The tag that came with sample reads as follows:
MarcasiteI traded this sample for a few of my strange copper nodules.
A dimorph of iron pyrite from Montezuma Co., Colorado
This specimen was found in the tailings of the THUNDER mine north of Mancos, Colorado. Iron Pyrite is FeS2 and so is Marcasite. However it is a separate mineral and often found in gold deposits.
Source: Calvin Webb
Contributor: Calvin Webb
Acquired: 1 September, 2003