Quantitative evaluation of metamictisation of columbite-(Mn) from rare-element pegmatites using Raman spectroscopy

Raman spectroscopic analysis was performed on columbite-(Mn) samples from a variety of previously studied rare-element pegmatites in Xinjiang, China, including the Jing'erquan No. 1 spodumene-subtype, Dakalasu No. 1 beryl-columbite-subtype and Kalu'an spodumene-subtype pegmatites, to quantify the relationship between the degree of metamictisation of columbite and Raman spectra. For all of the analysed columbites-(Mn), the position (p) and the full width at half maximum (FWHM) of the strongest band, A(1g) vibration mode related to the Nb/Ta-O bond, in the Raman spectra have a negative correlation. Combined with previously determined U-Pb isotopic data and major-minor-element data for the columbites-(Mn), the degree of metamictisation was quantified using the alpha-decay dose (D) and displacement per atom (dpa), both of which were corrected for effects caused by annealing. The results demonstrate that the columbite-(Mn) from Jing'erquan and Kalu'an are very crystalline, whereas those from Dakalasu are transitional between crystalline and amorphous stages. The main factor influencing the key parameters, i.e. band position and FWHM, of the strongest Raman band of columbite-(Mn) is metamictisation caused by radiation damage, whereas composition and crystal orientation have limited influence. A set of equations are established to quantify the degree of metamictisation of columbite using the band position and the full width at half maximum: FWHM = 8.309 x ln(D-a) + 30.11 (R-2 = 0.9861); p = -5.187 x ln(D-a) + 867.09 (R-2 = 0.966); FWHM = 8.1453 x ln((a)dpa) + 48.425 (R-2 = 0.9822); and p = -5.078 x ln((a)dpa) + 855.67 (R-2 = 0.9594).