Hydrated copper(II) sulfate 1: introduction

Hydrated copper(II) sulfate exists in nature as the mineral chalcanthite. It is one of only a few water-soluble sulfate minerals and so is most commonly found in arid regions. The crystals form very readily from solution and are an attractive brilliant blue, which makes it very easy to fake natural specimens. (Indeed most people have grown a copper sulfate crystal at some point in their school life.) Copper sulfate is poisonous though.

A fragment of the bulk structure of hydrated copper(II) sulfate is shown to the left, with a black wireframe indicating the triclinic unit cell. Copper ions are shown in brown, sulfur atoms in yellow, oxygens in red and the hydrogens in black. Both covalent bonds and the coordinating bonds to copper are shown in blue. Each copper ion in the bulk has six oxygen atoms coordinated to it: four from water in equatorial positions, and two from sulfate in axial positions. Sulfate ions in the bulk each coordinate to two copper ions. Of the five waters of crystallisation in the formula unit, one is not coordinated to the copper ion. See if you can spot these waters in the structure on the left.

The unit cell in this structure is in the triclinic class, which is the least symmetrical type. All of the lengths of the three types of cell edge are different; the three types of angle in the cell are also all different and not equal to 90°. It is not possible use the symmetry elements of rotation, reflection and improper rotation (see cyclohexane for what this means) alone to demonstrate that the copper ions, for example, are in equivalent positions in the lattice. Instead one needs to use symmetry operations that involve tranlations too. (Translations combine with reflections to form glide planes, and combine with rotations to form screw axes.)

Go to page 2 to focus on the waters of crystallisation in the unit cell that are not coordinated to the copper ions.

Other elements/compounds with a triclinic unit cell:

K₂Cr₂O₇, BF₃, FeSO₄.5H₂O, P₄, turquoise (CaAl₆(PO₄)₄(OH)₈.5H₂O), wollastonite (CaSiO₃), kaolinite (Al₂Si₂O₅(OH)₄), kyanite (Al₂SiO₅), sanidine (KAlSi₃O₈), anorthite (CaAl₂Si₂O₈), albite (NaAlSi₃O₈), okenite (CaSi₂O₄(OH)₂.H₂O), suolunite (Ca₂Si₂O₅(OH)₂.H₂O) and tarbuttite (Zn₂(PO₄)OH).

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