Pyrite
Iron sulfide — fool's gold — a mineral more valuable for the sulfur it yields and the acid it produces than for the iron in it.
Pyrite is brass-yellow, metallic and common enough to have disappointed prospectors for as long as there have been prospectors. The nickname is fair but the resemblance is superficial: pyrite is hard where gold is soft, brittle where gold is malleable, and leaves a black streak where gold leaves gold.
Its real significance is chemical rather than economic. Pyrite is the main sulfur reservoir in many rocks, and when mining or weathering exposes it to air and water it oxidises to sulfuric acid. Acid mine drainage — the orange, lifeless streams below old workings — is pyrite doing what pyrite does.
How to identify it
Hardness settles it. Pyrite scratches glass; gold, chalcopyrite and most other yellow metallic minerals do not. It is also brittle, shattering under a hammer where gold would flatten.
Its crystals are unusually well formed — cubes with fine parallel striations on the faces, or twelve-sided pyritohedra — and the striations run in different directions on adjacent faces, which is diagnostic. Chalcopyrite, the other common brassy sulfide, is distinctly softer and often iridescent.
Uses
Pyrite was for a long time the principal source of sulfur for sulfuric acid manufacture, roasted to drive off sulfur dioxide. That role has largely been taken over by sulfur recovered from oil and gas processing, which arrives already separated and without the arsenic that pyrite often carries.
It strikes sparks against steel, which is the origin of the name, from the Greek for fire, and gave it a role in early firearms. Today it is mostly encountered as a specimen mineral, as an unwanted contaminant in coal, and as a problem in construction where pyritic fill expands and cracks foundations.
How it forms
Pyrite forms across an unusually wide range of conditions: in hydrothermal veins, in magmatic sulfide deposits, and in ordinary muds where bacteria reduce sulfate in the absence of oxygen. That last route makes it a common accessory in black shale and coal.
It is also a marker of oxygen-poor conditions. Pyrite in a sediment says the water or the pore space was anoxic when it formed, which makes it useful for reconstructing ancient environments.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is an ore of
- Sulfur — element · historically the principal source of sulfur for sulfuric acid Wikidata
is commonly confused with
- Gold — element · the resemblance is superficial — pyrite is hard and brittle where gold is soft and malleable
- Arsenopyrite — mineral · both brassy metallic sulfides of iron; arsenopyrite is paler and harder, and smells of garlic when struck
is used as
- Fertiliser — application · roasted for the sulfur that becomes sulfuric acid, most of which makes phosphate fertiliser
is an input to
- Roasting — process · roasted for its sulfur as much as for its iron
is a component of
- Shale — rock · in the black shales, where oxygen-poor bottom water let iron and sulfur combine instead of oxidising — and where fossils are so often replaced by it
- Porphyry copper deposit — deposit type · abundant and worthless in itself, and the mineral whose oxidation generates the acid that makes the oxidised zone above
is a source for
- Oxidised zone — deposit type · pyrite is what drives it — oxidising to sulfuric acid in place, which then attacks everything else in the deposit
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain