FIRST OCCURRENCE OF STRONTIAN K-CR-LOPARI TE AND CR-CHEVKINITE IN DIAMONDS

Ti-bearing radioactive minerals with exotic compositions containing up to 40% REE and 11% Cr occur as inclusions in diamonds from the River Ranch kimberlite pipe (Limpopo Mobile Belt, SE Africa). These radioact ive inclusions are identified as strontian K-Cr-loparite and Cr-chevki nite; both are partially replaced by secondary phases. The strontian K -Cr-loparite differs in composition, physical properties and unit cell parameters from air known minerals of the perovskite group. Potassium and chromium, common in synthetic compounds with a perovskite structu re, are reported for the first time in a naturally-occuring mineral fr om the perovskite group. The inclusions are surrounded by dark green r adioactive damage haloes developed at temperatures less then 600 degre es C. The exotic titanates may have been created by metasomatism genet ically associated with processes of kimberlite formation. The chemistr y of the radioactive inclusions reflects a geochemical specialization of metasomites and kimberlites, with enrichment in Ti, Sr, Nd, Th, F, Nb, K, extreme enrichment in light REE, overprinted on the chemistry o f the parent mantle.