Nuclear magnetic resonance spectroscopy applied to minerals. Part 6.—Structural iron in kaolinites as viewed by proton magnetic resonance

Abstract

This paper examines to what extent the proton n.m.r. signal of hydroxyl groups can be used as an internal probe to investigate the distribution of structural iron in the framework of kaolinite. To this end, nine samples with iron contents varying between 0.06 and 1.86 wt% in Fe₂O₃ have been examined at 90 and 24 MHz. Measurements involved determinations of the various contributions to the second moment of the proton line by solid-echo techniques and also spin–lattice relaxation times (T₁). Although the lineshapes of all samples are fairly constant, the T₁ values vary considerably (over three orders of magnitude). These T₁ values can be accounted for by a paramagnetic induced relaxation mechanism using (apart for the purest sample) a multi-impurity approach. These experiments clearly show that part of the iron is not evenly distributed within the structure. The results are finally discussed in relation to the problem of structural disorder in kaolinites and are compared with other experimental approaches.