© 1998 by Udo J.A. Behner
© 1998 by Udo J.A. Behner
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 aquaeous 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.