The name marcasite comes from the Arabic word markaschatsa meaning "Firestone" because, like pyrite, marcasite emits a spark when struck by some rocks. This is unsurprising since marcasite and pyrite are polymorphs of each other. A polymorph is a mineral that has the same chemical formula as another, but a different crystal structure. Of the two, marcasite is the more unstable mineral, having a distressing tendency to decompose into melanterite (FeSO4 • 7H2O) and sulfuric acid (H2SO4).
| FeS2 |
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| Sulfide |
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| Orthorhombic |
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| 2/m 2/m 2/m |
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| 6-6.5 |
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| 4.89 |
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| {010} indistinct |
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| Uneven |
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| Brassy-yellow |
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| Gray-black |
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| Metallic |
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| Opaque |
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| Tabular, stalactitic, globular |
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| Magnetic after heating |
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| Notable Localities: |
Czech Republic; Dover, England; Tri-State Mining District, Missouri-Oklahoma-Kansas; Mineral Point, Wisconsin; Galena, Illinois |
| Uses: |
Mineral specimen |
Specimens:
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