Goethite after Pyrite

specimen number:
2062382

location:
Jackson Mine, Negaunee, Marquette Co., Michigan, USA

description:

Cubic crystal of what was once pyrite, this 0.9 cm on edge, embedded up on matrix and replaced by goethite. In good condition. Collected in 1998.

The Jackson Mine in Negaunee, Michigan, opened on July 23, 1845, marking the official birth of the iron mining industry in the Lake Superior region, and it ultimately closed in 1927. Geologically, the deposit is situated within the Marquette Iron Range, where high-grade hematite and goethite iron ores formed via the weathering, hydrothermal alteration, and chemical precipitation of Precambrian-aged iron formation rocks, specifically the Negaunee Iron Formation.

Pseudomorphs of goethite after pyrite occur when hydrothermal solutions or weathering processes oxidize and replace original crystals of iron sulfide (pyrite) with iron oxide-hydroxide (goethite) while preserving the external crystal shape, such as cubes or pyritohedrons.

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