A Guide to the Delicate Sulfate Minerals

The Twelve Apostles, Australia.
The limestone apostles in Australia are an example of sulfate minerals. © Marco Bottigelli / Getty Images
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Aluminum-potassium sulfate
Sulfate Mineral Pictures. Photo courtesy Dave Dyet through Wikimedia Commons

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.

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 colored white to flesh-red, like this specimen. 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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Lead sulfate
Sulfate Mineral Pictures. Courtesy Dave Dyet through Wikimedia Commons; specimen from Tombstone, Arizona

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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Calcium sulfate
Sulfate Mineral Pictures. Courtesy Alcinoe through Wikimedia Commons

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

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. This specimen was mined in Chihuahua, Mexico, and is in the Harvard Natural History Museum.

Other Evaporitic Minerals

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Barium sulfate
Sulfate Mineral Pictures. Photo (c) 2011 Andrew Alden, licensed to About.com (fair use policy)

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" like these. They're similar to gypsum roses, and sure enough, gypsum is also a sulfate mineral. Barite is much heavier, though; 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 (as seen in the gallery of mineral habits).

This specimen is massive barite from strongly metamorphosed dolomite marble in the Gavilan Range of California. 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.

Other Diagenetic Minerals

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Strontium sulfate
Sulfate Mineral Pictures. Photo courtesy Bryant Olsen of flickr under Creative Commons license

Celestine (or celestite) is strontium sulfate, SrSO4, found in scattered occurrences with gypsum or rock salt. Its pale blue color is distinctive.

Other Diagenetic Minerals

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Gypsum Rose

Hydrated calcium sulfate
Sulfate Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

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 -- 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. See other common mineral shapes in the ​mineral habits gallery

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, and household wallboard is filled with gypsum. Plaster of Paris is a roasted gypsum with most of its associated water driven off, so it readily combines with water to return to gypsum.

Other Evaporitic Minerals

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Selenite Gypsum

Clear crystalline gypsum
Sulfate Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

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