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°

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

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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).)

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, α = β = 90°, γ = 120°

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Modified by CombineZP

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

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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).)

View along the c axis.

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, α = β = γ = 90°

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

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Crystal structure (click on the picture 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).)

• 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