Frankamenite in charoite rocks of the Murun massif: comparative characterization of green and lilac-gray varieties

   Research subject. Two varieties of frankamenite, green and lilac-gray, were discovered in the charoite rocks of the Murun massif.

   Aim. The aim of this study is a comparative analysis of lilac-gray and green frankamenite samples.

   Materials and Methods. The mineral composition of frankamenite-containing charoite rocks was studied by optical petrographic method using polarization microscope, the chemical composition was studied on electron probe microanalyzer. Crystal structure of frankamenite was studied using single crystal automatic diffractometer, and absorption and photoluminescence spectra were obtained on spectrophotometer and spectrofluorometer respectively.

   Results. The morphogenetic features of rock samples containing frankamenite, its paragenetic associations and relationships with associated minerals were studied. In terms of chemical composition, the Na₂O and CaO contents in the samples are almost identical to the previously studied Murun samples, while the K₂O level for lilac-gray and green frankamenite is higher than in the analyses of other authors. The crystal structure of green frankamenite was also studied, compared with the lilac-gray sample, and absorption and photoluminescence spectra were analyzed.

   Conclusions. Both varieties of the mineral differ in their associations of minerals: green frankamenite is usually associated with charoite, aegirine, microcline and quartz, while lilac-gray frankamenite is associated with charoite, amphibole, quartz, steacyite and apatite. Using EPR, optical absorption and photoluminescence methods, it was established that the green color of frankamenite is associated with Fe/Ti and Fe²⁺/Fe³⁺ charge transfer transitions.

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