Halite
Rock salt — the only mineral people eat, and for much of history a commodity worth fighting over.
Halite is sodium chloride, and it is the one mineral that is a dietary necessity. Human physiology requires sodium and cannot store much of it, which made salt a genuine strategic commodity: taxed, fought over, used as payment, and the reason for the location of a good many old roads and towns.
It is also the textbook example of ionic bonding. Sodium and chlorine alternate in a cubic lattice, and the crystal cleaves along three planes at right angles — smash a piece of rock salt and the fragments are small cubes, which few other minerals do so unambiguously.
How to identify it
It tastes of salt, which is conclusive and is one of the few places in mineralogy where taste is an accepted test. Short of that: cubic cleavage, softness, and solubility in water — halite is one of very few common minerals that simply dissolves.
It is usually colourless or white, but impurities and radiation damage produce yellow, pink and a deep blue that is genuinely striking and entirely a defect in the lattice rather than a chemical difference.
Uses
Most salt produced is never eaten. The chemical industry consumes the majority as feedstock for chlorine and sodium hydroxide, which between them lead to PVC, solvents, paper, soap and a large share of industrial chemistry.
De-icing roads is the next largest use in cold countries, and it is seasonal enough to dominate winter freight in some regions. Food use — seasoning, and preservation by drawing water out of tissue faster than bacteria can tolerate — is small by tonnage and disproportionately large in history.
How it forms
Halite is an evaporite, precipitating when a body of saline water dries up. Because it is among the last salts to come out of solution, thick halite beds mean a basin evaporated almost completely — the Mediterranean did so repeatedly, leaving salt layers a kilometre thick beneath the seabed.
Buried halite behaves unusually: it is less dense than the rock above it and flows slowly under pressure, rising as salt domes that pierce overlying strata. Those domes trap oil and gas against their flanks, which is why salt geology and petroleum geology are closely connected.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is an ore of
is a source for
- Sodium hydroxide — compound · electrolysis of brine, which produces chlorine in fixed proportion
is an input to
- Chlor-alkali electrolysis — process · as brine; the salt is the feedstock for both products at once
is commonly confused with
- Sylvite — mineral · cubic, colourless and found together; sylvite tastes bitter where halite is simply salty
is a component of
- Evaporite — rock · 30–90% · the bulk of most evaporite sequences, and the reason the rock is called salt
has the same composition as
- Salt — compound · the same substance in two trades: halite is a cubic mineral with a cleavage and a type locality, and salt is a food, a de-icer and the feedstock of the chlorine industry
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain