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

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  • 0:03 Definition
  • 0:28 Pyrite
  • 1:34 Physical Properties
  • 3:27 Chemical Properties & Uses
  • 5:00 Lessin Summary
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Lesson Transcript
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 fool's gold comes from when novice gold prospectors mistook tiny pieces of pyrite for gold when panning for it during old mining days. However, on close inspection, the mineral really looks nothing like gold. And I suppose that's why they were considered fools.

Pyrite

Pyrite is a metallic mineral that is composed of iron and sulfur atoms bound together in a ratio of one iron to two sulfur, and it is arguably the most abundant sulfide mineral on Earth. The name pyrite comes from pyrite lithos, which, in Greek, means 'stone which strikes fire' in reference to the tendency of the mineral to spark when struck against steel. In fact, pyrite was used for that purpose in early flintlock rifles.

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, including gold. And yet pyrite is not mined for its iron, as the metal is difficult to extract from it.

In some rocks, usually shale, pyrite forms pyritized fossils, where the pyrite replaces shell material inside the fossil. An example of a pyritized fossil is shown in this picture of a cephalopod fossil.

Physical Properties

Pyrite rates a hardness of 6 to 6.5 on Mohs hardness scale, the scale used by geologists to describe a mineral's resistance to being scratched. For comparison, quartz has a hardness of 7, diamond has a hardness of 10, and gold has a hardness of 2.5.

Pyrite has a brassy yellow color, but it does not look as bright yellow as gold. It also sometimes displays greenish-black streaks. If pyrite is ground into powder, its color appears greenish-black, while powdered gold is still yellow.

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

When the mineral forms crystals, they can assume a number of possible shapes. One common form is a cube with six square faces, as in pyrite crystals in schist. Another form is an octahedron, which has eight triangular faces. A common form is the pyritohedron, a 12-sided crystal named after the mineral pyrite. Each pyritohedron has 12 pentagonal faces. Pyrite can also form combinations of these shapes, or lack any visible crystal forms and occur as irregular grains spread out through a rock. Pyrite can also fill in gaps in shale layers to form radiating growths called pyrite suns or pyrite dollars, as shown in this picture.

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.

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