The nature of the chalcogen colour centres in ultramarine-type solids
Blue minerals and gems have always been subjects of fascination to man. In particular lapislazuli is and was highly esteemed and also used as a precious stone and as a pigment. The host compound is the zeolite-type sodalite and the colour centres are S2– and S3–, whose amount ratio determines the specific hue, varying from violet blue to blue to green. But Se2– can also be substituted into the sodalite cages in high percentages, giving the pigment a brilliant red colour. It is even possible to incorporate the rather voluminous radical species Te2 and Te2– into the sodalite matrix. Because the otherwise unstable colour centres are completely screened from the atmosphere and isolated from each other in the cages, they can be fully characterised by various spectroscopic techniques.