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, α = β = γ = 90°
Picture: Rob Lavinsky, iRocks.com – CC BY-SA 3.0
Crystal structure (click on the picture to download the VESTA file):
- 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
The Fascination of Crystals and Symmetry 
In addition to occupying only 3/4 of the tetrahedral voids, the cations also seem to be displaced from the center of the voids to a different extent, or am I mistaken?
Furthermore, I’d like to add that “more order – lower symmetry” is quite a common phenomenon, for example when going from a phase where one site is fractionally occupied by two different species to another phase where the sites are occupied in an ordered fashion. The potassium feldspars sanidine and microcline are a textbook example of this (apologies if this is already covered elsewhere on this site).
Thanks, Michael, for your valuable comment. You are completely right, some cations are rather located in one of the trigonal planes of the tetrahedra (but I do not know exactly the reason for that).
Concerning the occurence or frequence of disorder-order transitions you are, of course, also right. The purpose of this blog entry was only to point to the – at first glance – slightly counter-intuitive recognition of both these categories, the relationship between “degree of order” and “amount of symmetry”.