Infrared and Raman spectroscopic characterization of the carbonate mineral huanghoite - and in comparison with selected rare earth carbonates.

Abstract

Raman spectroscopy complimented with infrared spectroscopy has been used to study the rare earth based mineral huanghoite with possible formula given as BaCe(CO3)2F and compared with the Raman spectra of a series of selected natural halogenated carbonates from different origins including bastnasite, parisite and northupite. The Raman spectrum of huanghoite displays three bands are at 1072, 1084 and 1091 cm^-1 attributed to the (CO3)^2- symmetric stretching vibration. The observation of three symmetric stretching vibrations is very unusual. The position of (CO3)^2- symmetric stretching vibration varies with mineral composition. Infrared spectroscopy of huanghoite show bands at 1319, 1382, 1422 and 1470 1091 cm^-1. No Raman bands of huanghoite were observed in these positions. Raman spectra of bastnasite, parisite and northupite show a single band at 1433, 1420 and 1554 1091 cm^-1 assigned to the m3 (CO3)^2- antisymmetric stretching mode. The observation of additional Raman bands for the m3 modes for some halogenated carbonates is significant in that it shows distortion of the carbonate anion in the mineral structure. Four Raman bands for huanghoite are observed at 687, 704, 718 and 730 1091 cm^-1 and assigned to the (CO3)^2- m2 bending modes. Raman bands are observed for huanghoite at around 627 1091 cm^-1 and are assigned to the (CO3)^2- m4 bending modes. Raman bands are observed for the carbonate m4 in phase bending modes at 722 1091 cm^-1 for bastnasite, 736 and 684 1091 cm^-1 for parisite, 714 1091 cm^-1 for northupite. Raman bands for huanghoite observed at 3259, 3484 and 3589 1091 cm^-1 are attributed to water stretching bands. Multiple bands are observed in the OH stretching region for bastnasite and parisite indicating the presence of water and OH units in their mineral structure. Vibrational spectroscopy enables new information on the structure of huanghoite to be assessed.