Formula: FeS2
Specific gravity: 4.887
Hardness: 6 to 6½
Streak: Dark grey to black
Colour: Pale brass-yellow, tin-white on fresh exposures.
Solubility: Insoluble in hydrochloric acid and sulphuric acid

Sedimentary environments
Metamorphic environments
Hydrothermal environments

Marcasite most frequently occurs as replacement deposits also often in concretions and in limestone, and often in concretions and replacing organic matter and forming fossils in sedimentary beds, particularly coal beds. It is also found in chemical sedimentary environments, massive and disseminated hydrothermal replacement deposits and hydrothermal replacement lodes and in epithermal (low temperature) hydrothermal veins.
Marcasite may be found in clay, marl, shale and dolostone.
In hydrothermal veins it may be associated with pyrite.
Marcasite is a mineral of low-temperature, near-surface, environments, forming from acid solutions. Pyrite, the more stable form of FeS2, forms in higher temperatures and lower acidity or alkaline environments.

At the Blue Points mine, Thunder Bay, Ontario, Canada, marcasite has been found with galena, chalcopyrite and quartz (R&M 94.4.325).

At Ladywash mine, Eyam, Derbyshire, England, UK, marcasite is associated with baryte (RES p119).

At Hampstead Farm quarry, Chipping Sodbury, South Gloucestershire, England, UK, marcasite is associated with calcite and baryte (RES p173).

Common impurities: Cu,As

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