Groutite

specimen number:
8311458

location:
Robert Mine, Crow Wing Co., Minnesota, USA

description:
Lustrous black crystals to 0.3 cm in size on matrix. This is excellent for this rare mineral. In good condition.

The Robert Mine, located near Cuyuna in Crow Wing County, Minnesota, is a notable part of the Cuyuna Iron Range. Geologically, the deposit is hosted within the Paleoproterozoic Trommald Formation, characterized by complexly folded and metamorphosed iron formation consisting of magnetite, hematite, and various manganese-bearing minerals. The mine is the type locality for groutite, a rare manganese oxide-hydroxide mineral that occurs as lustrous, jet-black, sub-metallic crystals with a distinct wedge-shaped habit, often found in vugs within the iron ore. Historically, mining and quarrying operations at this site were part of the broader industrial effort to extract manganiferous iron ore for steel production, with active operations taking place primarily between 1916 and 1953. As of 2026, the mine is no longer active and has transitioned into a water-filled pit, but it remains an essential reference point for mineralogists studying the specific crystallization of manganese minerals in the Lake Superior region.

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