Monthly Archives: December 2016

Orange-Red like Saffron: Crocoite

Crocoite

  • The name crocoite comes from the Greek “krokos” = saffron, alluding to the saffron-orange color of its powder
  • Formula: PbCrO4
  • Space group: P21/n (No. 14)
  • Crystal system: monoclinic
  • Crystal class: 2/m
  • Lattice parameters: a = 7.127 Å, b = 7.438 Å, c = 6.799 Å, α = 90°, β = 102.43°, γ = 90°

crocoite-360746_pd

Picture: Juergen Merzhttp://www.mindat.org/photo-360746.html
(This image has been released to the public domain.)


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

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

crocoite

View along the a axis.

Gray: Pb, red: O, orange: CrO4 tetrahedra

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Cinnabar: Red as dragon blood and optically active

Cinnabar

  • probably named after the Persian word for dragon blood because of its characteristic red colour
  • used since ancient times as a pigment
  • Formula: HgS
  • Space group: P3121 (No. 152) or P3221 (No. 154)
  • Crystal system: trigonal
  • Crystal class: 32
  • Lattice parameters: a = b = 4.1347(6) Å, c = 9.4451(3) Å, αβ = 90°, γ = 120°

Modified by CombineZP

Modified by CombineZP

Picture: JJ Harrison (jjharrison89@facebook.com) – CC BY-SA 3.0


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

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

cinnabar_01

View along the c axis.

cinnabar_02

View along the a axis.

  • the crystals structure is characteristic of chains with alternating sulfur and mercury atoms, which form helices with the helix axis running parallel to the c axis.
  • These helices are also the reason why crystals of cinnarbar are optically active (see: A.M. Glazer, K. Stadnicka, “On the origin of optical activity in crystal structures”. J. Appl. Cryst. 19 (2), 1986, 108–122. doi:10.1107/S0021889886089823

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