What does it look like?
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
53C the crystals will be dark blue-green, and above 53C 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.
NiSO4.6H20 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 0.5um. In a fascinating paper, Stadnicka, Glazer and
Koralewski  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
Where did the structure come from?
The structure of the lower-temperature phase was first determined in
1932 by Beevers and Lipson . 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.
 Stadnicka, K., Glazer, A.M. and Koralewski, M. (1987), Acta Cryst.B43, 319-325
 Beevers, C. A. and Lipson, H. (1932) Z. Kristallogr. 83, 123-135
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