Fresh and weathered pyrochlore studies by Fourier transform infrared spectroscopy coupled with thermal analysis

Fresh and weathered pyrochlore from the Lueshe carbonatite complex (in the northeast of the Democratic Republic of Congo) was studied by Fourier trans fom infrared (FTIR) spectroscopy and by a combination of FTIR spectroscopy and thermal analysis. The former was carried out in the spectral range 400- 4000 cm(-1). The spectra for fresh and weathered pyrochlores were very diff erent. For the weathered pyrochlore, two bands were identified as OH vibrat ion modes, one broad band with a maximum at 3413 cm(-1) and another finer b and at 1630 cm(-1). The fresh pyrochlore does not show OH absorption bands. The presence of OH confirms the hydrated state of the weathered pyrochlore suggested by previous microanalytical work. The combination of FTIR spectr oscopy and thermal analysis allows 'real-time' observation of gas emanation s and solid-state transformations taking place during heating up to 800 deg rees C. For fresh pyrochlore, no solid transformation was detected, except a CO2 emanation from 242-576 degrees C. For the weathered pyrochlore a dehy dration was observed between 234-565 degrees C followed by an exothermic pe ak in Differential Scanning Calorimetric (DSC) curve at 604 degrees C. This exothermic peak corresponds to the formation of a Nb oxide phase. At highe r temperatures the weathered pyrochlore is partly decomposed, forming a deh ydrated pyrochlore and a Nb oxide phase. The combination of FTIR and therma l analysis has provided useful information on both fresh and weathered pyro chlore transformations which has clarified our understanding of the water c ontrol of the structural stability of pyrochlore minerals.