Radiating sprays of cyanotrichite needles on matrix from the old Grand Canyon Mine now within Grand Canyon National Park (size: 3.8 x 3.7 x 1.2 cm)
|Crystal class||Prismatic (2/m)
(same H-M symbol)
|Unit cell||a = 10.16 Å, b = 12.61 Å,
c = 2.9 Å; Z = 1
|Crystal habit||Acicular crystals and fibrous aggregates; incrustations|
|Mohs scale hardness||1-3|
|Diaphaneity||Transparent to translucent|
|Optical properties||Biaxial (+)|
|Refractive index||nα = 1.588 nβ = 1.617 nγ = 1.655|
|Birefringence||δ = 0.067|
|Pleochroism||X = colorless; Y = pale blue; Z = bright blue|
|2V angle||Measured: 82°|
Cyanotrichite is a hydrous copper aluminium sulfate 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 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 nα=1.588 nβ=1.617 nγ=1.655.
Occurrence and discovery
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.
It was first described in 1839 from Moldova Nouă, Banat, Romania. The name is from Greek kyaneos 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.
- Palache, C., H. Berman, and C. Frondel (1951) Dana’s System of Mineralogy, (7th edition), v. II, p. 578–579. PDF file
- Mineral galleries
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