Illite
TOT 2:1 layer clay
KAl2(AlSi3O10)(OH)2
(Muscovite)
Presented by Paul Sandlin
Illite Structure
• Illite is a clay-grade mica resembling muscovite with more SiO2, less
K2O, and more H2O
• Most Illites are dioctahedral with polytypes 2M1, 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 H2O
• Broken bonds may take up H2O, i.e., Si-O-Si may become Si-OH OHSi to satisfy valence requirement
• Alternatively, deficient K+ may be replaced by H3O+
• Aruja (1944)--chemical formula relative to 12(O + OH) is (K,
Na)0.72(Al1.48Mg0.40)(Si3.19Al0.77Ti0.04)O9.18(OH)2.82 but the anion part is
O10(OH)1.18(H2O)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 1Md--1M--2M1 with increasing
hydrothermal run time and temperature
• In regional geologic studies, Reynolds (1963) and Maxwell
and Hower (1967) showed 2M/1Md 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
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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--
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– Horton (1983) documented a 1Md--1M--1M/3T--3T sequence evolution with
decreasing expandibility
– Shirozu and Higashi (1972) found 1Md--1M--2M1 set in similar geologic situation