Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).
'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 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.
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.
Chemical Properties and Uses
Because pyrite is often found near gold, gold atoms can substitute for the iron atoms in pyrite's atomic structure. Accordingly, 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.
When pyrite-bearing rocks are exposed to the elements, the mineral reacts readily with oxygen and water to form new iron oxide and sulfate minerals, as well as sulfuric acid. That chemical reaction causes the acid mine drainage that is a common problem in mining regions, including metal and coal mines.
The iron oxides in the water, not the sulfuric acid, coloring the drainage orange. On the other hand, that same chemical reaction has been used in commercial-scale sulfuric acid production since the 15th century. Pyrite is also used to make sulfur dioxide, which is used to bleach paper and in other industrial applications.
Some of you may own marcasite jewelry. It is actually pyrite that has been cut or faceted and polished. Just to confuse matters, the jewelry shares the same name of another iron sulfide mineral similar to pyrite that is not used for jewelry because it oxidizes more quickly than pyrite. Maybe 'fool's marcasite' is a better name?
Pyrite is the most common iron sulfide mineral on Earth and is found in shale, coal, limestone, schist, and many deposits of metallic ores. Its association with gold ore and its superficially similar appearance led to its confusion with the precious metal, and thus the name 'fool's gold.' It often forms brassy-yellow crystals of various shapes that have a shiny metallic luster. It is not mined for its iron content but is used to make sulfuric acid and other chemicals. Naturally-weathered pyrite creates acid mine drainage.
|Fool's gold||when novice gold prospectors mistook tiny pieces of pyrite for gold when panning during old mining days|
|Pyrite||a metallic mineral that is composed of iron and sulfur together|
|Pyritized fossils||the pyrite replaces shell material inside fossils|
|Mohs hardness scale||scale used by geologists to describe a mineral's resistance to being scratched|
|Pyritohedron||a 12-sided crystal named after the mineral pyrite|
Completing this lesson should prepare you to do the following tasks:
- Describe the features of fool's gold
- Recognize the history of fool's gold
- Explain the present use of pyrite
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