Groutite

specimen number:
8311513

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

description:
Lustrous black crystals of grout tight across the top of matrix. Good thumbnail for the species. In good condition.

The Robert mine, located near Kennedy in Crow Wing County within the Cuyuna North Range of Minnesota, United States, was a notable underground and later open-pit operation celebrated by collectors for producing some of the world's finest lustrous, jet-black crystals of the rare manganese oxide mineral groutite. Geologically, this deposit is hosted within the middle Precambrian Trommald Formation of the Animikie Group, where tightly folded, highly metamorphosed banded iron formations underwent extensive alteration and localized manganese enrichment. The mineralization created pockets, vugs, and late-stage fractures filled with secondary manganese and iron oxides, allowing groutite to form as distinct, highly lustrous, wedge-shaped or lens-shaped orthorhombic crystals associated with goethite, hematite, and manganite. Historically, the property was initially developed as an underground operation by the Rogers-Brown Ore Company during the early boom years of the Cuyuna Range, with shipments beginning around the 1910s and shifting to open-pit quarrying as technology advanced to extract critical manganese-rich iron ores for wartime steel production. The mine operated intermittently under various managers, including Pickands Mather and Company, through the mid-twentieth century until the depletion of high-grade natural ores caused operations to cease by the 1960s, mirroring the general decline of the district before its eventual incorporation into the local state recreation area.

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