OPTICAL-DATA ON MINERALS OF THE EUDIALYTE GROUP - DISCUSSION OF THE EUCOLITE-MESODIALYTE-EUDIALYTE TERMINOLOGY

21 samples of eudialyte minerals with high quality electron microprobe data have been investigated by means of the microrefractometer spindl e-stage technique. The object of the study is to determine the most si gnificant correlations between chemical composition and optical proper ties of this group and on this basis evaluate the eudialyte-mesodialyt e-eucolite terminology. The measured refractive indices and birefringe ncies fall within the ranges known from the literature. The majority o f the samples are optically negative with a low birefringence. The Na content has no detectable influence on the mean refractive index (n_me an), while Ca shows a negative correlation to n_mean, opposite that su ggested in connection to an alleged Na reversible arrow Ca isomorphous series in the group. The most significant correlations involving n_me an are positive ones to Dr and to Sigma Nb,REE,Mn,F the kentbrooksite end-member component. The birefringence shows a good positive correlat ion to the quantity Nb/(Nb + exSi) that signifies the occupation of th e sites in the middle of the nine-fold rings. This quantity can howeve r not be sorted out from the complex substitution found in the eudialy te-kentbrooksite series. It is concluded that the term eucolite, which has been used for an optically negative and Ca-rich eudialyte, is mea ningless and in some respect misleading, so the use of this term shoul d be discontinued. Mesodialyte as a name for an optically isotropic in termediate of an inferior if at all existing series should also be abo lished.