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 [1][2][3]

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.[1]

Crystallography and properties

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.[3]

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.[4]

Discovery and occurrence

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."[2]

It occurs as a hydrothermal mineral in silver bearing veins in association with native silver, pyrargyrite, acanthite, stromeyerite, tetrahedrite, allemontite, galena, calcite and baryte.[1]

See also

References

  1. 1 2 3 Anthony, J.W., Bideau, R.A., Bladh, K.W., and Nichols, M.C. (1990) Handbook of Mineralogy: Volume I: Elements, Sulfides, Sulfosalts. 588 p. Mineral Data Publishing, Tucson, Arizona. http://rruff.geo.arizona.edu/doclib/hom/dyscrasite.pdf
  2. 1 2 Mindat.org
  3. 1 2 Webmineral data
  4. ↑ Klein, C., and Dutrow, B. (2007) The Manual of Mineral Science, 23rd ed. 675 p. John Wiley & Sons, Inc. Hoboken, New Jersey.
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