Grunerite with Magnetite

specimen number:
2062205

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

description:
Radiating crystals in groupings to 0.6 cm in size frozen throughout matrix. Collected in June, 1990. A good example for the species.

The Michigamme Mine, situated near the western end of the Marquette Iron Range in Marquette County, Michigan, was a significant producer of iron ore from its opening in 1872 until its eventual closure in 1905. Geologically, the mine exploited the Negaunee Iron Formation, a massive Precambrian banded iron formation composed of alternating layers of iron oxides and silica. In this specific western portion of the range, the rocks underwent higher grades of metamorphism compared to the eastern end, leading to the development of unique silicate minerals. One such mineral is grunerite, a magnesium-iron amphibole that typically forms as radiating, fibrous, or needle-like crystal clusters. Grunerite is a characteristic mineral of metamorphosed iron formations and is often found at the Michigamme Mine in association with magnetite and garnet. Historically, the mine was a vital industrial site during the expansion of the Lake Superior iron trade, and today it remains a classic locality for collectors seeking well-crystallized specimens of metamorphic iron silicates.

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