Cyanotrichite is a hydrous copper aluminum sulfate mineral with formula Cu4Al2[(OH)12|SO4]·2H2O, also known as lettsomite. It is a rare, blue, orthorhombic copper aluminum sulfate mineral. It is formed from the oxidation of copper ore minerals along with other oxidation zone minerals. The color is very impressive for such tiny crystals. It is an aptly named mineral since cyano and trich are derived from the Greek for blue and hair, respectively.
It was first described in 1839 by Moldova Nouă, Banat, Romania. The name is from Greek kyaneos for “blue” and triches for “hair” referring to the typical color and habit.
- Category: Sulfate minerals
- Formula: Cu4Al2[(OH)12|SO4]·2H2O
- Crystal system: Monoclinic
- Crystal class: Prismatic (2/m) (same H-M symbol)
Cyanotrichite forms velvety radial acicular crystal aggregates of extremely fine fibers. It crystallizes in the orthorhombic system and forms translucent bright blue acicular crystal clusters or dry coatings. The Mohs hardness is 2 and the specific gravity ranges from 2.74 to 2.95. Refractive indices are nα=1.588 nβ=1.617 nγ=1.655.
- Color: Sky-blue, azure-blue
- Crystal habit: Acicular crystals and fibrous aggregates; incrustations
- Fracture: Uneven
- Mohs scale hardness: 1-3
- Luster: Silky
- Streak: Light blue
- Diaphaneity: Transparent to translucent
- Specific gravity: 2.76
- Optical properties: Biaxial (+)
It is an oxidation product of primary copper mineralization in a weathering environment with abundant aluminum and sulfate. Associated minerals include brochantite, spangolite, chalcophyllite, olivenite, tyrolite, parnauite, azurite, and malachite.
Cyanotrichite is an uncommon mineral that occurs in oxidized copper sulfide deposits in localities in Romania, France, England, Italy, Greece Russia, Kazakhstan, South Africa, the United States, Chile, and quite a few others.
Its earlier name, Lettsomite, is taken from the name of William Garrow Lettsom (1804–1887), co-author of the 1858 Manual of the Mineralogy of Great Britain and Ireland.