Structure
Ultramarine is based on the sodalite structure which is a 3 dimensional aluminosilicate cage containing 3 sulfur atoms bonded together to form an ion. These ions are charge balanced by cations of sodium in the natural material. The sodium ions can be ion exchanged with lithium and potassium as described above. The modification of the ions has a dramatic effect on the structure of the cages. Lithium being smaller than sodium causes the cage to contract whilst potassium being large causes the cage to expand. The modification of the cage structure and the interaction of the different cations with the central sulfur species modifies the coloration of the final pigment.
By treating blue ultramarine with silver nitrate solution, "silver-ultramarine" is obtained as a yellow powder. This compound gives a blue potassium- and lithium-ultramarine when treated with the corresponding chloride, and an ethyl-ultramarine when treated with ethyl iodide. Selenium- and tellurium-ultramarine, in which these elements replace the sulfur, have also been prepared.
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