Solid-state NMR analysis of Fe-bearing minerals: implications and applications for Earth sciences

Abstract

Solid-state nuclear magnetic resonance (NMR) is commonly used in the study of solid structures in Earth sciences; however, it suffers from the impossibility to analyse solid structures containing ferromagnetic particles or paramagnetic elements. We have attempted to decipher the effect of (1) ferromagnetic particles (Fe- Ti-bearing mineral phase) and (2) paramagnetic elements (Fe, Cr, Ni) on the signature of diamagnetic elements ( 1 H, 29 Si, 27 Al) in natural clino- and orthopyroxene from peridotite. The results obtained on these natural minerals have been compared with results obtained for a synthetic mixture of kaolinite + magnetite. The 29 Si, 27 Al Echo-MAS NMR spectra acquired for pyroxenes show signatures that are consistent with previous data. Weak additional anomalous peaks are detected in 29 Si spectra. Both elements show a broadening in the spectra, which is commonly observed when paramagnetic elements are present. The perturbations induced by paramagnetic elements are the result of several interactions: (1) pseudocontact shift and (2) Fermi contact shift. 1 H Echo-MAS NMR spectra for pyroxenes are dramatically affected by the presence of ferromagnetic impurities and are chemical shifted beyond the known range for 1 H in solids. The effect of ferromagnetic particles is also confirmed by the results obtained for the kaolinite + magnetite mixture showing increasing perturbation with increasing magnetite content. We suggest that the presence of paramagnetic elements and/or ferromagnetic particles is only weakly affecting the 29 Si and 27 Al NMR spectra. Thus, new perspectives on the use of NMR technique for mineralogy and geochemistry are envisaged.