Pyrite from Peru. (SMM DM14826, Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.

Pyrite is a common iron sulfide mineral (FeS2). It’s nickname is “fool's gold”. Pyrite has a metallic luster, brassy gold color (in contrast to the deep rich yellow gold color of true gold - www.flickr.com/photos/jsjgeology/sets/72157651325153769/), dark gray to black streak, is hard (H=6 to 6.5), has no cleavage, and is moderately heavy for its size. It often forms cubic crystals or pyritohedrons (crystals having pentagonal faces).

Pyrite is common in many hydrothermal veins, shales, coals, various metamorphic rocks, and massive sulfide deposits.

The beautiful crystalline pyrite mass shown above comes from the Huanzala Deposit in the Andes Mountains of Peru. There, several limestone horizons of the Lower Cretaceous Santa Formation have polymetallic mineralization. Origin interpretations vary - some consider this to be a contact metamorphic (metasomatic) deposit - a skarn, formed by the interaction between host limestones and Miocene dikes. Others consider this to be a high-temperature, Cretaceous-aged, volcanic sedimentary ore deposit subjected to Miocene remobilization of minerals.

Locality: Huanzala Mine, Huallanca District, Dos de Mayo Province, Huanuco Department, Peru

Some geologic info. synthesized from:

Crowley et al. (1997) - The Raura-Huanzala Group (mining districts in Peru). Mineralogical Record 28(4).

Imai et al. (1985) - Mineralization and paragenesis of the Huanzala Mine, central Peru. Economic Geology 80: 461-478.

Soler et al. (1986) - Mineralization and paragenesis of the Huanzala Mine, central Peru - a discussion. Economic Geology 81: 195-196.

Photo gallery of pyrite: