Groutite

specimen number:
2062188

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

description:

An excellent example for the species from the type locality with bright lustrous sharp crystals to 0.3 cm in size across the front of matrix. This is in great condition.

The Robert Mine, located near Cuyuna in Crow Wing County, Minnesota, was a notable iron ore producer within the Cuyuna Iron Range. Geologically, the mine exploited the Precambrian-aged Trommald Formation, a complex unit of iron-bearing rocks characterized by high manganese content and intense folding within the Animikie Group. This specific geochemical environment, rich in manganese and oxygen, facilitated the formation of exceptional secondary minerals, most famously groutite, which was first discovered and described from this district. The groutite from the Robert Mine typically occurs as brilliant, jet-black, sub-metallic crystals with a distinct acicular or bladed habit, often lining cavities within the hematite and goethite ore. Historically, the Robert Mine was operated by the Hanna Mining Company as an open-pit facility, with its primary period of activity spanning from the early 1950s through 1973. While the mine was primarily an industrial source of manganiferous iron ore, it remains a site of immense mineralogical importance as the type locality for groutite. Today, the flooded pit is part of the Cuyuna Country State Recreation Area, and the site is remembered for providing some of the finest examples of manganese oxide crystals ever found in the United States.

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