in form of a cube with small octahedron faces containing an octahedron of "chromium alum".
chemical formula
: KAl(SO4)2 · 12 H2O
crystal class
: cubic disdodekaedric
growing method
: evaporation of the aquaous solution
growing time
: several months
crystal size
: app. 4 centimeters
Alum is one of the oldest and best known substances for hobby crystal growth and even the ancient Romans knew that
alum not only crystallizes in octahedrons but also in cubes.
The common recipe was to ad some alkali to the solution to rise the pH-value. This often results in nothing
but no cubes and/or a slimy mess of aluminum hydroxide etc.
More powerfull and better to use are some organic compounds used as surfactants like sodium dodecylbenzolsulfonate.
Also some organic dyestuffs (which are sulfonated to make them more water soluble) have a strong capability of changing
crystal forms.
To be honest I have to admit that this specimen was not grown by me but by Gerald Bolanz who did a lot of research
in this field for his "Jugend Forscht" work. Only slow growth at temperatures arround or lower then 20 °C will allow the
surfactant to slow down material build up at the cubic crystal faces - as always the crystal faces with the slowest material
build up will grow biggest (It may be paradox on the first view but its the way it is !).
To grow this real unusual example and to make the change from octahedron to cube quite obvious a seed crystal of
"chromium alum" (e.g. a mixed crystal of potassium alum and chromium alum) was used.
This specimen is currently exhibited at the mineral museum in Pforzheim.
