Dioptase: emerald-green and visible cleavage planes

Dioptase

• The name dioptase comes from the Greek words “dia” (= through) and “optos” (= visible), alluding to the visible cleavage planes inside the often highly transparent or translucent crystals
• Formula: CuSiO₃ · H₂O
• Space group: R-3 (No. 148)
• Crystal system: trigonal
• Crystal class: -3
• Lattice parameters: a = b = 14.566 Å, c = 7.778 Å, α = β = 90°, γ = 120°

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Dioptase Tsumeb

Picture: Didier Descouens – 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).)

View along the c axis.

• six SiO₄ tetrahedra (orange) are corner-connected to build a ring, so the mineral belongs to the cyclo silicates
• the copper atoms (blue) are octahedrally coordinated by six oxygen atoms (red), four of them are oxygens involved in SiO₄ tetrahedra and they occupy the equatorial positions of the CuO₆ octahedra and two of them are oxygen atoms of water molecules, which occupy the axial positions of the CuO₆ octahedra
• the CuO₆ octahedra are edge-connected and build a continuous 3D net

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