheet |
| Anhydrite (anhydrous calcium sulfate) | CaSO4 | 0.2 g per 100 ml at 20 °C | ILO-WHO card 1589 | 2 g | Similar, at 20 °C | 3 to 3.5 |
| Calcite (calcium carbonate) | CaCO3 | 14 mg per litre at 25 °C | ILO-WHO card 1193 | 0.014 g | About 171 times less | 3 |
Is selenite water soluble? The number, and where it comes from
Selenite dissolves in water at 0.24 g per 100 ml at 25 °C, because selenite is gypsum and that is the figure on the ILO-WHO International Chemical Safety Card for gypsum (mineral). The International Gem Society defines selenite as colourless, transparent gypsum crystals and satin spar as the fibrous variety, and describes gypsum as water soluble. The Handbook of Mineralogy gives the formula CaSO4·2H2O, hardness 1.5 to 2; the IMA list writes it Ca(SO4)(H2O)2. There is no selenium in either formula.
Two cards, two temperatures, one answer. The card for the mineral gives 0.24 g per 100 ml at 25 °C; the card for the laboratory reagent, calcium sulfate dihydrate, gives 0.2 g at 20 °C and calls it very poor. This page quotes both and does not derive a temperature trend from two rounded figures.
The quotable line: selenite is about 171 times more soluble than calcite and 150 times less soluble than salt. The first ratio is 2.4 g against the 14 mg per litre on the calcium carbonate card; the second is 36.0 against 0.24, from PubChem's sodium chloride record, whose figure is per 100 g of water rather than per 100 ml, so that ratio is approximate.
What 2.4 g per litre does to a selenite wand: the worked example
A litre of standing water can take 2.4 g of gypsum before it is saturated, and then it takes no more. Take a satin spar or selenite wand 15 cm long and 2 cm square. With the Handbook's measured density of 2.317, it weighs 139.0 g and has 128 square centimetres of surface. The 2.4 g is 1.036 cubic centimetres of gypsum; spread evenly over that surface it is 81 micrometres off every face, or 1.73% of the mass.
Substitute your own numbers. Mass lost per litre is 2.4 g at most; thickness lost is 2.4 ÷ 2.317 ÷ surface area in square centimetres, in centimetres. A second litre of fresh water takes another 2.4 g, so dissolving the whole 139 g wand would need about 58 litres, each taken to saturation. Running water is replaced before it saturates, so a rinse under a tap is not the same as a bowl.
What the arithmetic does not say: how fast any of this happens. Solubility is a limit, not a rate, and nothing verified for this page gives a dissolution rate for a specimen. In our judgement dissolution is also not even across faces, edges and the fibres of satin spar, which would explain a wetted face ending up frosted rather than uniformly thinner; no source verified here measures that. What that looks like on a specimen is on why is my selenite turning yellow.
Is selenite like salt? What the reference sheets record about solubility
No: the gypsum sheet does not record gypsum as soluble at all. Across the 176 sheets in this site's reference set, converted with pdftotext -layout and searched case-insensitively, 6 record solubility in water: borax, chalcanthite, epsomite, halite, portlandite and sylvite, each as soluble in H2O. Four other sheets use the word only as insoluble in an analysis. The word needs no ligature handling.
What the gypsum sheet does record is how it forms: in marine salt deposits, in soils formed directly by evaporation, and as efflorescences in mines. Four of the 176 sheets record efflorescences: borax, epsomite, gypsum and halite; the search allowed for the ff ligature, and a control word typeset that way matched 24 sheets. The sheet records gypsum growing from evaporating water; the ILO-WHO figure measures the reverse.
A different set gave a different count before. This site's page on how to store soluble mineral specimens found 3 of 109 sheets in an earlier, smaller set; the counts are from different denominators and do not conflict. That page sets out humidity bands, which this page does not repeat.
When the answer changes: running water, fibres, heat and water-safe
Four conditions change what slightly soluble means for a specimen. Running or repeatedly changed water keeps dissolving where a still bowl saturates. Fibrous satin spar exposes more surface for its mass than a compact crystal. Heat drives off the 20.93% of the mass that is crystal water, which the ILO-WHO card puts at an apparent melting point of 100 to 150 °C; that is dehydration, not dissolving. And the gypsum sheet records hardness 1.5 to 2, so a softened, frosted face also scratches easily.
Water-safe, in the sense of putting selenite in drinking water, is not a question this page answers. The physics above says it dissolves slightly; any health question belongs with a clinician, and workplace limits for dusts are in HSE EH40. This is not health, safety or legal advice.
Care, without a recipe. The International Gem Society advises against mechanical cleaners that use heat or vibration on any gypsum. This page gives no cleaning or drying method of its own; the general guide is how to clean mineral specimens. The conservation pages are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.