Far-Infrared Study of the Interlayer Torsional-Vibrational Mode of Mixed-Layer Illite/Smectites

Abstract

Abstract--Investigation of mixed-layer illite/smectites with far-infrared (FIR) spectroscopy indicates the presence of torsional mode absorption bands associated with interlayer fixed-K sites. By contrast, hydrated montmorillonitic interlayer cation sites are transparent in the far IR. The presence or absence of bands for interlayer cation sites appears to be related to both the magnitude and site of negative layer charge within the 2:1 layer structure. The bimodal nature of illite/smectite spectra leads to the suggestion that two different fixed-K environments occur within illite/smectite structures. These two environments are controlled by the composition of the octahedral sheet. The torsional modes at 112 and 89 cm ~ represent fixed-K sites influenced, respectively, by an Al-rich, high-charge dioctahedral layer and a heterogeneous A1-Fe-Mg-bearing, low-charge layer. A general trend of increasing absorption of the 112 cm -1 band, relative to the 89 cm 1 band, is observed in a typical diagenetic illite/smectite sequence of Miocene shales from the Gulf of Mexico sedimentary basin. The absorbance strength of both torsional bands is also seen to increase with increasing degree of illitization and the amount of fixed potassium in the illite/smectite. These observations are consistent with the concept of shales undergoing illitization during burial diagenesis by both the collapse of high-charge smectite layers to form illite layers (i.e., transformation) and the formation of new high-charge (-0.9) illite layers at the expense of smectite layers (i.e., dissolution/ neoformation).

A Chemical, XRD, and 27Al MAS NMR Investigation of Miocene Gulf Coast Shales with Application to Understanding Illite/Smectite Crystal-Chemistry

“…These results must first be considered in light of the mineralogical and crystallographic changes that also occur with depth. Figure 10 illustrates the relative quantitative mineralogy of the <0.2 um fraction of the shales using the data of Schroeder (1992a). The kaolinite content remains relatively unchanged, which should be noted with regard to the observations of Hower etaL, (1976), who show a decrease in kaolinite with depth in the 0.1-0.5 um fraction.…”

“…Mineral specimens used in this study have been characterized in detail using XRD, electron microprobe, and far-infrared (IR) analyses (Schroeder, 1990(Schroeder, , 1992a. Natural mixed-layer I/S from a well located in Caillou Island Oil Field, Terrebonne Parish, Louisiana, was studied as representative of materials from a burial diagenetic environment.…”

“…Repository reference materials Syn-1 (synthetic mica-montmorillonite), IMt-1 (Silver Hill illite), ISMt-1 (Mancos Shale), an Fe-bearing muscovite (East Morris, Connecticut), phengitic muscovite (Zacapa, Guatemala, American Museum of Natural History MVJ84-52-1), and a randomly ordered mixed layer I/S with 50% illite layers (I/S, I = 50, R = 0, Cameron, Arizona). All clay samples were pretreated for removal of labile organics, carbonates, and both iron oxide and hydroxide coatings (Schroeder, 1992a). Samples were then sized to the <0.2 ~m fraction.…”

“…Consequently, quantitative clay mineralogy of the <0.2 #m fraction is required. The procedure for quantitative phase analysis utilizes full pattern XRD and calculated mineral intensity factors (Schroeder, 1992a). Accuracy of the XRD estimates of mineral weight percentages is about + 10% of the values reported.…”

Quantification of Mixed-Layer Illite-Smectite in Glass Matrices by Fourier-Transform Infrared Spectroscopy

“…The Fourier-transform infrared (FTIR) technique has been used for both qualitative and quantitative determination in mineral species (McMillan et al, 1988;Pandya et al, 1992;Schroeder, 1992;Yanagisawa et al, 1997;Zhang et al, 1997). The method presents a number of important advantages: simple preparation, high sensitivity, and rapid analysis.…”