Cyanotrichite is a copper aluminium sulfate mineral, Cu4Al2(OH)12SO42H2O, also known as lettsomite, very similar to halotrichite, and a member of the same group. However, it is much rarer. It forms velvety aggregates of extremely fine fibres and radial aggregates. It crystallizes in the orthorhombic system and forms translucent bright blue acicular crystal clusters or drusey coatings. The Mohs hardness is 2 and the specific gravity ranges from 2.74 to 2.95. Refractive indices are na=1.588 n=1.617 n?=1.655. It is an oxidation product of primary copper mineralization in a weathering environment with abundant aluminium and sulfate. Associated minerals include brochantite, spangolite, chalcophyllite, olivenite, tyrolite, parnauite, azurite and malachite.

The main deposits are Cap la Garrone in the Var (France), Romania and Arizona (USA). Because of its beautiful colouring, it is very popular with collectors.

It was first described in 1839 from Moldova Noua, Banat, Romania. The name is from Greek kyaneos for "blue" and triches for "hair" referring to the typical color and habit.