Mineralogical and Morphological Evidence for the Formation of Illite at the Expense of Illite/Smectite

Abstract

Abstract--The conversion of smectite to illite by way of a mixed-layer illite/smectite (I/S) series was found to be the major depth-related reaction in clay-mineral assemblages from two cored sedimentary sequences in the Rocky Mountains. The I/S reaction occurred in both interbedded sandstone and shale of Upper Cretaceous and lower Tertiary age in the Green River basin, Wyoming, and in chalk and chalky shale of the Upper Cretaceous Niobrara Formation, Denver basin, Colorado. As the proportion of illite layers in I/S increased with depth in these rocks, the amount of I/S in the clay fraction decreased, and the amount of discrete illite increased. Scanning electron microscopy revealed that the morphologies of highly expansible, randomly interstratified I/S clay (samples from shallow cores) exhibited no distinctive intergrowth or overgrowth textures. In deeply buried rocks containing highly illitic, ordered I/S and abundant discrete illite, however, fibers or laths of illite were formed on earlier I/S substrates. Less commonly, I/S of low expandability shows morphological features of both smectite and illite whereby rigid laths of illite appear to have formed diagenetically from the wall surfaces of I/S honeycombs. This combined morphology suggests some dissolution and reprecipitation (or some reorganization) of materials from the I/S substrate as the substrate was transformed into a more illitic mixed-layer day.These data suggest that some I/S clay was destroyed by the selective cannibalization of smectite layers in I/S to provide the components needed to make a more illitic I/S. Moreover, discrete illite and other minerals apparently were formed during the reaction. Also, coarser mineral phases, such as potassium feldspar and detrital micas, may not have been required to supply the chemical components in the reaction. The observations provide an explanation for late diagenetic I/S reactions that occurred in restricted or relatively closed geochemical systems, as in early cemented rocks having extremely low permeability and little or no potassium feldspar.

Changes in Particle Morphology During Illitization: An Experimental Study

“…Within a coalescing crystallite, individual particle boundaries became difficult to discern. Although a few lath-shaped particles formed near 50% expandability in our experiments, there was no significant change in morphology to lathshaped or fibrous particles like those seen in some natural materials (Inoue et al, 1987(Inoue et al, , 1988Pollastro, 1985).…”

“…The reaction of smectite to illite via an intermediate interstratified illite/smectite (I/S), commonly called illitization, may be a series of two or more reactions with different reaction rates and reaction mechanisms (Inoue et aL, 1988;Whitney and Northrop, 1988;Pollastro, 1985). The first stage of the reaction may produce randomly interstratified I/S, and the second stage may produce ordered (R1 or R3) I/S.…”

Diagenesis and Reservoir Properties of the Permian Ecca Group Sandstones and Mudrocks in the Eastern Cape Province, South Africa

“…Therefore, the occurrence of illite or illite-smectite mixed layers lies merely on the transformation/conversion of smectite clay mineral during a shallow burial diagenetic process at increased temperature between 90 • C and 130 • C [37]. Smectite Clay.…”

“…Thus, destruction of smectite layers during illitization can release significant amounts of these elements, depending on the initial composition of the rock. Pollastro (1985) [37] documented that, for smectite to be converted to illite, it is essential that K + must be added in the interlayer space and the amount of tetrahedral Al 3+ must be increased. The mathematical expression is shown below:…”

Diagenesis of Dioctahedral and Trioctahedral Smectites from Alternating Beds in Miocene to Pleistocene Rocks of the Niigata Basin, Japan

“…In addition to the potassic feldspar, therefore, discrete illite, which occurs throughout the well, can be considered as another source of K for the illitization. Futhermore, some I-S may provide the components required to make a more illitic I-S (Pollastro, 1985).…”