As seawater evaporates, dissolved ions do not precipitate simultaneously. USGS summaries of marine evaporite systems describe a typical sequence of gypsum or anhydrite, followed by halite, and at still higher concentration, potash minerals such as sylvite.
You can explore related material in the Rock Salt Chemistry section.
Early Sulfates
Calcium-sulfate minerals can begin precipitating before the brine reaches sodium-chloride saturation.
Depending on conditions, hydrated gypsum or anhydrous anhydrite may dominate.
Halite Stage
As salinity continues to increase, sodium chloride reaches saturation and halite begins to precipitate.
Thick rock-salt successions can form where this stage persists or recurs for long periods.
Late Bittern Salts
After major halite precipitation, residual brines become enriched in magnesium, potassium and other conserved components.
USGS Earth MRI summaries highlight potash and Mg-Li enrichment in these residual brines.
Related article: Why Do Gypsum and Halite Precipitate in Different Parts of the Same Basin?.
Sources
- USGS – Earth MRI marine evaporite systems table
- USGS – Origin and chemical composition of evaporite deposits
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