Mineral Information and Data:

Alloclasite
CoAsS [CNMNC approved formula]

Named from the Greek for "other", and "to break," because its cleavage was believed to be different from marcasite, which it resembles.

IMA status : Valid species (Pre-IMA 1866) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
02.12.06.02  
  (02) Sulfides - Including Selenides and Tellurides
  (02.12) where Am Bn Xp, with (m+n):p=1:2
  (02.12.06)
  
Nickel-Strunz
(10th edition) :
02.EB.10b
  (02) Sulfides
  (02.E) Metal Sulfides, M:S = 1:2
  (02.EB) M:S = 1:2, with Fe, Co, Ni, PGE, etc.
  
Crystal system: Monoclinic System
Point group (H-M): 2 — sphenoidal
Unit cell: a = 4.661 Å, b = 5.602 Å, c = 3.411 Å, b = 90.2°
Crystal Habit: Crystals short prismatic; massive; columnar to hemispherical aggregates.
  
Color: Steel-gray to silver
Diaphaneity: Opaque
Luster: Metallic
Hardness (Mohs): 5
Measured Density: 5.91–5.95 g/cm3
Cleavage: 2; {101} perfect; {010} distinct.
Tenacity: Brittle
Fracture: Uneven to subconchoidal
Streak: Nearly black
  
Polymorphism and Series: Dimorphous with glaucodot.
Geologic Setting: In calcite or quartz veins of apparently low-temperature, late-stage hydrothermal origin. Also in silicified, recrystallized metamorphic rock.
Mineral Association: Gold, silver, glaucodot, cobaltite, cobaltian arsenopyrite, emplectite, bismuthinite, sphalerite, calcite, quartz
  

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