Dufrénoysite

Formula:   Pb₂As₂S₅

Origin of name:   For Ours Pierre Armand Petit Dufrenoy (1792-1857), French mineralogist, National School of Mines, Paris

Crystal system:   Monoclinic

Crystal class:   2

Twinning:   On {001}

Unit cell: a = 8.41 Å, b = 25.85 Å, c = 7.88 Å, β = 90.5°

Colour:   Steel-gray to lead gray

Diaphaneity: Subtranslucent

Luster:   Metallic

Habit:   Crystals tabular, somewhat elongated, sometimes striated parallel elongation

Hardness:   3

Specific gravity:   5.53

Cleavage:   1; {010} perfect

Tenacity:   Conchoidal fracture; brittle

Streak:   Reddish brown to chocolate brown

Synonyms/varieties:   Gottardite, scleroclase

Comments: Found as an uncommon mineral of moderate to low-temperature hydrothermal origin, in association with minerals such as chalcopyrite, sphalerite, realgar, orpiment and tetrahedrite, also  occurs in crystalline dolomite (Binntal, Switzerland)