Appendix: Semidisplacive Mechanism for Diagenetic Alteration of Montmorillonite Layers to Illite Layers

219

129

Abstract--This paper compares mechanisms of the reaction of smectite to illite, in light of structural models for interstratified illite/smectite (US). The crystal structure of US has been described previously by a nonpolar and polar 2:1 layer model. In a nonpolar model, individual 2:1 layers are chemically homogeneous, whereas a polar model assumes a 2: l layer can have a smectite charge on one side and an illite charge on the other side. Several kinds of data support the polar model; however, more determinations of the negative charge of expandable sites in US are needed to confirm such a model.Assuming a polar 2:1 layer model for I/S, we compare the mineralogical and geochemical consequences of several reaction mechanisms for smectite illitization: 1) solid-state transformation (SST), 2) dissolution and crystallization (DC) and 3) Ostwald ripening (OR). Features of an SST model are the replacement of smectite interlayers by illite intedayers, resulting in gradual changes in interlayer ordering, polytype, chemical and isotopic composition and crystal size and shape. Several SST models are possible depending on the nature of the reaction site (framework cations, polyhedra or interlayers). In contrast, DC models allow for abrupt changes in the structure, composition and texture of I/S as illitization proceeds. Several DC models are possible depending on the nature of the rate-controlling step, for example, diffusional transport or surface reactions during crystal growth. The OR model represents the coarsening of a single mineral where the smallest crystals dissolve and nucleate onto existing larger crystals, allowing for evolution in the overgrowth but not in the template crystal.An SST mechanism, involving either reacting polyhedra or reacting interlayers, seems to best model illitization in rock-dominated systems such as bentonite. A DC mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Both DC and SST mechanisms can occur in shale. Differences in reaction mechanism may be related to permeability. An OR model poorly describes illitization because of the progressive mineralogical and chemical changes involved. For many geologic environments, it is important to consider alternate origins for US such as kaolinite illitization and detrital. Further work is needed to clarify the DC crystal growth process in terms of a structural model of I/S and to determine which specific SST or DC model best characterizes illitization in geologic systems.

Section: Reaction Mechanisms For Smectite Illitizationmentioning

confidence: 99%

Comparison of Structural Models of Mixed-Layer Illite/Smectite and Reaction Mechanisms of Smectite Illitization

Altaner

1997

219

129

“…Solid-state transformation hyCopyright 9 1986, The Clay Minerals Society potheses call on ionic substitutions of A1 for Si within an intact silicate sheet, and K for exchangeable cations in the interlayer position (Towe, 1962;Weaver and Beck, 1971;Hower et al, 1976). Dissolution of K-feldspar and perhaps detrital mica may supply A1 and K. Pollard (1971) favored a solid-state transformation because of likely energy barriers to smectite dissolution at sub-metamorphic temperatures, and he presented a model of ionic diffusion in the interlayer region and distortion of silica tetrahedra, allowing A1 substitution for St. K/Ar dating studies that indicate that radiogenic Ar is retained within I/S minerals during illitization (Weaver and Wampler, 1970;Perry, 1974;Aronson and Hower, 1976) also support a transformation that preserves illite interlayers.…”

Section: Possible Reaction Mechanismsmentioning

confidence: 99%

Layer-By-Layer Mechanism of Smectite Illitization and Application to a New Rate Law

Bethke

1986

129

Abstract--A layer-by-layer mechanism explains important features of mixed-layer clay minerals formed during the illitization of smectite, including the occurrence of randomly interstratified illite/smectite, the transition to ordered interstratifications, and the development of long-range ordering. A variety of solidstate transformation mechanisms were tested with a stochastic model, which accounts for interactions among clay layers. The model produces most successful results when the reaction of smectite layers with one illite nearest neighbor is favored over smectites with no illite neighbors by a factor of about two, and over those with two illite neighbors by a factor often or more. Synthetic X-ray powder diffraction patterns calculated from model results compare well with those of iUite/smectite minerals. These results suggest a new kinetic rate law. Solutions to this rate law for reaction within sediments undergoing burial give mineralogical profiles with depth similar to those observed in subsiding sedimentary basins.

“…They enumerated three possible mechanisms for converting expanding into nonexpanding clay: substitution of A1 for Si, substitution of Mg and/or Fe 2 § for A1 and/or reduction of Fe 3 § They concluded, however, that there is insufficient evidence to decide which chemical reactions are involved. Pollard (1971) proposed a semi-displacive mechanism for structural changes occurring in the course of conversion of montmorillonite into illite layers. In the first stage interlayer A1 enters hexagonal holes.…”

Section: Possible Applications To Clay Diagenesismentioning

confidence: 99%

Montmorillonite-Alkali Halide Interaction: A Possible Mechanism for Illitization

Heller-Kallai

1975

Abstract--The reaction products obtained when montmorillonites react with potassium halide at elevated temperatures, which were described in a previous publication, are further characterized. On the basis of their X-ray powder diffraction patterns, i.r. spectra, CEC and chemical composition they could be regarded as montmorillonite~illite interstralifications. Changes in morphology of various montmorillonites heated with and without K halktc arc related to the size, charge and position of interlayer cations. Scanning electron-micrographs of samples heated with KBr resemble those of wellcrystallized illite. It is speculated that reactions of clay minerals with halides or other proton acceptors may account for some diagenetic processes in nature, e.g. the conversion of montmorillonite to illite on deep burial.