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 precio us stone and as a pigment. The host compound is the zeolite-type sodalite a nd the colour centres are S-2(-) and S-3(-), whose amount ratio determines the specific hue, varying from violet blue to blue to green, But Se-2(-) ca n also be substituted into the sodalite cages in high percentages, giving t he pigment a brilliant red colour. It is even possible to incorporate the r ather voluminous radical species Te-2 and Te-2(-) 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 full y characterised by various spectroscopic techniques.