Nagyagite from Nagyag (Săcărâmb), Romania (image width: 1.5 mm)
Category Sulfosalt mineral
Chemical formula Pb5Au(Te,Sb)4S5-8[1] or AuPb(Sb,Bi)Te2-3S6[2] or (Te, Au)Pb(Pb, Sb)S2[3]
Strunz classification 02.HB.20a
Dana classification
Colour Blackish lead-grey; pale grey in polished section
Crystal habit Tabular crystals (often bent), also massive granular
Crystal system Monoclinic, pseudotetragonal. Point Group: 2/m.
Twinning Crossed twin lamellae observed on (001) sections
Cleavage Perfect on {010}, excellent on {101}
Fracture Hackly
Tenacity Flexible, slightly malleable
Mohs scale hardness 1.5
Luster Metallic, bright on fresh cleavage
Streak Blackish lead-grey
Diaphaneity Opaque
Specific gravity 7.35–7.49
Pleochroism Weak
References [3][1][2]

Nagyagite (Pb5Au(Te,Sb)4S5-8) is a rare sulfide mineral with known occurrence associated with gold ores. Nagyagite crystals are opaque, monoclinic and dark grey to black coloured.

It was first described in 1845 for an occurrence at the type locality of the Nagyag mine, Sacarîmb, Hunedoara, Romania.[1][2]

It occurs in gold–tellurium epithermal hydrothermal veins. Minerals associated with nagyagite include: altaite, petzite, stutzite, sylvanite, tellurantimony, coloradoite, krennerite, native arsenic, native gold, proustite, rhodochrosite, arsenopyrite, sphalerite, tetrahedrite, calaverite, tellurobismuthite, galena and pyrite.[3]