Kathy Feick Pyrite group of minerals Pyrite’s name comes from the Greek, pyrites lithos, “the stone which strikes fire.” The crystals form in the Isometric System; cubes, octahedrons, pyritohedrons and combinations of these and other forms. It also may be found in radiating disks, hair-like crystals, concretions and massive lumps in sulphide ore deposits. Berner, “Sedimentary Pyrite Formation: An Update,” Geochimica et Cosmochimica Acta, Vol. Agua Nueva Formation; Mexico; Sedimentary Pyrite. The Formation of LowTemperature Sedimentary Pyrite and Its Relationship with BiologicallyInduced Processes 1. Pyrite formation and burial in sedimentary envi. Sedimentary Pyrite: A Window into the Microbial Past. And thrive in the reducing sediments that favor pyrite formation may. Sedimentary pyrite grains. Magnetic investigations of framboidal greigite formation. 1984: Sedimentary pyrite formation: an update. Geochimica et Cosmochimica Acta 48, 605-15. Pyrite sometimes also contains small amounts of cobalt, nickel, silver or gold. Pyrite Cubes. Langoro, Spain. University of Waterloo Earth Sciences Museum Collection. Crystals of pyrite are easy to collect or purchase. It is interesting to start a collection illustrating the various crystal forms. Large clean lusterous cubes are available from Spain, pyritohedrons from Washington State, a variety of forms from Peru, and Pyrite “suns” from Sparta, Illinois. Occurrence: Pyrite forms in almost all types of environments including sedimentary, igneous, and metamorphic, as well as hydrothermal veins. Uses: The uses of pyrite are declining. The main uses today include: • Production of sulphur dioxide for the paper industry • Production of sulphuric acid for the chemistry industry and the fertilized industry • Pyrite is most often mined for the gold, copper or other elements associated with it • Pyrite used to be polished by Native Americans in early times and used as mirrors • Ornamental stone • Collector stone Fool’s Gold and Black Gold Richard P. Wells, reprinted with permission of the National Driller’s Buyers Guide, July 1995 Iron pyrite, the fool’s gold of antiquity, turns out to be a useful indicator for those of us seeking the black gold of modern times, crude oil. Pyrite is a common accessory mineral in sedimentary rocks, particularly in limestone, sandstone and carbonaceous siltstones or shales. Some times we wonder who and why it got there; and what does it mean for petroleum exploration. Originally the iron came from the weathering of older igneous or metamorphic rocks. ![]() Iron is a common minor constituent of all continental igneous rocks; and occurs in minerals such as ilmenite, magnetite and pyrite, and ferro-magnesian silicates like olivine, pyroxene, amphibole, and biotite mica. Deep weather over long periods of geologic time releases the iron and soluble iron salts form. The dissolved iron then travels in solution to the sea, where ferrous iron is oxidised and deposited. Much of the pyrite contained in sediments and sedimentary rocks is authigenic, formed in the depositional environment, or early diagenetic, formed during the transformation of the sediment into rock (lithification). The formation of pyrite requires the presence of organic matter in the sediment, sulphate in solution in the pore water, and locally anerobic (reducing) chemical environment. It is the presence of decaying organic matter in the sediment that creates the reducing chemical environment. In marine environments, decay of organic matter occurs most rapidly just below the bottom of the sea, before more than a few centimetres of other sediment have accumulated on top of it. With deeper burial, most of the reactive organic matter has already been consumed and no more pyrite can form. Pyrite seldom forms in fresh-water environments. The formation of pyrite crystals depends mainly on the iron content of the sediment. The process of pyrite formation in sediments results from the action of bacteria, which reduce sulphate ions (dissolved in the pore water) to sulphide. If there is iron present, iron sulphide crystals begin to grow. These sulphate –reducing bacteria also need other nutrients to live, which are provided by organic carbon in the sediment. Pyrite can also precipitate in reducing sulphate – rich environments created by the migration of crude oil into the rock system in deeper layers. This pyrite generally forms larger crystals, which tend to fill pre-existing pore spaces in the rock and overlay any primary cement present. The significance of pyrite for oil-finders is that its presence proves the chemically reducing conditions (rather than oxidising conditions) prevailed at some time in the past. Reducing conditions allow organic carbon to be preserved, whether in the form of plant or animal remains, coaly sediments or petroleum. The pyrite –sulphur content of the sediment correlates directly with the total organic carbon content of the rock, which is a measure of petroleum source rock richness. Incidentally, the expression “fools gold” doesn’t refer to just any old fool, but originally referred to the Queen of England. During colonial times, some intrepid British explorers wanted to establish a new colony on the coast of Labrador, but were denied the funding because they had not found any gold there. Not to be deterred, they collected some fine specimens of pyrite and sent them back as proof of a gold discovery, and the ruse worked!
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