Halite
Halite is hardness 2 to 2.5, density 2.168, in cubes to 1 m. It deliquesces above 75.3% relative humidity at 25 °C, which a British room reaches often.
Halite is sodium chloride, hardness 2 to 2.5 and density 2.168, in cubes to 1 m that are often skeletal with hopper-shaped faces, with perfect cubic cleavage and a saline taste. It is soluble in water, and it deliquesces — draws water out of the air and dissolves in it — above 75.3% relative humidity at 25 °C.
In short
- Halite dissolves in its own display case above 75.3% relative humidity at 25 °C. That figure is the equilibrium humidity over saturated sodium chloride, and a British room in an unheated month sits above it routinely.
- Cubes reach 1 metre. The Handbook of Mineralogy also records skeletal growth with hopper-shaped faces, a habit produced by faster growth at the edges of a face than at its centre.
- Hardness 2 to 2.5 and density 2.168, with perfect cleavage on {001} in three directions at right angles. A dropped halite does not chip, it comes apart into smaller cubes.
- Sylvite is the look-alike and it is safer. KCl deliquesces only above 84.3% RH, is lighter at 1.993, and tastes salty with a distinct bitter edge. Taste is a legitimate test on these two species and on almost nothing else.
- It rarely fluoresces red under shortwave ultraviolet, one of only two shortwave-only responses among the 19 fluorescent species in the 74 Handbook entries we surveyed for our UV lamp note.
| Species | Formula | Deliquescence RH at 25 °C | Density | What this means in a British house |
|---|---|---|---|---|
| Halite | NaCl | 75.3% | 2.168 | Reached in most unheated rooms in winter |
| Sylvite | KCl | 84.3% | 1.993 | Reached in a damp room, not an average one |
| Chalcanthite | CuSO4·5H2O | Not applicable — it dehydrates instead | 2.286 | Fails in dry air rather than damp |
| Target for storage | — | Below 50% | — | Needs active control, not a closed cupboard |
The number that decides whether you can keep halite at all
Advice on storing salt minerals usually says “keep them dry”. There is a measured figure behind that, and it is specific enough to act on.
The equilibrium relative humidity over saturated sodium chloride is 75.3% at 25 °C. The value comes from Greenspan's standard determination of humidity fixed points, published in the Journal of Research of the National Bureau of Standards in 1977, which fits 44 separate measurements between 0 and 80 °C. We evaluated the paper's own polynomial at 25 °C to get 75.30%.
Above that humidity, halite does not merely get damp. It pulls water out of the air and dissolves in it. The specimen liquefies from its surfaces inward, the cube edges round off first, and what is left when the room dries again is a recrystallised crust that is no longer the crystal you had. The process is not reversible in any sense a collector cares about.
Set against that, a British sitting room runs at 45 to 60% RH when heated and can sit in the 70s for weeks in an unheated room in winter. Halite is therefore not a species you keep in an ordinary cabinet in this country — it needs a sealed container with active desiccant, checked, not a closed cupboard. Our note on storing a collection in a damp house covers the general problem; halite is the species that sets the hardest limit in it.
Hoppers, cubes and why halite looks the way it does
Halite is cubic and its commonest habit is the cube, to a recorded metre. The habit that makes specimens interesting is the hopper: a skeletal cube whose faces are stepped funnels rather than flat planes.
The mechanism is straightforward. In a rapidly evaporating brine the edges and corners of a growing face reach fresh solution before the centre does, so they grow faster and the centre is left behind as a recess. A hoppered halite is a record of how fast the brine it grew in was drying, which is more information than most habits carry.
Colour is the other feature worth understanding, because the blue and violet halite from Stassfurt and from the Carlsbad potash district is not coloured by an impurity in the ordinary sense. It is coloured by defects in the crystal structure itself, which is why the colour can be patchy, banded, or concentrated along cleavage planes rather than following growth zones. We do not assert a specific mechanism here, because the literature on colour centres in halite is more contested than a species note should pretend, and what a collector needs to know is only that the colour is structural and can fade.
Taste is a legitimate test here, and almost nowhere else
The Handbook records halite's taste as saline and sylvite's as salty, with bitter overtones. Those are published diagnostic properties, and on these two species a careful taste is a real test that separates them instantly where colour, habit and hardness all struggle.
It is also the one place on this site where we suggest putting a mineral near your mouth, and the caveats are not optional. Do it only on a specimen you have already established is a simple halide, on a clean fresh surface, with a damp fingertip rather than the tongue on the specimen, and never on anything from a mine dump, anything with associated minerals you have not identified, or anything that has been cleaned with acid. Our note on toxic minerals in a collection sets out why that list is not excessive caution.
The physical properties back the taste up. Halite is density 2.168 against sylvite 1.993, and hardness 2 to 2.5 against a flat 2. Both cleave perfectly in three directions at right angles, so cleavage separates neither. Sylvite is the lighter, softer, more bitter and slightly more stable of the pair, and it is the one that survives a damp room longer.
Where the specimens come from
Halite is worldwide and most of it is rock salt rather than a specimen. The Handbook names a short list of deposits that have produced crystals worth keeping.
Wieliczka and Bochnia in Poland for large crystals — the historic mines south-east of Kraków; Stassfurt-Leopoldshall in Saxony-Anhalt, the classic German potash locality; Hallstadt in Salzburg and Hall near Innsbruck in the Tyrol, whose names are the word for salt; Bex in Vaud; Girgenti and Racalmuto in Sicily; the Salt Range in the Punjab; and in the United States the Michigan Basin, the Gulf Coast salt domes, and large crystals from the Potash Corporation of America mine in the Carlsbad district of Eddy County, New Mexico, which is where most of the fine blue material originates.
British occurrences are industrial rather than collectable: the Cheshire salt field around Northwich and Winsford, and the Teesside deposits. Cheshire rock salt is a real and historic British occurrence and it produces almost nothing a collector would want, which is worth saying plainly because it is often assumed otherwise.
We keep no stock list of any kind. If you hold old Polish or Stassfurt material with its original labels, the wanted list sets out the documentation we look for — and for halite specifically, how it has been stored matters as much as where it came from. Our species notes cover the rest of the soluble and unstable group.