Optical and m�ssbauer study of minerals of the eudialyte group

Abstract

Twelve eudialyte specimens from the Khibini alkaline complex (Kola peninsula, USSR) have been studied by optical and Mossbauer spectroscopy methods. It has been found that in optically positive eudialytes Fe is mainly represented by Fe2+ ions in a rare planar four-fold coordination (Mossbauer hyperfine parameters — isomer shift IS=1.08 mm/s and quadrupole splitting QS=0.34 mm/s). It is these ions, not Mn3+ as it was assumed before, that are responsible for the crimson-red colour of eudialyte. In the optically negative red-brown and yellow-brown eudialytes (eucolites) Fe2+ ions are contained also in a tetragonal pyramid based on the above planar quadrangle (IS=1.33 mm/s and QS=2.24 mm/s). Major attention has been paid to an explanation of the colour and crystal-chemical features of optically positive in comparison with optically negative varieties of eudialytes.