| Illite | |
|---|---|
 | |
| General | |
| Category | Phyllosilicateminerals,micagroup,muscovitevariety |
| Formula | (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] |
| IMA symbol | Ilt[1] |
| Strunz classification | 9.EC.60 |
| Dana classification | 71.02.02d.02 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | C2/m (no. 12) |
| Identification | |
| Color | Grey-white to silvery-white, greenish-gray |
| Crystal habit | Micaceous aggregates |
| Cleavage | {001} Perfect |
| Mohs scale hardness | 1–2 |
| Luster | Pearly to dull |
| Streak | white |
| Diaphaneity | Translucent |
| Specific gravity | 2.6–2.9 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.535 – 1.570 nβ = 1.555 – 1.600 nγ = 1.565 – 1.605 |
| References | [2][3][4] |
Illite, also calledhydromica orhydromuscovite,[6] is a group of closely related non-expandingclay minerals. Illite is a secondary mineral precipitate, and an example of aphyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) –alumina octahedron (O) – silica tetrahedron (T) layers.[5] The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar tomuscovite with slightly moresilicon,magnesium,iron, and water and slightly less tetrahedralaluminium and interlayerpotassium. The chemical formula is given as(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)],[3] but there is considerable ion (isomorphic) substitution. It occurs as aggregates of smallmonoclinic grey to white crystals. Due to the small size, positive identification usually requiresx-ray diffraction or SEM-EDS (automated mineralogy) analysis. Illite occurs as an altered product of muscovite andfeldspar inweathering andhydrothermal environments; it may be a component ofsericite. It is common in sediments, soils, and argillaceoussedimentary rocks as well as in some low grademetamorphic rocks. The iron-rich member of the illite group,glauconite, in sediments can be differentiated by x-ray analysis.[7]
Thecation-exchange capacity (CEC) of illite is smaller than that ofsmectite but higher than that ofkaolinite, typically around 20 – 30 meq/100 g.
Illite was first described for occurrences in the Maquoketashale inCalhoun County, Illinois, US, in 1937. The name was derived from its type location in Illinois.[2]
Brammallite is a sodium rich analogue. Avalite is achromium bearing variety which has been described fromMt. Avala,Belgrade,Serbia.[8]
Zipao 'jade' is an ornamental form of illite showing bands of red-purple and pale yellow-green. It may be carved into pendants and other ornaments.
The crystallinity of illite has been used as an indicator of metamorphic grade in clay-bearing rocks metamorphosed under conditions betweendiagenesis and low-grade metamorphism.[9] With increasing temperature, illite is thought to undergo a transformation intomuscovite.[10]
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