Evaporite
Rock precipitated where a body of salt water dried up — the source of table salt, of potash fertiliser, of gypsum for plaster, and of borates.
An evaporite is what remains when a body of salt water evaporates faster than it is replenished. The salts dissolved in it come out of solution and accumulate as rock, and because different salts saturate at different concentrations they come out in a predictable order — which makes an evaporite sequence readable as a record of how far the drying went.
The economic consequence is that four unrelated industries draw on one rock type. Table salt, potash fertiliser, plasterboard and the borates used in glass and detergent all come out of evaporite deposits, and often out of the same basin at different levels.
How it forms
The sequence follows solubility. Carbonates precipitate first, then gypsum and anhydrite as the water reduces to roughly a fifth of its original volume, then halite, and finally the potassium and magnesium salts — sylvite and carnallite — which need the brine concentrated to a few per cent of where it started. Reaching that last stage requires near-total desiccation, which is why potash deposits are far rarer than salt ones.
The volumes involved are difficult to produce by evaporating a lake once. A metre of halite requires tens of metres of seawater, so the great deposits record basins repeatedly refilled and re-evaporated over long periods — most dramatically the Messinian salinity crisis, when the Mediterranean is thought to have dried almost completely and left kilometre-scale salt beneath its floor.
Salt also flows. Buried under later sediment it deforms plastically and rises as domes, and those domes trap oil against their flanks — which is why salt geology and petroleum geology have been intertwined since the beginning of both.
Economic significance
Salt was the first mineral commodity to be traded at scale and taxed as a matter of state policy, and evaporite basins have shaped settlement patterns accordingly — Salzburg, Hallstatt, Cheshire, the Punjab.
Potash matters differently. Potassium is one of the three macronutrients a crop removes from soil and must have returned, and there is no substitute for it: nitrogen can be fixed from the air and phosphorus recovered from several sources, but potassium comes from evaporite. A handful of basins in Canada, Russia and Belarus hold most of what is accessible, which makes fertiliser supply a geological fact before it is a political one.
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 composed of
- Gypsum — mineral · 5–40% · precipitated early, when the water has reduced to roughly a fifth of its volume
- Halite — mineral · 30–90% · the bulk of most evaporite sequences, and the reason the rock is called salt
- Sylvite — mineral · one of the last salts to precipitate, needing near-total desiccation — which is why potash deposits are far rarer than salt ones
- Borax — mineral · in the continental playa evaporites rather than the marine ones, where borate-rich springs fed a closed basin
is sourced from
- Seawater — material · what is left when a body of seawater evaporates faster than it is replenished — the salts come out in order of solubility, which is why the sequence is readable
is a source for
- Salt — compound · mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing