Pyrite is the most common sulfide mineral with a chemical composition of iron disulfide (FeS2). Pyrite often occurs as cubes or octahedrons and its brilliant metallic luster and bright yellow to golden color makes it a distinctive and attractive mineral. Massive pyrite can be very similar to other yellow, metallic minerals such as chalcopyrite or gold, so it has a famous nickname of “Fool’s Gold”, but its greater hardness is the key feature that distinguish it from those much softer minerals. Pyrite is a very hard metallic mineral because it is capable of scratching glass and will only be scratched by the best metal files; even knives will leave its surface unmarked.
Pyrite can be a very dangerous mineral because it is the main source of acid mine waters associated with sulfide mine tailings. Oxidation of pyrite, which is catalyzed by certain types of bacteria and archaea, release trace quantities of toxic metals and metalloids like Arsenic(As). Acid waters associated with pyrite oxidation can cause acid rain and solubilization of toxic heavy metals, resulting in their dispersal in the environment. Arsenic-containing pyrite in coals is a major health problem in the Guizhou province in southwest China, affecting millions of people.
The only common mineral that has similar properties with pyrite is marcasite, a dimorph of pyrite with the same chemical composition but an orthorhombic crystal structure. Marcasite has a pale brass color, sometimes with a slight tint of green. Marcasite is more brittle than pyrite and also has a slightly lower specific gravity at 4.8. Pyrite is called the “Fool’s Gold” because it appears very similar to gold, but these two can easily be distinguished. Gold is softer and has a much higher specific gravity while pyrite is a very hard mineral with a lower specific gravity.
Iron does not easily dissolve in pure water or in dry air, but when both water and oxygen are present (moist air), iron will start to corrode. Weathering processes release the element of iron into the waters, so both mineral water and drinking water contain iron carbonate. Iron is a dietary requirement for most organisms, including humans. However, when high concentrations of iron are absorbed, it can create serious health issues; the iron can be stored in the pancreas, the liver, the spleen and the heart. These vital organs can be severely damaged and can be complex to cure. Iron compounds can be a more serious problem to health than elements; water-soluble binary iron compounds (i.e. FeCl2, FeSO4) may cause toxic effects when concentrations are over 200 mg, the effects can be lethal.
