What is Clay mineral, kaolinite, illite, smectite, chlorite? — Earth and Space Sciences

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Definition 19.10 University Earth and Space Sciences — Year 1 · অধ্যায় 19 — Weathering and Clays

A clay mineral is a hydrous aluminium phyllosilicate, usually in grains a few micrometres across or less, built of layers that combine a tetrahedral sheet (T) of $\ce{SiO4}$ tetrahedra sharing three corners and an octahedral sheet (O) of aluminium or magnesium surrounded by oxygen and hydroxyl. In kaolinite, $\ce{Al2Si2O5(OH)4}$ , each layer is one T and one O sheet (a 1:1 or T–O layer), uncharged, and the layers are held by hydrogen bonds. In the 2:1 or T–O–T clay minerals an O sheet lies between two T sheets pointing toward it: illite is a non-expanding 2:1 clay mineral with potassium ions between the layers, like a mica with less potassium; a smectite is a 2:1 clay mineral with hydrated, exchangeable cations and water between its layers, which can take up more water and swell; a chlorite is a 2:1 mineral whose negatively charged layers alternate with positively charged octahedral sheets of brucite type, $\ce{(Mg,Fe)(OH)2}$ with some aluminium.

The four families of clay minerals (schematic, two layers of each). The vertical scale is proportional to the basal spacing, the repeat distance from one layer to the next measured by X-ray diffraction: 7\, Å for kaolinite, 10\, Å for illite, about 14\, Å for an air-dried smectite and for chlorite. — The four families of clay minerals (schematic, two layers of each). The vertical scale is proportional to the basal spacing, the repeat distance from one layer to the next measured by X-ray diffraction: $7\,\text{Å}$ for kaolinite, $10\,\text{Å}$ for illite, about $14\,\text{Å}$ for an air-dried smectite and for chlorite.

mineral	layer	interlayer	air-dried	glycol	heated	CEC	typical setting
kaolinite	T–O	none	$7\,\text{Å}$	$7\,\text{Å}$	destroyed at $550\,{}^{\circ}\mathrm{C}$	3–15	feldspars leached by acid water; intense tropical weathering
illite	T–O–T	$\ce{K+}$	$10\,\text{Å}$	$10\,\text{Å}$	$10\,\text{Å}$	10–40	alkaline water rich in K; from feldspars and micas; the main clay of mudstones
smectite	T–O–T	hydrated $\ce{Na+}$ , $\ce{Ca^2+}$	about $14\,\text{Å}$	$17\,\text{Å}$	$10\,\text{Å}$ at $400\,{}^{\circ}\mathrm{C}$	70–100	weathered basic rocks; poorly drained ground, K scarce
chlorite	T–O–T	brucite-like sheet	$14\,\text{Å}$	$14\,\text{Å}$	$14\,\text{Å}$	–	low-grade metamorphic and altered igneous rocks; mudstones

Table 19.1. The four clay-mineral families: structure, basal spacing measured on oriented mounts air-dried, after solvation with ethylene glycol and after heating, cation-exchange capacity (CEC, milliequivalents per

100\,\mathrm{g}

; the value for smectite is that of montmorillonite) and where they typically form.

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