Original paper

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

Begaudeau, Karine; Morizet, Yann; Florian, Pierre; Paris, Michael; Mercier, Jean-Claude

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 (1H, 29Si, 27Al) 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 29Si, 27Al Echo-MAS NMR spectra acquired for pyroxenes show signatures that are consistent with previous data. Weak additional anomalous peaks are detected in 29Si 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. 1H Echo-MAS NMR spectra for pyroxenes are dramatically affected by the presence of ferromagnetic impurities and are chemical shifted beyond the known range for 1H 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 29Si and 27Al NMR spectra. Thus, new perspectives on the use of NMR technique for mineralogy and geochemistry are envisaged.

Keywords

solid-state nmrfe-bearing mineralsparamagnetic elementsferromagnetic particlespyroxene