Marcasite (#53)

A flattened marcasite "dollar" on black shale matrix.
North America Sparta, Illinois, USA 11 ¼" x 9 ½" x 2"

 

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This strange looking specimen may resemble some sort of fossil to you, perhaps a sand dollar, but its origin is not organic. It formed between layers of coal and black shale about 300 million years ago. The crystals grew radially, like rays between the coal and shale layers, and so these are sometimes called "suns" rather than "dollars." Those from this locality have been found to be mostly pyrite, with only minor marcasite. It is possible that they were originally marcasite and have been transformed into pyrite.

Pyrite and marcasite have the same chemical formula: FeS2, which makes them sulfides, but they have different crystal structures (arrangements of atoms). Such minerals are called polymorphs. (Okay, if there are only two forms, then "dimorphs" is the technically correct term - but almost everyone says "polymorphs" anyway.) At conditions prevalent at the surface of the earth, pyrite is the more stable of the two.

Left in storage for years, say, in a museum or collection drawer, pyrite will be unchanged, while marcasite will oxidize and begin to disintegrate into powder. (A brief introduction to polymorphism is found in the Learn More for specimen #32.) These flattened suns or dollars, whether pyrite or marcasite, are sometimes used for ornamental purposes. As the coal mines around Sparta, Illinois, gradually cease production and close, specimens like these may become rarer on the mineral collectors' market.