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Marcasite [FeS2]
Structure Orthorhombic
Space Group Pnnm (No. 58)
a=4.436 Å, b=5.414
Å,
c=3.381 Å
Z=2
Atomic Positional Parameters
Fe 2a 0.0000 0.0000 0.0000
S 4g 0.2000 0.3780 0.0000
Reference
MJ Buerger (1931) American Mineralogist,
16, 361
Mineral Chemistry
Marcasite is the metastable dimorph of pyrite, and rapidly inverts to pyrite when heated in vacuum above about 400oC. It is formed at low temperatures from acidic sulphidic aqueous fluids, and occurs mostly in near-surface deposits. Marcasite nodules are found in limestones.
Minerals with the marcasite structure have the formula AX2, where A = Fe, Co, Ni, Ru, Os and X = S, Se, Te, As and Sb. Names of marcasite-type minerals include löllingite (FeAs2) and nisbertite (NiSb2).
The Chime figure shows sulphurs as yellow spheres, and irons as ochre spheres. Note the sulphur-sulphur bonds in the structure. The unit cell is outlined.