Abstract
Solid state 27Al and 29Si MAS n.m.r. spectroscopy has proven useful for studying the transformation of mixed-layer minerals in Upper Jurassic source rocks from the North Sea and the Danish onshore area. The transformation of smectite to illite layers in the mixed-layer minerals is accompanied by tetrahedral substitution of Al 3+ for Si 4+ in the tetrahedral layer of the smectite. The 27Al n.m.r. results indicate that the regularly interstratified illite-smectite mixed-layers with ≈ 85–95% illite correspond to a n.m.r. Al(4) Al(6) ratio of about 0.27–0.38. For most samples the 29Si n.m.r. spectra show separate Si( nAl) resonances according to the number ( n = 0,1 and 2) of neighbouring Al(4) atoms and thus allow estimation of Si Al(4) (or Si (Si +Al(4))) ratios in the tetrahedral layers of these samples. The n.m.r. data are in good accordance with the data obtained from chemical analysis combined with calculations of ideal mineral formulae. Ultra-high speed magic angle spinning is shown to greatly improve the experimental results for natural mixed-layer clay minerals.
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