Chamosite after Almandine

specimen number:
2062329

location:
Michigamme Mine, Marquette Co., Michigan, USA

description:

A loose sharp well-formed crystal of what was once almandine, this now replaced by chamosite. In super condition all around. Collected around 1950.

The Michigamme mine, located near the village of Michigamme in Marquette County, Michigan, was an underground iron mine operating on the Marquette Iron Range. Geologically, the deposit is situated within the Paleoproterozoic Negaunee Iron-Formation, where secondary iron enrichment produced high-grade zones of magnetite and hematite ore flanked by extensive layers of chlorite schist waste rock. Operations at the site commenced in 1872, initially extracting ore from seven shallow open pits before transitioning into deep underground shafts that eventually reached depths exceeding 1000 feet. The primary mining operations ceased in 1901 due to economic challenges and declining ore quality, though shipments from surface stockpiles and waste rock processing continued until the site was completely abandoned in 1905.

A pseudomorph is a mineral specimen that has undergone a total chemical replacement by another mineral species while completely preserving the distinct crystal shape and external geometry of the original mineral. At the Michigamme mine, this process is demonstrated by dark greenish-black chamosite, an iron-rich chlorite group mineral, replacing almandine garnet within the local chlorite schist. These specimens, historically referred to by local miners as black diamonds, retain the sharp, twelve-sided dodecahedral crystal habit of the original almandine garnets despite now consisting entirely of microcrystalline chamosite.

Questions about this specimen?
Send an email or call us: 314-341-1811.