Macfallite
specimen number:
2062238
location:
Manganese Mine, Keweenaw Co., Michigan, USA
description: The Manganese mine, situated just south of Copper Harbor in Grant Township within Keweenaw County, Michigan, was a small, highly unusual historic exploration and mining attempt situated at the northernmost tip of the Keweenaw Peninsula. Geologically, unlike the surrounding native copper deposits typical of the region, this locality consists of a narrow hydrothermally mineralized fissure vein cutting through the Mesoproterozoic Copper Harbor Conglomerate. The unique geochemical environment concentrated manganese and iron carbonates and oxides, which filled fissures and replaced calcite in lenses within the host basaltic and sedimentary rock. Historically, the deposit was first discovered in the 1830s by Michigan state geologist Douglass Houghton, but active development did not occur until 1881 when the Cambria Iron Company excavated an open cut to a depth of 75 feet, extracting roughly 800 tons of material. In 1883, a formal shaft was sunk to remove another 400 tons of ore for assaying, but metallurgical testing revealed the high-silica manganese ore was completely unsuitable for commercial steel production, causing the operations to be permanently abandoned later that year. Through 2026, while the historic open cut and shaft are entirely overgrown and inactive, the locality remains internationally famous among mineralogists as the global type locality for macfallite, a rare calcium manganese silicate hydroxide mineral officially named in 1979 that occurs on the old mine dumps as compact, radiating fibrous aggregates.
Rich crystallized macfallite throughout matrix. Collected in 1996.
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