Endellionite

Bournonite
Bournonite from Viboras mine, Machacamarcha, Bolivia, 95 mm x 74 mm, largest crystal size: 29 mm
General
Category	Sulfosalt mineral
Formula (repeating unit)	PbCuSbS3
Strunz classification	2.GA.50
Dana classification	3.4.3.2
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Pn21m
Identification
Color	Steel-gray to iron-black
Crystal habit	Crystals short prismatic to tabular, typically striated; commonly as subparallel aggregates. Also massive, granular to compact
Twinning	On {110}, commonly forming cross or cogwheel aggregates
Cleavage	[010] Imperfect
Fracture	Subconchoidal to uneven
Mohs scale hardness	2.5 - 3.0
Luster	Brilliant to dull
Streak	Steel-gray to iron-black
Diaphaneity	Opaque
Specific gravity	5.7 - 5.9
Pleochroism	Very weak
References

Bournonite is a sulfosalt mineral species, trithioantimoniate of lead and copper with the formula PbCuSbS₃.

It was first mentioned by Philip Rashleigh in 1797 as an ore of antimony and was more completely described in 1804 by French crystallographer and mineralogist Jacques Louis, Comte de Bournon (1751–1825), after whom it was named. The name given by Bournon himself (in 1813) was endellione, since used in the form endellionite, after the locality in Cornwall where the mineral was first found.

The crystals are orthorhombic, and are generally tabular in habit owing to the predominance of the basal pinacoid; numerous smooth bright faces are often developed on the edges and corners of the crystals. They are usually twinned, the twin-plane being a face of the prism (m); the angle between the faces of this prism being nearly a right angle (86° 20′), the twinning gives rise to cruciform groups and when it is often repeated the group has the appearance of a cog-wheel, hence the name Rãdelerz (wheel-ore) of the Kapnik miners. The repeated twinning gives rise to twin-lamellae, which may be detected on the fractured surfaces, even of the massive material.

It is a mineral in medium temperature hydrothermal vein deposits. It commonly occurs with galena, tetrahedrite, sphalerite, chalcopyrite, pyrite, stibnite, zinkenite, siderite, quartz, rhodochrosite, dolomite and barite.