Crystals you can grow at home – Nickel Sulphate Hexahydrate

What does it look like?

Spot the helix! Drawn using Jmol.

What is it?

Nickel sulphate hexahydrate (NiSO₄.6H₂O) forms large, perfect crystals that can easily be grown at home – just dissolve plenty of NiSO⁴ in water and let the water evaporate. You can see a beautiful example here. As an extra bonus, by choosing the appropriate temperature you can get slightly different coloured crystals – up to 53 °C the crystals will be dark blue-green, and above 53 °C they will be clear green. These colours correspond to slightly different structures, and this is an apparently unique situation, as similar sulphate compounds are only stable as a single structure type. As might be expected for something so easily grown, it is found in nature as the mineral retgersite.

NiSO₄.6H₂O is also interesting because of its ability to rotate the direction of polarisation of light – and the strength of this rotation depends on the wavelength of the light, even switching rotation direction at 0.5 μm. In a fascinating paper, Stadnicka, Glazer and Koralewski [1] were able to relate the direction and strength of the optical rotation to the properties of the structural helix formed by the O atoms. The helix goes from top to bottom on the structural diagram above.

Where did the structure come from?

The structure of the lower-temperature phase was first determined in 1932 by Beevers and Lipson [2]. It was sufficient to classify X-ray reflections into 8 intensity classes ranging from "very strong" to "absent" in order to derive and verify their model. Both forms have since been extensively studied by X-ray and neutron diffraction.

[1] Stadnicka, K., Glazer, A.M. and Koralewski, M. (1987), Acta Cryst. B43, 319-325 http://dx.doi.org/10.1107/S0108768187097787

[2] Beevers, C. A. and Lipson, H. (1932) Z. Kristallogr. 83, 123-135 (http://rruff.info/doclib/zk/vol83/ZK83_123.pdf)

Further reading

Angel, R. J. and Finger, L. W. (1988) “Polymorphism of Nickel Sulfate Hexahydrate”, Acta Cryst. C44, 1869-1873 http://dx.doi.org/10.1107/S0108270188006717

Tags: crystals   sulfate   inorganic