Groutite

specimen number:
8311291

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

description:
Lustrous black crystals of groutite measuring to 0.5 cm in size on matrix. In good condition and quite nice for the species.

The Robert Mine, situated near Cuyuna in Crow Wing County, Minnesota, is a notable historic operation within the Cuyuna Iron Range. Geologically, the deposit is part of the Trommald Formation, a Paleoproterozoic banded iron formation characterized by layers of iron oxides, carbonates, and silicates that underwent significant folding and metamorphism. The ore bodies at the Robert Mine were particularly rich in manganese-bearing iron oxides, which formed through the secondary enrichment of the primary iron formation by circulating groundwaters. Historically, the mine was operated as an open pit for the extraction of manganiferous iron ore, with primary production taking place between 1951 and 1953. Although its period of industrial operation was relatively brief compared to larger mines in the region, the Robert Mine remains famous among mineralogists as the type locality for several rare species discovered within its manganese-rich mineral pockets.

Groutite is a rare manganese oxide mineral and is a member of the diaspore group. It was first described as a new species in 1947 from specimens found in the Cuyuna Range, including those from the Robert Mine. Geologically, it forms as a secondary mineral in weathered manganese-rich iron formations, typically occurring in vugs and cavities within hematite or goethite. The mineral is prized by collectors for its sharp, submetallic, jet-black to dark steel-grey crystals that often exhibit a distinctively brilliant luster and a characteristic wedge-shaped or bladed morphology. While groutite is chemically related to manganite, it crystallizes in the orthorhombic system, and its discovery at the Robert Mine provided critical insights into the complex mineralogy of the oxidation zones within Minnesotas iron deposits.

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