Tag Archives: Buntkuperkies

Bornite: Why order sometimes means less symmetry

Bornite

  • named after the Austrian mineralogist Ignaz von Born
  • also known as peacock ore because in air this mineral forms very quickly an iridescent coverage on its surface
  • Formula: Cu5FeS4
  • Space group: Pbca (No. 61)
  • Crystal system: orthorhombic
  • Crystal class: mmm
  • Lattice parameters: a = 10.950 Å, b = 21.862 Å, c = 10.950, αβγ = 90°

bornite_irocks

Picture: Rob Lavinsky, iRocks.com – CC BY-SA 3.0


Crystal structure (click on the picture to download the VESTA file):

bornite

(K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data,” J. Appl. Crystallogr., 44, 1272-1276 (2011).)

  • yellow: Sulfur, blue: Copper, orange: Iron
  • The structure is based on a cubic-closest packing of sulfide ions; the copper and iron ions are located in 3/4 of the tetrahedral voids of the packing. At temperatures above 228 °C the cations are completely randomly distributed over these sites (the crystallographer names it disorder), forming a genuine (isometric) cubic phase, but in the low temperature modification the cations are more ordered, which is accompanied with the symmetry reduction to the orthorhombic crystal system