Kaolinite

Kaolinite (/ˈk.ə.lɪˌnt/)[4][5][6] is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4
) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6
) octahedra.[7] Rocks that are rich in kaolinite are known as kaolin /ˈk.ə.lɪn/ or china clay.[8]

Kaolinite
General
CategoryPhyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)
Al
2
(OH)
4
Si
2
O
5
Strunz classification9.ED.05
Crystal systemTriclinic
Crystal classPedial (1)
(same H-M symbol)
Space groupP1
Unit cella = 5.13 Å, b = 8.89 Å
c = 7.25 Å; α = 90°
β = 104.5°, γ = 89.8°; Z = 2
Identification
ColorWhite to cream, sometimes red, blue or brown tints from impurities and pale-yellow; also often stained various hues, tans and browns being common.
Crystal habitRarely as crystals, thin plates or stacked, More commonly as microscopic pseudohexagonal plates and clusters of plates, aggregated into compact, claylike masses
CleavagePerfect on {001}
TenacityFlexible but inelastic
Mohs scale hardness2–2.5
LusterPearly to dull earthy
StreakWhite
Specific gravity2.16–2.68
Optical propertiesBiaxial (–)
Refractive indexnα = 1.553–1.565,
nβ = 1.559–1.569,
nγ = 1.569–1.570
2V angleMeasured: 24° to 50°, Calculated: 44°
References[1][2][3]
Kaolinite
Traditional Chinese高嶺石
Simplified Chinese高岭石
Literal meaning"Gaoling stone"

The name kaolin is derived from Gaoling (Chinese: 高嶺; pinyin: Gāolǐng; lit. 'High Ridge'), a Chinese village near Jingdezhen in southeastern China's Jiangxi Province.[9] The name entered English in 1727 from the French version of the word: kaolin, following François Xavier d'Entrecolles's reports on the making of Jingdezhen porcelain.[10]

Kaolinite has a low shrink–swell capacity and a low cation-exchange capacity (1–15 meq/100 g). It is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar. In many parts of the world it is colored pink-orange-red by iron oxide, giving it a distinct rust hue. Lighter concentrations yield white, yellow, or light orange colors. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, United States. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle, or liquid slurry.