Evaporite Minerals and Halides

Close up of gypsum formation.

Ra'ike / Wikimedia Commons / CC BY 3.0

Evaporite minerals form by coming out of a solution where seawater and the waters of large lakes evaporate. Rocks made of evaporite minerals are sedimentary rocks called evaporites. Halides are chemical compounds that involve the halogen (salt-forming) elements fluorine and chlorine. The heavier halogens, bromine and iodine, make quite rare and insignificant minerals. It's convenient to put all these together in this gallery because they tend to occur together in nature. Of the assortment in this gallery, the halides include halite, fluorite, and sylvite. The other evaporite minerals here are either borates (borax and ulexite) or sulfates (gypsum).

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Borax

Borax crystals on a black background.

Rock Currier / Wikimedia Commons / CC BY 3.0

Borax, Na2B4O5(OH)4·8H2O, occurs at the bottom of alkaline lakes. It is also sometimes called tincal.

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Fluorite

Calcium fluoride
Evaporite Minerals and Halides.

ThoughtCo / Andrew Alden

Fluorite, calcium fluoride or CaF2, belongs to the halide mineral group. 

Fluorite isn't the most common halide, as common salt or halite takes that title, but you'll find it in every rockhound's collection. Fluorite (be careful not to spell it "flourite") forms at shallow depths and relatively cool conditions. There, deep fluorine-bearing fluids, like the last juices of plutonic intrusions or the strong brines that deposit ores, invade sedimentary rocks with lots of calcium like limestone. Thus, fluorite is not an evaporite mineral.

Mineral collectors prize fluorite for its very wide range of colors, but it's best known for purple. It also often shows different fluorescent colors under ultraviolet light. Some fluorite specimens display thermoluminescence, emitting light as they are heated. No other mineral displays so many kinds of visual interest. Fluorite also occurs in several different crystal forms.

Every rockhound keeps a piece of fluorite handy because it's the standard for hardness four on the Mohs scale.

This is not a fluorite crystal, but a broken piece. Fluorite breaks cleanly along three different directions, yielding eight-sided stones — that is, it has perfect octahedral cleavage. Usually, fluorite crystals are cubic-like halite, but they can also be octahedral and other shapes. You can get a nice little cleavage fragment like this at any rock shop.

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Gypsum

Calcium sulfate
Evaporite Minerals and Halides.

ThoughtCo / Andrew Alden

Gypsum is the most common evaporite mineral. It's one of the ​sulfate minerals.

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Halite

Sodium chloride on a black background.

Piotr Sosnowski / Wikimedia Commons / CC BY 4.0, 3.0, 2.5, 2.0, 1.0

Halite is sodium chloride (NaCl), the same mineral you use as table salt. It is the most common halide mineral.

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Sylvite

Chunk of sylvite on a gray table.

Darth vader 92 / Wikimedia Commons / CC BY 4.0

Sylvite, potassium chloride or KCl, is a halide. It's usually red but can also be white. It can be distinguished by its taste, which is sharper and more bitter than halite.

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Ulexite

Alkali borate
Evaporite Minerals and Halides.

ThoughtCo / Andrew Alden

Ulexite combines calcium, sodium, water molecules, and boron in a complicated arrangement with the formula NaCaB5O6(OH)6∙5H2O. 

This evaporite mineral forms in alkali salt flats where the local water is rich in boron. It has a hardness of about two on the Mohs scale. In rock shops, cut slabs of ulexite like this one are commonly sold as "TV rocks." It consists of thin crystals that act like optical fibers, so if you place it on a paper, the printing appears projected on the upper surface. But if you look at the sides, the rock is not transparent at all.

This piece of ulexite comes from the Mojave Desert of California, where it is mined for many industrial uses. On the surface, ulexite takes the shape of soft-looking masses and is often called "cotton ball." It also occurs beneath the surface in veins similar to chrysotile, which features crystal fibers that run across the thickness of the vein. That's what this specimen is. Ulexite is named after the German man who discovered it, Georg Ludwig Ulex.