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

Abstract

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.

Juvenile chemical sediments and the long term persistence of water at the surface of Mars

“…The conversion of smectite into either illite (from dioctahedral smectite) or chlorite (from trioctahedral smectite) through burial diagenesis and/or metamorphism requires liquid water (Whitney, 1990;Altaner and Ylagan, 1997;Robinson et al, 2002). A recent examination of chloritization reaction pathways indicates that the fluid to rock ratio is key to reaction progress (Robinson et al, 2002).…”

“…At the molecular level, the hydration of cations to and from smectite, dissolution/reprecipitation of smectite/illite layers, and the introduction of excess OH -(in chloritization) show that the reaction may not progress unless water is present (Whitney, 1990;Altaner and Ylagan, 1997;Robinson et al, 2002). Perhaps the most convincing evidence for water's role in catalyzing smectite conversion comes from experiments on the illitization of smectite as a function of rock:water ratio (Whitney, 1990): the extent of illitization decreased significantly with decreasing amounts of water.…”

“…For example, pore fluid chemistry (e.g., pH, K + , Al 3+ and H 4 SiO 4 0 ) plays a significant role, as does the composition of the initial smectite (Altaner and Ylagan, 1997;Meunier, 2005). The former point deserves underscoring.…”

Influência da pressão e temperatura na cristalinidade da illita em sequências Proterozoicas: Norte do Distrito Federal e Goiás, Brasil

“…Em sequências sedimentares, a illita ocorre como uma partícula deposicional ou sob a forma de cimento, ou pela illitização da esmectita (Hower et al 1976;Boles & Franks 1979;Altaner & Ylagan 1997), ou ainda pela alteração do feldspato detrítico (Morad & Aldahan 1987;Bjorkum & Gjelsvik 1988;De Ros et al 1994). À medida que aumenta a profundidade de soterramento, acontece a illitização da esmectita (Boles & Franks 1979), e a reflexão 001, característica da illita sob a difração de Raios X (d = 10Å), torna-se cada vez mais bem definida.…”

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

“…The reaction of dioctahedral smectite to illite is a continuous series of I-S; the proportion of illite layers in I-S increases with depth in response to increasing temperature or time. The mechanism of the smectite-to-illite reaction has been examined over the past few decades (Nadeau et al, 1984;Altaner and Ylagan, 1997). In addition, the study of I-S diagenesis has been stimulated partly by an economic interest in finding more efficient methods to prospect for petroleum.…”