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Evaporite Minerals: How Drying Water Grows Gypsum, Halite and Desert Roses

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: evaporites are minerals that crystallize when salty water loses more to evaporation than it gains from rain and rivers. As seawater shrinks, calcite comes out first, then gypsum once only about 15% of the water is left, halite at about 10%, and potassium and magnesium salts at about 5%. Gypsum (the mineral behind selenite and many desert roses) and halite dominate the record, and their softness and solubility are the reason specimens need dry storage.

What makes a mineral an evaporite?

The word describes how a mineral formed, not what it is made of. The Alex Strekeisen petrography site defines evaporites as layered crystalline sedimentary rocks that form from brines in areas where the amount of water lost by evaporation exceeds the total amount of water from rainfall and influx via rivers and streams. Almost 100 mineral species are possible in these rocks, yet fewer than a dozen are volumetrically important: carbonates such as calcite, dolomite and aragonite; the sulfates gypsum and anhydrite; and the chlorides halite, sylvite and carnallite.

The chemistry is simple concentration. In the Perkins mineralogy text, evaporation raises the concentration of everything dissolved in the remaining water until it becomes oversaturated and crystals start to precipitate. Inland lakes and seas commonly precipitate calcite, halite and gypsum, and in some places they have laid down salt beds thicker than 300 m. Geology.com puts the starting stock in practical terms: every liter of seawater evaporated in a solar salt pond yields about 35 grams of salt (Geology.com).

The same mineral can also grow by other routes. Gypsum precipitates in hot springs and caves, and barite builds rosettes in sandstone from groundwater. Calling something an evaporite is a claim about its setting, which matters when you read a label.

In what order do minerals crystallize from drying seawater?

Seawater carries a mix of ions, and each salt waits until the brine is concentrated enough to exceed its own solubility. The least soluble go first. The sequence below comes from a sedimentary geology course text by Michael Rygel and Page Quinton at SUNY Potsdam (LibreTexts), framed as a thought experiment: start with a 1,000 m column of seawater and dry it out completely.

Mineral Brine left when it starts to form Thickness from 1,000 m of seawater
Calcite 50% (500 m of water) 0.10 m
Gypsum 15% (150 m) 0.61 m
Halite 10% (100 m) 13.30 m
Potassium and magnesium salts 5% (50 m) 2.99 m
Total about 17 m

Three things stand out. A kilometer of ocean yields only about 17 m of rock. Halite makes up most of that, roughly three quarters of the total. And the potash salts, the last and most soluble, form only if the basin dries almost to nothing. Those late minerals include sylvite (KCl), carnallite, kieserite and polyhalite; the same text calls them less common but economically important. The USGS potash page explains why: potash is used primarily as an agricultural fertilizer because it is a source of soluble potassium.

Real basins rarely follow the table cleanly. Fresh seawater spills in, partly dissolves earlier layers and resets the clock, so many deposits show stacked cycles of carbonate, sulfate and salt rather than one neat sequence.

Where do evaporites form today and in the past?

The SUNY text groups the settings into three families. The one requirement for all of them: evaporation has to outrun recharge.

Coastal flats and lagoons

Modern marine evaporites are largely restricted to coasts. Along the Arabian Gulf, low flat shorelines called sabkhas are wetted by storm tides and by seawater seeping in through the ground. Strekeisen describes gypsum and anhydrite growing inside the sediment while a crust of halite forms on top, with anhydrite favoring the hotter, drier flats and gypsum the cooler ones or those with some fresh continental water. In restricted lagoons, large crystals of selenitic gypsum grow upward from the lagoon floor.

Whole basins: the saline giants

The thickest deposits need a basin cut off from the ocean by a barrier that overflows from time to time, refilling and drying again for long periods. The Mediterranean is the best known case: it holds a thick evaporite succession beneath the seafloor, formed when the basin was periodically isolated from the Atlantic between 5 and 6 million years ago in the Messinian Salinity Crisis. Twinned Messinian selenite crystals from the Piedmont Basin of Italy are one of Strekeisen's illustrated examples.

Playas and closed desert basins

On land, playa lakes and other basins with no outlet produce smaller deposits, and because the water came from rivers and springs rather than the sea, their mineral mix can differ. Trona, a sodium carbonate mined for soda ash, forms in this kind of non-marine setting. Perkins uses Utah's Great Salt Lake and the shores of the Dead Sea as examples of salt being deposited at present.

Why does gypsum turn into anhydrite and back again?

Gypsum is calcium sulfate with water locked into the structure, CaSO4·2H2O, while anhydrite is the same compound without the water, CaSO4. The SUNY text notes that with increased burial depth gypsum commonly loses its water and transforms into anhydrite, and that the change can reverse when anhydrite returns to wet conditions near the surface. Together they often form nodular masses with a distinctive "chicken wire" texture, a pattern geologists use to recognize ancient sabkhas.

Temperature drives the swap. The anhydrite to gypsum balance point is around 58 °C: anhydrite is the more stable of the two above it, gypsum below, according to a 2018 report on the Naica research in ACS Central Science. Gypsum is also soft. Geology.com lists Mohs hardness 2, specific gravity 2.3, perfect cleavage and the monoclinic crystal system.

Are all gypsum crystals evaporites?

No, and the most famous gypsum on Earth proves it. The beams in the Cave of the Crystals at Naica, Chihuahua, reach about 11 meters long and 1 meter thick (Live Science). They did not grow from a drying lake. Magma heated the groundwater, anhydrite first deposited in the hot water, and as the water cooled below 58 °C over thousands of years the anhydrite began to dissolve while gypsum nucleated and grew. The dissolving anhydrite kept the solution barely supersaturated, which is the recipe for a few huge crystals instead of many small ones.

That growth was astonishingly slow. Researchers estimated the crystals formed between about 54 and 58 °C and that a beam 1 m thick would have needed nearly 1 million years at 55 °C, the ACS article reports. Live Science quotes the slowest rate measured as 14 femtometers per second. Juan Manuel García-Ruiz of the University of Granada called it the slowest growth rate ever measured, not only in nature but in general (University of Granada).

So gypsum can be an evaporite, a cave mineral or a low-temperature hydrothermal mineral. Perkins also pictures gypsum veins in Utah's red Moenkopi Formation, deposited by slowly moving groundwater filling cracks.

How do desert roses grow inside sand?

A desert rose is a rosette of flat, bladed crystals that grew within loose sand rather than in open water. As groundwater in a dune or dry lakebed evaporates near the surface, gypsum crystallizes between the grains and engulfs them. Minerals.net notes that sand trapped this way turns gypsum brown or gray and opaque, sometimes in hourglass patterns, and that desert roses are plentiful in the Great Salt Plains near Jet, Alfalfa County, Oklahoma, as well as the Mojave Desert and several places in the Sahara in Algeria and Morocco.

The name also covers a different mineral. Geology.com's barite page describes "barite roses" as clusters of bladed barite crystals that grew in sand, incorporating many grains, and up to several inches long. The Minerals.net barite entry records perfect barite roses from the Norman area of Cleveland County, Oklahoma, and adds that gypsum roses are much lighter, thinner and more brittle. The detailed comparison belongs on the desert rose guide; for evaporite purposes, only the gypsum rose is the drying-water product.

What does evaporite growth look like in a specimen?

The textures record how the crystal grew:

  • Swallowtail and fishtail twins. Minerals.net notes gypsum crystals frequently twin into these forked shapes, typical of crystals growing up from a lagoon floor or in clay.
  • Curved "ram's horn" growths. Gypsum crystals and fibrous masses can bend, sometimes severely; Minerals.net cites Terlingua, Brewster County, Texas, for curved fibrous masses.
  • Sand inside the crystal. Brown, opaque blades or hourglass shadows mean the crystal grew in sediment, not water.
  • Fibrous satin spar. Parallel fibers fill cracks, which is vein growth from groundwater; see selenite vs satin spar.
  • Hopper cubes. Stepped, hollow-faced cubes are fast halite growth at the brine surface, covered on the halite guide.

What do people get wrong about evaporites?

  • "Salt beds mean the sea was that deep." The 1,000 m table shows how little rock a column of water yields; thick beds need repeated refilling over long periods, not one deep sea drying once.
  • "Selenite is always an evaporite." Naica's cave gypsum grew from cooling hydrothermal water, and many vein selenites grew from groundwater.
  • "Evaporites stay put." The SUNY text explains that halite, gypsum and anhydrite are less dense than most rocks and flow plastically when buried, rising as salt diapirs that can pierce the layers above.

What to look for when you buy evaporite minerals

Treat every evaporite as a soft, water-sensitive mineral. Gypsum at Mohs 2 marks under a fingernail, and both gypsum and halite dissolve in water, so ask about the locality, check for re-crystallized crusts or rounded edges that show past wetting, and plan for dry display (the humidity-sensitive minerals guide covers storage). Natural growth features (twins, sand inclusions, crisp terminations) add more interest than polish.

  • clear selenite gypsum cluster, 11.27 lb, Morocco: a large natural cluster of clear gypsum blades, the best piece here for seeing how selenite crystals intergrow; at about 5 kg, check every tip on arrival.
  • Desert Rose: sandy rosettes that show the sand-engulfing growth described above; compare weight in the hand to tell gypsum from barite.
  • Ram's Horn Selenite, Rare Mineral Specimen: curved gypsum, the bending habit Minerals.net lists under "Ram's Horn"; handle it gently, since curved fibers split easily.
  • Halite - Pink: the listing gives Trona and Searles Lake, California, a modern dry-lake evaporite; keep it away from humidity.
  • Golden Selenite (UT): an inexpensive gypsum piece for comparing color and luster against the clear Moroccan material.

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Frequently asked questions

What are the most common evaporite minerals?

Gypsum, anhydrite and halite make up the overwhelming majority of evaporite deposits. Calcite and dolomite form early in the sequence, while potassium and magnesium salts such as sylvite and carnallite form last and are much less common.

How long does it take for evaporites to form?

A shallow pond can grow salt crystals within days, but thick deposits need a basin that keeps refilling and drying for very long periods. The Mediterranean's Messinian salts accumulated over episodes between 5 and 6 million years ago.

Is selenite an evaporite?

Often, but not always. Gypsum crystallizes from evaporating seawater once about 15% of the water remains, and selenite forms in lagoons and dry lakes. Some selenite, including the giant Naica crystals, grew instead from slowly cooling hydrothermal water.

Why are evaporite minerals so soft and fragile?

Their bonds are weak and partly ionic, and gypsum holds water in its structure. Gypsum rates 2 on the Mohs scale with perfect cleavage, and both gypsum and halite dissolve in water, so rough handling or damp air damages them.

What is the difference between gypsum and anhydrite?

Both are calcium sulfate, but gypsum contains two water molecules and anhydrite none. Gypsum is stable below about 58 °C and anhydrite above it. Burial converts gypsum to anhydrite, and water near the surface can turn it back.

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