Nagyágite

Nagyágite (Pb₅Au(Te,Sb)₄S_5-8) is a rare sulfide mineral with known occurrence associated with gold ores. Nagyágite crystals are opaque, monoclinic and dark grey to black coloured.

Nagyágite
Nagyágite from Nagyág (Săcărâmb), Romania (image width: 1.5 mm)
General
Category	Sulfosalt mineral
Formula (repeating unit)	Pb5Au(Te,Sb)4S5-8[1] or AuPb(Sb,Bi)Te2-3S6[2] or (Te, Au)Pb(Pb, Sb)S2[3]
Strunz classification	2.HB.20a
Dana classification	02.11.10.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Identification
Colour	Blackish lead-grey; pale grey in polished section
Crystal habit	Tabular crystals (often bent), also massive granular, pseudotetragonal
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
Lustre	Metallic, bright on fresh cleavage
Streak	Blackish lead-grey
Diaphaneity	Opaque
Specific gravity	7.35–7.49
Pleochroism	Weak
References	[1][2][3]

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

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

References

1. http://www.mindat.org/min-2830.html Mindat.org
2. http://webmineral.com/data/Nagyagite.shtml Webmineral data
3. http://rruff.geo.arizona.edu/doclib/hom/nagyagite.pdf Handbook of Mineralogy