Dickite

Dickite
General
Category	Phyllosilicates Kaolinite-serpentine group
Formula (repeating unit)	Al2Si2O5(OH)4
Strunz classification	9.ED.05
Dana classification	71.01.01.01
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Cc
Unit cell	a = 5.150, b = 8.940 c = 14.424 [Å]; β = 96.8°; Z = 4
Identification
Color	White, with coloration from impurities
Crystal habit	Pseudohexagonal crystals, aggregates of platelets and compact massive
Cleavage	Perfect on {001}
Tenacity	Flexible but inelastic
Mohs scale hardness	1.5 - 2
Luster	Satiny to pearly
Streak	White
Diaphaneity	Transparent
Specific gravity	2.6
Optical properties	Biaxial (+)
Refractive index	nα = 1.561 - 1.564 nβ = 1.561 - 1.566 nγ = 1.566 - 1.570
Birefringence	δ = 0.005 - 0.006
2V angle	Measured: 50° to 80°
References

Dickite (Al₂Si₂O₅(OH)₄) is a phyllosilicate clay mineral named after the metallurgical chemist Allan Brugh Dick, who first described it. It is chemically composed of 20.90% aluminium, 21.76% silicon, 1.56% hydrogen and 55.78% oxygen. It has the same composition as kaolinite, nacrite, and halloysite, but with a different crystal structure (polymorph). Dickite sometimes contains impurities such as titanium, iron, magnesium, calcium, sodium and potassium.

Dickite occurs with other clays and requires x-ray diffraction for its positive identification. Dickite is an important alteration indicator in hydrothermal systems as well as occurring in soils and shales.

Dickite’s type location is in Pant-y-Gaseg, Amlwch, Isle of Anglesey, Wales, UK, where it was first described in 1888. Dickite appears in locations with similar qualities and is well spread among countries like China, Jamaica, France, Germany, UK, US, Italy, Belgium and Canada.

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