Alums are a distinct class of double salts described by a general formula M(I)M(III)(SO4)2x12H2O, where M(I) is a monovalent and M(III) is a trivalent metal. In addition to the formula, what all types of alum also have in common is the crystal structure – all alum types have cations, anions and water molecules distributed in space in the same way. That is why the shape of the crystals they build is identical, so all alums from aqueous solutions crystallize in the shape of a regular octahedron.
Two alums that can be mixed to produce interesting crystals are chromium alum, KCr(SO4)2x12H2O and aluminium alum, KAl(SO4)2x12H2O. Given that chromium(III) ions and aluminum(III) ions are approximately the same size, they can randomly replace each other in the crystal structure without disturbing the crystal lattice or the shape, i.e. crystal morphology. Substances whose crystal structures match so perfectly that they enable this phenomenon are called isomorphs.
In our example, by mixing different proportions of saturated solutions of aluminium alum and chromium alum, it is possible to obtain mixed-composition crystals (crystals in which there are both chromium ions and aluminum ions). Their composition can be expressed by the formula KAl1-xCrx(SO4)2x12H2O, where ‘x’ represents the mass fraction of chromium ions, and can range from 0 to 1.
Given that aluminium alum is colourless, and chromium alum is dark purple, by mixing them we can prepare solutions from which beautiful octahedral crystals of various shades will grow – from pale pink to deep purple, depending on the proportion of chromium alum in the mixture.
Some other mixed-composition crystals can be prepared in this way, which can be useful in the industry of producing filters that selectively remove some colours from the visible part of the spectrum, that is, they filter them. By varying the proportion of salts used, we can modify or “fine-tune” such filters as needed, and in addition to this, some grow them purely as a hobby, because as you can see in the photos, they are really beautiful.
Author of the text and photos: Alen Bjelopetrović, Dr. Sc.