Calcite with Marcasite

specimen number:
2013464

location:
Linwood Mine, Buffalo, Scott Co., Iowa, USA

description:
A lustrous colorless gem twinned crystal of calcite 2.2 centimeter in length coming off matrix with other smaller crystallized non-twinned calcite and bright golden crystals of marcasite. Good examples of twinned calcite are not common at this locality, matrix examples of such a really quite rare. This is a very good one and in great condition.

The Linwood Mine, located in Scott County, Iowa, is a large limestone quarrying operation that works Mississippian age carbonate rocks for aggregate and industrial use. Quarrying has exposed numerous cavities and fracture zones where secondary mineralization has occurred, making the site well known among collectors. The locality is especially noted for its calcite, including rare and unusual twinned crystals that display complex and often highly symmetrical forms. These twins form when calcite crystals grow in specific repeated orientations due to the internal crystal structure and changing conditions during growth. Linwood calcite can occur as clear to white crystals, sometimes with excellent transparency and sharp faces, and the twinning adds both aesthetic and scientific interest.

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