Cyanotrichite is a hydrous copperaluminiumsulfate mineral with formula Cu4Al2[(OH)12|SO4]·2H2O, also known as lettsomite. Cyanotrichite forms velvety radial acicular crystal aggregates of extremely fine fibers. It crystallizes in the monoclinic 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 nα = 1.588 nβ = 1.617 nγ = 1.655.
It was first described in 1839 from Moldova Nouă, Banat, Romania. The name is from Greekkyaneos for "blue" and triches for "hair" referring to the typical color and habit. 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.
^ abAnthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2005). "Cyanotrichite"(PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved 3 August 2022.