Antarcticite

• named after the type locality – the Don Juan Pond in Antarctica
• Antarcticite is very hygroscopic and is formed only in very arid regions and precipitates only from highly saline brines
• Formula: CaCl₂  · 6 H₂O
• Space group: P321
• Crystal system: trigonal
• Crystal class: 32
• Lattice parameters: a = b =  7.8759(2) Å, c = 3.9545(2) Å, α = β = 90°, γ =  120°

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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).)

• every Ca ion is surrounded by nine water molecules
• the CaO₉ polyhedra (blue) are face-connected and stacked along the c axis
• these faces are made up of three water molecules  acting as bridging ligands between two Ca ions
• every Cl anion is involved in 6 H-bonds (dotted lines)
• Oxygen (red)
• Chlorine (green)
• Hydrogen (white)

For a 3D interactive version, see here:

https://skfb.ly/6PJXV

Refs:

[1] T. Torii, J. Ossaka, Science 1965, 149, 975-977.
DOI: 10.1126/science.149.3687.975

[2] P. A. Agron, W. R. Busing, Acta Crystallogr. C 1986, 42, 141-143.
DOI: 10.1107/S0108270186097007

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