Infrared and Raman spectroscopic characterization of the carbonate mineral weloganite - Sr3Na2Zr(CO3)6 3H2O and in comparison with selected carbonates.

Abstract

The mineral weloganite Na2Sr3Zr(CO3)6_3H2O has been studied by using vibrational spectroscopy and a comparison is made with the spectra of weloganite with other carbonate minerals. Weloganite is member of the mckelveyite group that includes donnayite-(Y) and mckelveyite-(Y). The Raman spectrum of weloganite is characterized by an intense band at 1082 cm_1 with shoulder bands at 1061 and 1073 cm_1, attributed to the CO2_ 3 symmetric stretching vibration. The observation of three symmetric stretching vibrations is very unusual. The position of CO2_ 3 symmetric stretching vibration varies with mineral composition. The Raman bands at 1350, 1371, 1385, 1417, 1526, 1546, and 1563 cm_1 are assigned to the m3 (CO3)2_ antisymmetric stretching mode. The observation of additional Raman bands for the m3 modes for weloganite is significant in that it shows distortion of the carbonate anion in the mineral structure. The Raman band observed at 870 cm_1 is assigned to the (CO3)2_ m2 bending mode. Raman bands observed for weloganite at 679, 682, 696, 728, 736, 749, and 762 cm_1 are assigned to the (CO3)2_ m4 bending modes. A comparison of the vibrational spectra is made with that of the rare earth carbonates decrespignyite, bastnasite, hydroxybastnasite, parisite, and northupite.