IlliteTOT 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 OH-Si 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

• 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 muscovitesericite (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 1Md--1M--1M/3T--3T sequence evolution with decreasing

expandibility

– Shirozu and Higashi (1972) found 1Md--1M--2M1 set in similar geologic situation