Clays and Clay Minerals, Vol. 45, No. 4, 517-533, 1997. COMPARISON OF STRUCTURAL MODELS OF MIXED-LAYER ILLITE/SMECTITE AND REACTION MECHANISMS OF SMECTITE ILLITIZATION STEPHEN P. ALTANER AND ROBERT E YLAGAN Department of Geology, University of Illinois, 1301 W. Green St., Urbana, Illinois 61801 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 deter- minations 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 mech- anisms 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 in- volved. 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. Key Words--Illite, Illite/Smectite, Mixed-Layer, Reaction Mechanism, Smectite. INTRODUCTION Smectite Illitization Documenting the degree of reaction of smeetite to illite, termed "smeetite illitization", is frequently used as an independent geothermometer to allow recon- structions of the thermal and tectonic history of sedi- mentary basins (Weaver and Beck 1971; Hoffman and Hower 1979; Schoonmaker et al. 1986) and active and fossil hydrothermal systems (Inoue and Utada 1983; Jennings and Thompson 1986). The chemical conse- quences of this reaction may influence the develop- ment of geopressures (Freed and Peacor 1989), growth faults (Bruce 1984), oil migrations (Burst 1969), min- eral cements mad porosity (Lahann 1980). Radiometric ages of illite can date regional overthrusting (Hoffman et al. 1976) and migrations of hydrothermal fluids, oil and natural gas (Aronson and Burtner 1983; Lee et al. 1985; Hay et al. 1988). Smectite illitization is considered to proceed through mixed-layer illite/smectite (US) intermediates in which the percentage of illite interlayers typically increases with increasing temperature (Hower et al. 1976), geologic time (Pytte and Reynolds 1989), K concentration (Huang et al. 1993) and water/rock ratio (Whitney 1990). As I/S becomes illitic, the interlayer arrangements change from random (R0) to short-range (R1) ordered, and then to long-range (R3) ordered, as inferred from computer-generated X-ray diffraction (XRD) patterns (Bethke et al. 1986) and as directly observed using high-resolution transmission electron microscopy (HRTEM; Veblen et al. 1990; ~rodofi et al. 1990). Polytypes generally evolve from turbostratic stacking in smectitic US to 1M~ or 1M in iUitic US to 2M1 in pure illite (Inoue et al. 1987; Reynolds 1993). The octahedral vacancy can occur in either the M1 (trans) or M2 (cis) octahedral site (Drits et al. 1993; Reynolds 1993). Chemical changes in I/S include an increase in K and A1, and a decrease in Si, Fe, Mg, Na, Ca and HzO (~rodofi et al. 1992). I/S as MacEwan Crystallites and Fundamental Particles The crystal structure of mixed-layer I/S can be de- scribed using either the Markovian model or the fun- Copyright 9 1997, The Clay Minerals Society 517