Cyanotrichite

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.

Cyanotrichite
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)
General
CategorySulfate minerals
Formula
(repeating unit)		Cu4Al2[(OH)12|SO4]·2H2O
Strunz classification7.DE.10
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 10.16 Å, b = 12.61 Å,
c = 2.9 Å; Z = 1
Identification
ColorSky-blue, azure-blue
Crystal habitAcicular crystals and fibrous aggregates; incrustations
FractureUneven
Mohs scale hardness1-3
LusterSilky
StreakLight blue
DiaphaneityTransparent to translucent
Specific gravity2.76
Optical propertiesBiaxial (+)
Refractive indexnα = 1.588 nβ = 1.617 nγ = 1.655
Birefringenceδ = 0.067
PleochroismX = colorless; Y = pale blue; Z = bright blue
2V angleMeasured: 82°
References[1][2][3][4]

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.[1]

The main deposits are Cap la Garrone in the Var (France), Romania and Arizona (US).

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.

References
  1. Cyanotrichite in the Handbook of Mineralogy
  2. Cyanotrichite on Mindat.org
  3. Cyanotrichite data on Webmineral
  4. Mineralienatlas
  • 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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