Argentopyrite

Argentopyrite is a moderately rare sulfide mineral with the chemical formula AgFe₂S₃. It is one of the natural compounds of the MFe₂S₃ type, with M being caesium in very rare pautovite, copper in relatively common cubanite, potassium in rare rasvumite and thallium in rare picotpaulite. The type locality is Jáchymov in Czech Republic. Chemically similar mineral include sternbergite (dimorphous with argentopyrite), lenaite, AgFeS₂, and argentopentlandite, Ag(Fe,Ni)₈S₈.^[2]

Argentopyrite
Argentopyrite (from Schneeberg, Erzgebirge, Saxony, Germany)
General
Category	Sulfide mineral
Formula	AgFe2S3
IMA symbol	Agpy[1]
Strunz classification	2.CB.65
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P1121/n
Identification
Color	Bronze-brown or gray-white; iridescent (many colors)
Crystal habit	Tabular prisms, pseudohexagonal or pseudo-orthorhombic, possibly pyramidally-terminated
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	3.5-4
Luster	Metallic
Streak	Grey
Diaphaneity	Opaque
Density	4.25-4.27
References	[2][3][4]

Crystal structure

Although previously assumed orthorhombic,^[5] argentopyrite was later shown to be monoclinic, with structural relationship to cubanite. The most important feature of the argentopyrite structure are:^[6]

• hexagonal close-packing of sulfur atoms
• presence of AgS₄ and FeS₄ tetrahedra in sheets displaying corner-sharing
• presence of a cluster of four FeS₄ tetrahedra that share edges
• presence of two iron sites, instead of one as in related species
• ordered-disordered ferrous-ferric nature of the mineral