Mineral Information and Data:

FeAsS [CNMNC approved formula]

Named for its composition, a contraction of the antiquated term "arsenical pyrite" (1817). The mineral was known before IMA publication (considered grandfathered), but under other names: mispickel, arsenical pyrite. The current name is IMA approved (1960).

IMA status : Valid species (Pre-IMA 1960) Grandfathered
(8th edition) :  
  (02) Sulfides - Including Selenides and Tellurides
  (02.12) where Am Bn Xp, with (m+n):p=1:2
  (02.12.04) Arsenopyrite Group (Monoclinic: P21/c (Pseudo-orthorhombic))
(10th edition) :
  (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/m — prismatic
Unit cell: a = 5.74 Å, b = 5.68 Å, c = 5.79 Å, β = 112.3°
Crystal Habit: Crystals short prismatic, may be elongated parallel to c-axis or b-axis; massive, compact, granular
Twinning: Common on {100} or {001} on {101}, contact or penetration, on {012}, cruciform, star-shaped trillings
Color: Silver-white to steel-gray
Diaphaneity: Opaque
Luster: Metallic
Hardness (Mohs): 5.5 to 6
Measured Density: 6.07 g/cm3
Cleavage: 2; {101} distinct, {010} in traces
Tenacity: Brittle
Fracture: Uneven
Streak: Black
Geologic Setting: Of hydrothermal origin, typically one of the earliest minerals to form. Found in pegmatites, high-temperature gold-quartz and tin veins, and in contact metamorphic sulfide deposits; less commonly of low-temperature hydrothermal origin. Also in gneisses, schists and other metamorphic rocks.
Mineral Association: Pyrrhotite, pyrite, chalcopyrite, galena, gold, scheelite, cassiterite
Ore deposits: The most common arsenic mineral. The relatively low demand for arsenic as compared to the amount of arsenic mined that is associated with other metals means it can be supplied from the waste streams of other ore processing.
Synonyms/varieties: Arsenical Pyrites, Dalarnite
Comments: Arsenopyrite is closely related to alloclasite, but is not an exact analogue, because of different As-S and S-As ordering.


Arsenopyrite crystal: Sixling twin
Origin: Double Rainbow mine, Galena, Lawrence Co., South Dakota, U.S.A.
Sample size: Microscopic image
Photo and Owner: Lou Perloff


Arsenopyrite crystal: Cyclic twin
Origin: Alexander mine, Madoc, Ontario, Canada
Sample size: Microscopic image
Photo and Owner: Lou Perloff


Arsenopyrite with calcite, sphalerite and pyrite
Origin: Huanzala mine, Huallanca district, Dos de Mayo Province, Huanuco Department, Peru
Sample size: 13 x 20 x 6 cm matrix
Photo: Dan Weinrich


(zoom-in of above)


Origin: Zacatecas, Zacatecas, Mexico
Sample size: 6.3 x 3.4 x 3.3 cm
Photo: Dan Weinrich


(zoom-in of above)