"Rock of the Month # 36, posted for June 2004" --- GCW sample 2420, courtesy of Queen's University. Digital image prepared 07 July 2004. The Canadian quarter is some 24 mm in diameter.

The pyrite displayed here is remarkable for its smooth, almost oily and mirror-like faces, superposed on the forms of two distorted cubes roughly 25 to 30 mm on a side. Pyrite ("fool's gold", FeS₂) is the best-known sulphide mineral in the world. A quick scan of a major gallery, such as the mineral hall at the Smithsonian in Washington, D.C., reveals pyrite in diverse forms: subspherical masses (Zacatecas, Mexico), smooth cubes (Navajun), and coarse pyrite with striated faces (Peru). Despite the simple basic (cubic) symmetry, surprisingly complex forms occur, such as the well-known pyrite pseudomorphs after marcasite from the former Nanisivik mine, northwest Baffin Island, Nunavut, Arctic Canada (Gait and Dumka, 1986). Pyrite crystal habits make the mineral a rockhound favourite, along with calcite and quartz. Researchers know the mineral for its complex internal zonations and multiple generations seen in some ore deposits. The cubic structure is present in grains as small as 0.05 microns or as large as 30 cm (Craig et al., 1999).

The dominant rock types in the area of Navajun are Jurassic sandstones and marls: the pyrite is typically hosted by the marls, which are >50% clay and at least 15% CaCO₃. Pyrite is the only sulphide (Calvo and Sevillano, 1989). Microscopic studies reveal silicate mineral inclusions in the pyrite, including near-pure Fe-endmember chloritoid (Lodders et al., 1998).

References

Calvo,M and Sevillano,E (1989) Pyrite crystals from Soria and La Rioja provinces, Spain. Mineral.Record 20 no.6, 451-456.

Craig,JR, Vokes,FM and Solberg,TN (1999) Pyrite: physical and chemical textures. Mineralium Deposita 34, 82-101.

Gait,RI and Dumka,D (1986) Morphology of pyrite from the Nanisivik Mine, Baffin Island, Northwest Territories. Can.Mineral. 24 Pt.4 (Ferguson Volume), 685-688.

Lodders,K, Klingelhofer,G and Kremser,DT (1998) Chloritoid inclusions in pyrite from Navajun, Spain. Can.Mineral. 36, 137-145.