Mineral Information and Data:

AgFe2S3 [CNMNC approved formula]

Named for its composition and physical similarity to pyrite.

IMA status : Valid species (Pre-IMA 1866) Grandfathered
(8th edition) :  
  (02) Sulfides - Including Selenides and Tellurides
  (02.09) where Am Bn Xp, with (m+n):p=1:1
(10th edition) :
  (02) Sulfides
  (02.C) Metal Sulfides, M:S = 1:1 (and similar)
  (02.CB) With Zn, Fe, Cu, Ag, etc.
Crystal system: Tetragonal System
Point group (H-M): 4/m — dipyramidal
Unit cell: a = 6.64 Å, b = 11.46 Å, c = 6.45 Å
Crystal Habit: Minute pseudo-hexagonal crystals
Twinning: pseudo-hexagonal twins parallel [100] by interpenetration of {120}, lamellar twinning also present
Color: Steel-grey to tin-white; tarnishes bronze-brown, green
Diaphaneity: Opaque
Luster: Metallic
Hardness (Mohs): 3.5 - 4
Measured Density: 4.25 g/cm3
Tenacity: Brittle
Fracture: Uneven
Polymorphism and Series: Dimorphous with sternbergite
Geologic Setting: Rare in silver-bearing hydrothermal veins.
Mineral Association: Arsenic, proustite, pyrargyrite, pyrostilpnite, xanthoconite, sternbergite, stephanite, pyrite, nickel-skutterudite, dolomite, calcite, quartz


This specimen contains several crystals of argentopyrite to about 1 mm showing cyclic twinning producing stout pseudohexagonal crystals with a violet tarnish. The pryramidal terminations are sometimes rough - but not all are rough, some are perfect. The sternbergite is also fabulous as thick tabular, striated crystals to 1 mm with a goldish tarnish. It is obviously much softer than the argentopyrite and foil like.
Origin: Jáchymov, Erzgebirge, Západoèeský Kraj , Bohemia, Czech Republic
Sample size: 3.5 x 3 x 2.3 cm
Photo: Tom Loomis


(zoom-in of above)