You got familiar with chromium alum and aluminium alum in the previous article, where we explained how, due to the property of isomorphism of these two substances, we can prepare crystals of different intensity of purple colour, which comes from chromium (III) ions, by mixing their saturated solutions.
Today we are continuing to talk about these two chemical compounds. We will make crystals from them again, but these will be a bit more special and, certainly, more beautiful. The focus is on the so-called epitaxial crystals, i.e. crystals in which one substance crystallizes on a crystal surface of the other substance with which is isomorphous.
Sounds complicated? Actually, it isn’t at all… In order to get such crystals, all you need is first to grow a crystal of chromium alum, which will be dark purple, almost black. You will get it by leaving a saturated solution of chromium alum in an uncovered glass for several days at room temperature.
We take thus obtained crystal of chromium alum out of its solution, remaining after the crystallization, wipe it and move to a saturated solution of aluminium alum, which is colourless. We would expect thus placed crystal to start dissolving, however, something completely different will happen. A thin colourless layer will first form on its surface, which will grow in the following days, forming the same geometric body as the initial chromium alum crystal – a regular octahedron.
Over time, this colourless layer will become thicker and thicker, and as a final result we will get what we call a real epitaxial crystal of chromium alum and aluminium alum – the one that is dark purple, almost black, on the inside, and colourless on the outside.
Author of the text and photos: Alen Bjelopetrović, Dr.Sc.