Illite TOT 2:1 layer clay KAl 2 (AlSi 3 O 10 )(OH) 2 (Muscovite) Presented by Paul Sandlin

Illite Structure Illite is a clay-grade mica resembling muscovite with more SiO 2, less K 2 O, and more H 2 O Most Illites are dioctahedral with polytypes 2M 1, 1M, 1Md, and 3T. However, trioctahedral illites are known Difficult to determine chemical and structural characteristics, particularly 1Md types Many illites are mixed-layer structures and the mixed-layering is not obvious when the proportion of admixed layers is small, 10% or less, usually montmorillonite Weaver and Pollard (1973) -- chemical analyses data suggests that illites may resemble phengites rather than muscovites.

Chemical Composition Srodon and Eberl (1984)

Interlayer Cations Interlayer stratification with montmorillonite will decrease the number of interlayer cations and increase the water content sorbed water on surface replacing K+ at interlayer sites Large surface area may account for excess sorption of H 2 O Broken bonds may take up H 2 O, i.e., Si-O-Si may become Si-OH OH- Si to satisfy valence requirement Alternatively, deficient K+ may be replaced by H 3 O+ Aruja (1944)--chemical formula relative to 12(O + OH) is (K, Na) 0.72 (Al 1.48 Mg 0.40 )(Si 3.19 Al 0.77 Ti 0.04 )O 9.18 (OH) 2.82 but the anion part is O 10 (OH) 1.18 (H 2 O) 0.82 indicating that about 1/2 OH is replaced by water molecules

1. Yoder and Eugster (1955) 2. Levinson (1955) 3. Levinson (1955) 4. Levinson (1955) 5. Nagelschmidt and Hicks (1943)

Srodon (1984)

Interparticle Diffraction McHardy et al. (1982), Nadeau et al. (1984) Pure smectite is 10 Å thick corresponding to one 2:1 layer Ordered I/S is composed primarily of particles 20 Å thick corresponding to two 2:1 layers coordinated by a single plane of K ions ISII-ordered averaged four 2:1 layers coordinated by three planes of fixed K ions about 40 Å thick Illite having no detectable expanding component averaged 70 Å thick Evolution of I/S towards pure illite can be perceived as a growth of illite crystals parallel to c axis

Polytypism Yoder and Eugster (1955), Yoder (1959), and Velde (1965) Found sequence of 1M d --1M--2M 1 with increasing hydrothermal run time and temperature In regional geologic studies, Reynolds (1963) and Maxwell and Hower (1967) showed 2M/1M d increases with increasing metamorphic grade in limestones and shales Frey et al. (1983) found 3T, coarse-grained micas in regionally meta rocks from the Alps, but this polytype has not been found in diagenetic or low-grade meta sequences

Illitization Processes Detrital, diagenetic, and metamorphic processes Illitization of smectite by wetting and drying process, Na replaced by K Most marine illite is detrital, however, has been found in deeper sed cores and elevated temps of around 50°C In lakes it is thought to be detrital smectite undergoing wetting and drying process Kaolinite is illitized completely before smectite with no known reason From muscovite with increasing temperature, the reaction sequence is detrital muscovite  sericite (illite or I/S)  phengite  muscovite which suggests that illite has a stability field with respect to muscovite Neoformation of illite in sandstone pores (hairy illite) Hydrothermal environments-- most studied from active geothermal fields-- –Horton (1983) documented a 1M d --1M--1M/3T--3T sequence evolution with decreasing expandibility –Shirozu and Higashi (1972) found 1M d --1M--2M 1 set in similar geologic situation