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Pyrite Suns from Sparta, Illinois
The mineral pyrite, also known as iron pyrite, is iron sulfide, the most common of the sulfide minerals. Pyrite crystallizes in the isometric crystal system and is most commonly found associated with other sulfides or oxides in quartz veins, metamorphic rocks, sedimentary deposits including coal beds, and as a replacement mineral in fossils. Pyrite is the paradigm "Fool's Gold". While there are other shiny, brassy colored minerals, pyrite is by far the most common and most frequently mistaken for gold. Pyrite is a favorite of mineral collectors due to its golden, metallic luster and interesting crystals. Pyrite is so common in the Earth's crust that it is found in almost every possible environment in numerous forms and varieties.
One of the most unusual and striking varieties of pyrite are the disc-shaped pyrite "suns", also known as pyrite dollars, which exhibit striations radiating out from the center like rays of the sun. These unique specimens are recovered from underground coal mines near Sparta, Randolph County, Illinois USA, where they occur in narrow, compacted seams of slate interbedded in 300 million-year-old Pennsylvanian Period coal deposits. The origin of pyrite suns has been a subject of speculation and debate. While some authorities have speculated they originated as fossils that were subsequently replaced with pyrite, the current consensus is they are concretions spread out under pressure and forced to grow in a laterally compressed, radiating manner in the seams of slate.
Marcasite, a polymorph of pyrite, has also been identified in pyrite suns. While marcasite has the same chemical formula as pyrite, it crystallizes in the orthorhombic crystal system, making it a distinct mineral. Marcasite is difficult to distinguish from pyrite without laboratory analysis when clear indicators such as well formed crystals are lacking.
Much sought after by mineral collectors and particularly by those with an affinity for pyrite, pyrite suns will become increasingly rare and difficult to obtain as the Illinois coal mines cease production and are shut down one by one.