A microspecimen of the extremely rare Ag-Au-telluride mineral muthmannite from the type locality in Nagyag (Sacaramb, Romania) in an analyzed specimen by SEM/EDS. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Muthmannite
Muthmannite is an IMA-recognized mineral species in the sulfide class, with the chemical formula AuAgTe₂.
It crystallizes in the monoclinic crystal system (space group P2/m).
Key properties: Mohs hardness 2.5; specific gravity 11.04; Black to gray-black streak; Bright metallic luster.
IMA symbol Mma, grandfathered status — known before the IMA’s founding, named in 1911.
Found in close intergrowths with other tellurides, especially krennerite, in epithermal hydrothermal veins, and in ores enriched by secondary processes (Săcărı̂mb, Romania); replacing calaverite (Baia-de-Arieş, Romania).
It is found associated with Krennerite, stibnite, hessite, nagyágite, petzite, pyrargyrite, sylvanite, calaverite, pyrite, sphalerite, altaite, tetrahedrite–tennantite, alabandite.
The mineral was named to honor the chemist and crystallographer, Professor Friedrich Wilhelm Muthmann (1861–1913), Technische Hochschule, Munich, Germany.