Marcasite
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The mineral marcasite, sometimes called "white iron pyrite", is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide
Marcasite with tarnish (8×6 cm)
Massive aggregate (c. 9 × 7 × 5.5 cm) of marcasite from the steep coast near Kühlungsborn, Mecklenburg-West Pomerania, Germany
Marcasite crystals forming a cockscomb-shaped aggregate. Reocín Mine, Reocín, Cantabria (Spain).
Iridescent cluster of marcasite crystals (3.3 × 2.1 × 1.4 cm)
Melanterite
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Melanterite is a mineral form of hydrous iron sulfate: FeSO4·7H2O. It is the iron analogue of the copper sulfate chalcanthite. It alters to siderotil by loss of water. It is a secondary sulfate minera
Melanterite as found in nature
Cuprian melanterite