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Forêtite (IMA2011-100)

Posted by Stuart Mills  
Stuart Mills June 25, 2012 10:28PM
Forêtite, a new secondary arsenate mineral from, the Cap Garonne mine, France.
Mineralogical Magazine, June 2012, Vol. 76(3), pp. 769–775


Forêtite, ideally Cu2Al2(AsO4)(OH,O,H2O)6, is a new secondary arsenate mineral from the Cap Garonne mine, Var, France (IMA2011‒100). Forêtite has also been identified at the Salsigne gold mine, Languedoc-Roussillon, France. Initially identified as a potentially new mineral in 1993, a formal description was only made possible through a new find in a chamber located near the entrance of the Cap Garonne mine called "Annex S". Forêtite occurs in pale sky blue to aqua aggregates, up to about 0.1 mm across, comprised of minute plates, no more than 20 µm in length. Forêtite has a very pale blue streak and crystals have a vitreous lustre and are transparent, whereas clusters appear translucent. Crystals are brittle, with an irregular fracture and have a hardness of ca 3‒4 on the Mohs scale. The calculated density is 3.286 g/cm3. Forêtite is found in direct association with bariopharmacoalumite, cyanotrichite, parnauite, chalcophyllite and mansfieldite in an Al-rich assemblage presumed to have formed under acidic conditions. The mineral is biaxial; nav was measured in white light on aggregates of forêtite crystals and gave 1.620(5). The empirical formula (based on 10 O apfu) is Cu1.94(Al1.96Fe0.04)Σ2.00(As0.84S0.09Si0.04)Σ0.97O10H5.19. Raman spectroscopy confirms the presence of OH and H2O in the mineral. Forêtite is triclinic, space group P-1, with the unit cell parameters: a = 6.969(9), b = 7.676(9), c = 8.591(11) Å, α = 82.01(9), β = 71.68(8), γ = 102.68(8)°, V = 415(1) Å3 and Z = 2. The five strongest lines in the X-ray powder diffraction diagram are : 7.307, (100), (010, 0-10); 3.141, (24), (200, -200); 2.818, (24), (2-20, -220); 2.343, (22), (1-31). The mineral is named in honour of Dr Jean-Paul Forêt, who co-founded the project that turned the Cap Garonne mine into a protected site and museum.

Edited 3 time(s). Last edit at 06/26/2012 11:36PM by Stuart Mills.
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