Dickite
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 | |
|---|---|
| General | |
| Category | Phyllosilicates Kaolinite-serpentine group |
| Formula (repeating unit) | Al2Si2O5(OH)4 |
| IMA symbol | Dck |
| 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 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, United Kingdom, where it was first described in 1888. Dickite appears in locations with similar qualities and is found in China, Jamaica, France, Germany, United Kingdom, United States, Italy, Belgium and Canada.