Macfallite Locality: Manganese Mine, Copper Harbor , Keweenaw County , Michigan , USA ( Locality at mindat.org ) Size: miniature, 4 x 2.3 x 2.1 cm Macfallite A mostly massive specimen with some embedded crystals as well, showing. Rare old material, this is a calciam manganese, aluminum silicate. Never seen another for sale. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Macfallite
Macfallite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₂Mn³⁺₃(SiO₄)(Si₂O₇)(OH)₃.
It crystallizes in the monoclinic crystal system (space group P2_1/m).
Key properties: Mohs hardness 5–5.5; specific gravity 3.43; Brown with a reddish tint streak; Silky to subadamantine luster; optically biaxial.
IMA symbol Mcf, IMA status: approved, named in 1974.
Found in abundance, replacing calcite in fissures and in lenses in basalt (Manganese Lake, Michigan, USA); replacing braunite under low-temperature metamorphic conditions (Faggiona, Italy).
It is found associated with Manganite, braunite, orientite, pyrolusite (Manganese Lake, Michigan, USA); braunite, quartz, manganoan richterite, carbonates (Faggiona, Italy).
The name refers to Russell P. MacFall, American amateur mineralogist.