Macfallite

specimen number:
2062282

location:
Manganese Mine, Keweenaw Co., Michigan, USA

description:

Rich crystallized in massive macfallite throughout matrix.. An example for the species. Collected in 1996.

The Manganese mine, situated just south of Manganese Lake near Copper Harbor in Keweenaw County, Michigan, was a small-scale, specialized underground exploration and mining venture focused on extracting manganese oxides rather than the native copper typical of the region. Geologically, the deposit consists of a narrow, steeply dipping hydrothermal fissure vein cutting through the Mesoproterozoic Copper Harbor Conglomerate. Low-temperature, manganese-rich hydrothermal fluids filled these fissures, replacing early calcite and cementing the conglomerate pebbles with dense assemblages of manganite, pyrolusite, and braunite. The vein was first discovered in 1863, leading to the organization of the Buffalo Mining Company, which conducted minor exploration before selling the property. Commercial extraction occurred primarily during a brief period of development between 1881 and 1883 when a shaft was sunk to a depth of roughly 100 feet to extract ore for steel production, but operations were quickly abandoned due to the limited extent of the vein and high transportation costs.

Macfallite is a rare manganese silicate mineral that crystallizes in the monoclinic system, typically forming dense, radiating aggregates of acicular or prismatic crystals. Geologically, it occurs as a secondary mineral within highly oxidized, strata-bound manganese deposits, typically developing via low-grade metamorphism or hydrothermal alteration of primary manganese oxide minerals. The mineral is characterized by a sub-metallic to vitreous luster, a Mohs hardness of approximately 5.5, and a distinct reddish-brown to dark brown or near-black coloration. It was originally discovered in the Keweenaw Peninsula of Michigan, USA, and officially described in 1979, with its type locality situated at the Manganese mine near Copper Harbor, where it closely associates with other specialized manganese minerals like braunite, orientite, and pyrolusite.

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