Abstract: This study assesses the distribution of Al and Fe in mixed-layer illite/smectites (I/S) in shales undergoing burial diagenetic changes, using evidence from 27Al NMR, XRD, and chemical analyses. Samples studied include a sequence of mixed-layer I/S (ranging from 40% to 68% illite layers) in shales from a well located in the Caillou Island Oil Field, Terrebonne Parish, Louisiana, as well as synthetic mica-montmorillonite (Syn-1), Silver Hill illite (IMt-1), K-benonite (ISMt-1), an Fe-bearing muscovite, phengitic muscovite, and a randomly interstratified mixed-layer I/S with 50% illite layers. Using a simplified model, where Fe3+ isomorphously substitutes randomly for Al in the dioctahedral 2:1 structure, the 27Al NMR signal intensities are examined with regard to the paramagnetic deshielding effect of the Fe3+. The rapid decrease in paramagnetic deshielding with distance allows for a spherical “wipeout” model with a radius of 6 Å, over which there is complete effective paramagnetic line broadening (i.e., Al within the sphere is not “seen”). Using the average dimensions of a dioctahedral mica, the expected relative intensities of the octahedral and tetrahedral Al signal are determined as a function of Fe2O3 content.
Observed 27Al signals, normalized per unit weight of Al2O3 and relative to the lowest Fe-bearing phase, show a clear trend of decreasing intensity with increasing Fe2O3 content. Normative fitting of oxide data to structural formulae reveals a similar trend of decreasing 27Al intensity with increasing fraction of dioctahedral site occupied by Fe3+. Agreement between the observed 27Al intensities of low Fe-bearing 2:1 phyllosilicates and 27Al intensities predicted using the wipeout model indicate regular ordering of Fe and Al within the low Fe-bearing phases. However, observed 27Al intensities for the I/S specimens fall into a region where the amount of Al seen is in excess for the given XFe, thus indicating segregation of Al and Fe domains.
The second order quadrupole effect for the Al site in the I/S fraction of shales decreases very slightly with increasing depth and percent of illite in the I/S, but not enough to effect site quantitation. Quantitative apportionment of elements into the I/S phase of the <0.2 µm fraction using NMR constraints shows directly a trend of increasing number of Al sites and no change in the number of Al sites with increasing degree of illitization for samples from the Gulf Coast diagenetic environment. Stoichiometry indicates an approximate 1:1 substitution of tetrahedral Al for Si over the 40–68% range of illite in I/S examined.