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MAS NMR investigation of kaolinite-smectite structure using ⁶Li and ²⁹Si with Mn exchange

J. CUADROS^1,* and T. WING-DUDEK^1,

¹ Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK

^* E-mail: j.cuadros{at}nhm.ac.uk

(Received 2 January 2007; revised 19 March 2007)

Kaolinite-smectite mixed-layers have been found to have a complex structure with smectite and kaolinite domains within layers. Here we further investigate this structure in samples with 0–80% kaolinite layers, as determined by X-ray diffraction, by means of magic angle spinning nuclear magnetic resonance (MAS NMR) of ²⁹Si and ⁶Li. The ²⁹Si NMR experiments were carried out on two samples (55 and 80% kaolinite layers), before and after their exchange with Mn²⁺, a paramagnetic ion that causes NMR signal loss from neighbouring nuclei, in order to investigate the distance between Mn ions and Si atoms in kaolinite sites. The ²⁹Si NMR intensity from such sites (at ~–91 ppm) was reduced upon Mn exchange, indicating that some Mn ions are located near kaolinite Si sites. The position of the ⁶Li peak changes slightly (–1.3 to –1.8 ppm) but progressively with increasing kaolinite content (0–80% kaolinite layers) of four K-S specimens, suggesting two slightly different chemical environments for interlayer Li, one related to smectite and the other to kaolinite. The two sets of experiments are consistent with a complex structure of kaolinite-smectite, including smectite and kaolinite domains within layers and/or interlayers of varying smectitic and kaolinitic character.

KEYWORDS: Kaolinite-smectite, ⁶Li, MAS NMR, ²⁹Si