Semseyite

Semseyite
Semseyite - Brioude-Massiac (Massif Central) France (11x3.5cm)
General
Category	Sulfosalt mineral
Formula (repeating unit)	Pb9 Sb 8 S21
Strunz classification	2.HC.10d
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 13.64 Å, b = 11.96 Å c = 24.46 Å; β = 105.87°; Z = 4
Identification
Color	Dark gray to black
Crystal habit	Tabular, elongated prismatic crystals, commonly twisted; rosettelike groups
Cleavage	Perfect on {112}
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Metallic
Diaphaneity	opaque
Specific gravity	6.15
References

Semseyite is a rarely occurring sulfosalt mineral and is part of the class of lead antimony sulfides. It crystallizes in the monoclinic system with the chemical composition Pb₉Sb₈S₂₁. The mineral forms dark gray to black aggregates.

Etymology and history

Semseyite was first described for an occurrence in the Felsöbánya mine in Baia Sprie, Romania in 1881 by József Sándor Krenner (1839–1920). The mineral was named after Hungarian mineralogist Andor von Semsey (1833–1923).

Occurrence

Semseyite forms in hydrothermal solutions at temperatures between 300 and 350 °C. It occurs in association with bournonite, jamesonite, sphalerite, zinkenite, sorbyite, guettardite, jordanite, diaphorite, galena, pyrite, chalcopyrite, tetrahedrite, arsenopyrite and siderite.

Images

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  Semseyite from Baia Sprie, Maramures County, Romania
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  Cluster of semseyite crystals on massive sulfide matrix
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  A spray of semseyite crystals perched, freestanding, at the apex of a cluster of quartz crystals
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  Vug filled with clusters of sprays of lightly iridescent, metallic-lustre semseyite blades on sphalerite matrix