Smectite Illitization

Dear Colleagues,

Smectite illitization has been a widely researched process in different geological contexts: burial diagenesis, low-grade and contact metamorphism, hydrothermal and pedogenic alteration, etc. This process has been used as a prograde and retrograde marker in siliciclastic rocks and as a paleoenvironment and paleoclimatic proxy in various geological contexts. It has also been useful in soil fertility management research, in hydrocarbon exploration or in research on the long-term performance of bentonite barriers in radioactive waste deposits.

The illitization reaction produces, in general terms, an increase in the TOT layer charge and the subsequent K fixation in the interlayer space. Therefore, structural adjustments are needed which affect not only the magnitude of the charge of the 2:1 expandable layers, but also their localization in the octahedral or tetrahedral layers. The process comprises a series of chemical reactions and structural changes that give rise to coherent domains intermediate between smectite and illite. These are the so-called I/S mixed-layer minerals, which consist of different stacking sequences of the 2:1 layers of illite and smectite.

This process leads to a greater structural order and compositional homogeneity of the phases involved, such as the reaction progresses. The nature, structure, composition, and textural relationships of the different phases involved in the process can be revealed using XRD and TEM techniques in various forms, among others.

The mechanism advocated to explain smectite illitization more frequently, are solid state transformation, dissolution–recrystalization, and grain coarsening by Ostwald’s step rule.

Although contributions from several of these is possible in different environments, solid state transformation seems to predominate in close-systems, while dissolution–recrystallization and grain coarsening seem more efficient in fluid-dominated open systems. Microbially-promoted smectite illitization represents a relevant mechanism and should be considered in more detail.

We encourage you to submit original papers on identification, description, modeling, applications, and any other subject related to smectite illitization.

Dr. Javier Arostegi García
Guest Editor

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• Clay minerals
• Mixed layers
• Geological environments
• Reaction mechanisms
• Proxy
• Applications