Dyscrasite

Dyscrasite
Twinned dyscrasite crystals from the Czech Republic (size: 4.5 x 4.5 x 3.3 cm)
General
Category	Antimonide minerals
Formula (repeating unit)	Ag3Sb
Strunz classification	2.AA.35
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Pmm2
Unit cell	a = 3.008, b = 4.828 c = 5.214 [Å]; Z = 1
Identification
Color	Silver-white (tarnishes to lead-gray, yellowish, or black)
Crystal habit	Pyramidal crystals also cylindrical, prismatic to platy, striated; granular, foliated or massive
Twinning	On {110} produces pseudohexagonal forms
Cleavage	Distinct on {001} {001}, imperfect on {110}
Fracture	Irregular or uneven
Tenacity	Sectile
Mohs scale hardness	3½ - 4
Luster	Metallic
Streak	Silver-white
Diaphaneity	Opaque
Specific gravity	9.4 - 10
Birefringence	Very Weak
Pleochroism	Very Weak
References

The silver antimonide mineral dyscrasite has the chemical formula Ag₃Sb. It is an opaque, silver white, metallic mineral which crystallizes in the orthorhombic crystal system. It forms pyramidal crystals up to 5 cm (2.0 in) and can also form cylindrical and prismatic crystals.

Dyscrasite is a metal ore and is opaque. In reflected light, however, it demonstrates weak anisotropism. Dyscrasite’s color under plane polarized light is most likely dark grey/black. When spun on a rotatable stage of a microscope (under plane polarized light), dyscrasite’s color should slightly change shades. This property is called pleochroism. Dyscrasite exhibits very weak reflected light pleochroism.

Dyscrasite belongs to the orthorhombic crystal class, meaning all three of its axes (a, b, and c) are unequal in length and are 90° to each other.

It was first described for an occurrence in 1797 in the Wenzel Mine, Black Forest, Germany. The name dyscrasite comes from the Greek word δυσκράσις, meaning "a bad alloy."

It occurs as a hydrothermal mineral in silver bearing veins in association with native silver, pyrargyrite, acanthite, stromeyerite, tetrahedrite, allemontite, galena, calcite and baryte.