What Is Pyrite (Fool's Gold)? - Properties, Definition & Facts

Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

The last thing you want is to be considered a fool the next time you go panning for gold. To avoid that terrible fate, read this lesson and learn what the common mineral pyrite looks like, then take the quiz to make sure you won't be fooled.

Definition

Fool's Gold is an expression used to describe the mineral pyrite (sometimes called iron pyrite). The name is derived from the old mining days, when (apparently) many novice gold prospectors mistook tiny pieces of the mineral for gold in their pans. Strangely enough, the mineral really looks nothing like gold. And I suppose that's why they were considered fools.

Pyrite

Cubic pyrite crystals in schist. Image by Wikimedia user CarlesMillan. Creative Commons Attribution Share Alike 3.0 Unported license.
pyrite cubes

Pyrite is a metallic mineral that is composed of iron and sulfur atoms bound together in a ratio of 1:2 (FeS2), and it is arguably the most abundant sulfide mineral on Earth. The name comes from a Greek word for fire in reference to the tendency of the mineral to spark when struck against steel (in fact, it was used for that purpose in early flintlock rifles).

Pyrite sometimes replaces shell material in fossils like this cephalopod. Image by Wikimedia user Didier Descouens. Creative Commons Attribution Share Alike 3.0 Unported license.
pyritized fossil

The mineral is found in many sedimentary rocks, such as limestone, shale and coal, as well as in metamorphic rocks like schist. It is a very common mineral in ore deposits, where it is found along with other metal-containing minerals. And yet it is not mined for its iron (the metal is difficult to extract). In some rocks, usually shale, pyrite replaces shell material to form what are known as pyritized fossils.

Physical Properties

Pyrite rates a hardness of H=5.5 on Moh's hardness scale, the scale used by geologists to describe a mineral's resistance to being scratched. (For comparison, quartz is H=6, diamond is H=10 and gold is H=2.5). It has a brassy yellow color, but it does not look as bright yellow as gold. If the mineral is ground into powder - a physical property known as the streak- the color appears greenish-black (powdered gold is still yellow in case you're wondering).

Pyrite Suns are radiating growths of pyrite that form in some shale rocks. Image by Wimikedia user cobalt123, Creative Commons Attribution-Share Alike 2.0 generic license.
pyrite sun

Pyrite crystallizes from iron and sulfur-bearing water solutions, sometimes heated, in which there is very little free oxygen (that's known as a reducing environment in chemistry). Bacteria may also be involved in some pyrite deposits, particularly in shale.

When the mineral forms crystals they can assume the geometric shapes of a cube (with six square faces), octahedron (eight triangular faces) or pyritohedron (twelve pentagonal faces), or even combinations of those shapes. It also can be massive, lacking any visible crystal forms, and occur as irregular grains spread out through a rock.

Pyritohedrons are twelve-sided crystals named after the mineral pyrite. Image by Wikimedia user Hannes Grobe Creative Commons Attribution-Share Alike 2.5 Generic
pyritohedron

Whether in crystal or massive form, pyrite has a metallic luster. However, because the mineral does react chemically in air, it often has a dull or tarnished appearance.

Chemical Properties and Uses

Gold atoms can substitute for the iron atoms in pyrite's atomic structure, and some gold deposits do contain pyrite that contains several tenths of a percent gold in it. That likely is the source of the confusion between the two; however, it hardly makes pyrite as valuable as pure gold. Nickel also substitutes in the structure, forming a closely-related sulfide mineral called bravoite.

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