Gypsum
A soft sulfate mineral that gives up its water when gently heated and takes it back on mixing — the basis of plaster.
Gypsum is calcium sulfate with water built into its crystal structure, and that water is the entire point. Heat gypsum gently and it gives up most of it, becoming the powder sold as plaster of Paris. Add water back and the powder recrystallises into interlocking needles, setting hard in minutes.
The reaction is reversible, cheap, and happens at a temperature a wood fire can reach — which is why plaster is one of the oldest manufactured building materials, and why it remains the lining of most interior walls built today.
How to identify it
It is the softest common mineral: a fingernail scratches it easily, and it defines hardness 2 on the Mohs scale. That alone separates it from calcite, which it otherwise resembles, and calcite fizzes in acid where gypsum does not.
It takes three distinctive forms. Selenite is clear and splits into flexible-looking sheets; satin spar is fibrous with a silky sheen; alabaster is fine-grained and massive, soft enough to carve with hand tools, which is why it has been a sculptor's stone since antiquity.
Uses
Plasterboard consumes most gypsum produced. A gypsum core between paper facings makes a wall lining that is fire-resistant — the bound water absorbs heat as it is driven off, and a plasterboard partition buys time in a fire for exactly that reason.
Gypsum is also added to Portland cement, where a few per cent slows the setting reaction enough to make the concrete workable. Without it cement would flash-set in the mixer. Agricultural gypsum conditions heavy soils, and alabaster remains a carving stone.
How it forms
Gypsum is an evaporite: it precipitates when seawater or a saline lake evaporates, and it comes out of solution before halite does, so gypsum beds commonly underlie rock salt in the same sequence.
That origin means it occurs in thick, laterally extensive beds that are simple and cheap to quarry. Gypsum also forms where sulfuric acid from oxidising sulfide minerals meets limestone, and enormous selenite crystals have grown in a few water-filled caves where conditions stayed stable for a very long time.
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 input to
- Calcination — process · gently calcined to plaster of Paris, a far lower temperature than lime burning
contains
is used in
- Construction — industry · plasterboard and plaster, and as the set retarder in cement
is a component of
- Evaporite — rock · 5–40% · precipitated early, when the water has reduced to roughly a fifth of its volume
is an alternative to
- Quicklime — compound · as plaster. Lime plaster sets by absorbing carbon dioxide over months and stays permeable and repairable; gypsum sets in an hour and does not — which is why historic masonry must be repaired with the slow one
is a source for
- Gypsum plaster — material · calcined to drive off three quarters of the water, and it takes it back when mixed
is commonly confused with
- Chalk — rock · blackboard and sidewalk chalk have been gypsum since the nineteenth century, because it casts into sticks from a slurry and makes less dust — so the most familiar association with the word is not the rock at all
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