Get to Know the 7 Delicate Sulfate Minerals

Rock formations along a sandy beach on a sunny day.
The limestone apostles in Australia are an example of sulfate minerals.

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Sulfate minerals are delicate and occur near the Earth's surface in sedimentary rocks such as limestone, gypsum rock, and rock salt. Sulfates tend to live near oxygen and water. There is a whole community of bacteria that make their living by reducing sulfate to sulfide where oxygen is absent. Gypsum is by far the most common sulfate mineral.

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Alunite

Chunk of alunite on a black background

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Alunite is a hydrous aluminum sulfate, KAl3(SO4)2(OH)6, from which alum is manufactured. Alunite is also called alumite. It has a Mohs hardness of 3.5 to 4 and coloring in white to flesh-red. Usually, it's found in the massive habit rather than as crystalline veins. Therefore, bodies of alunite (called alum rock or alumstone) look very much like limestone or dolomite rock. You should suspect alunite if it's completely inert in the acid test. The mineral forms when acid hydrothermal solutions affect bodies rich in alkali feldspar.

Alum is widely used in industry, food processing (especially pickling) and medicine (most notably as a styptic). It's great for crystal-growing lessons, too.

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Anglesite

Close up of alunite on a black background.

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Anglesite is lead sulfate, PbSO4. It is found in lead deposits where the sulfide mineral galena is oxidized and is also called lead spar.

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Anhydrite

Anhydrite on a black background.

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Anhydrite is calcium sulfate, CaSO4, similar to gypsum but without its water of hydration.

The name means "waterless stone," and it forms where low heat drives the water out from gypsum. Generally, you won't see anhydrite except in underground mines because, at the Earth's surface, it swiftly combines with water and becomes gypsum.

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Barite

Chunk of barite on a black backgound.

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Barite is barium sulfate (BaSO4), a heavy mineral that commonly occurs as concretions in sedimentary rocks. 

In the loose sandstones of Oklahoma, barite forms "roses." They're similar to gypsum roses, and sure enough, gypsum is also a sulfate mineral. Barite is much heavier, however. Its specific gravity is around 4.5 (by comparison, that of quartz is 2.6) because barium is an element of high atomic weight. Otherwise, barite is hard to tell apart from other white minerals with tabular crystal habits. Barite also occurs in a botryoidal habit.

Barium-bearing solutions entered the stone during this metamorphism, but conditions did not favor good crystals. The weight alone is the diagnostic feature of barite: its hardness is 3 to 3.5, it does not respond to acid, and it has right-angled (orthorhombic) crystals.

Barite is widely used in the drilling industry as a dense slurry (drilling mud) that supports the weight of the drill string. It also has medical uses as a filling for body cavities that is opaque to x-rays. The name means "heavy stone," and it's also known by miners as cawk or heavy spar.

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Celestine

Celestine chunk on a black background.

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Celestine (or celestite) is strontium sulfate, SrSO4. It's found in scattered occurrences with gypsum or rock salt and has a distinctive, pale blue color.

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Gypsum Rose

Gysum roses formation on a dark background.

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Gypsum is a soft mineral, hydrous calcium sulfate or CaSO4·2H2O. Gypsum is the standard for hardness degree 2 on the Mohs mineral hardness scale

Your fingernail will scratch this clear, white to gold or brown mineral, and that's the simplest way to identify gypsum. It's the most common sulfate mineral. Gypsum forms where seawater grows concentrated from evaporation, and it's associated with rock salt and anhydrite in evaporite rocks.

The mineral forms bladed concretions called desert roses or sand roses, growing in sediments that are subjected to concentrated brines. The crystals grow from a central point, and the roses emerge when the matrix weathers away. They don't last long at the surface, just a few years, unless someone collects them. Besides gypsum, barite, celestine, and calcite also form roses.

Gypsum also occurs in a massive form called alabaster, a silky mass of thin crystals called satin spar, and in clear crystals called selenite. But most gypsum occurs in massive chalky beds of rock gypsum. It's mined for the manufacture of plaster. Household wallboard is filled with gypsum. Plaster of Paris is roasted gypsum with most of its associated water driven off, so it readily combines with water to return to gypsum.

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Selenite Gypsum

Clear selenite gypsum on a black background.

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Selenite is the name given clear crystalline gypsum. It has a white color and soft luster that is reminiscent of moonlight.