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Forêtite, a new secondary arsenate mineral from the Cap Garonne mine, France

Published online by Cambridge University Press:  05 July 2018

S. J. Mills ,
A. R. Kampf ,
A. M. McDonald ,
G. Favreau  and
P.-J. Chiappero
S. J. Mills*
Affiliation:
Geosciences, Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia
A. R. Kampf
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
A. M. McDonald
Affiliation:
Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada
G. Favreau
Affiliation:
421 Avenue Jean Monnet, 13090 Aix-en-Provence, France
P.-J. Chiappero
Affiliation:
Département Histoire de la Terre, Muséum National d’Histoire Naturelle, 61 Rue Buffon, 75005 Paris, France
*
E-mail: smills@museum.vic.gov.au
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Abstract

Forêtite, ideally Cu2Al2(AsO4)(OH,O,H2O)6, is a new secondary arsenate mineral from the Cap Garonne mine, Var, France (IMA2011-100). It has also been identified at the Salsigne gold mine, Languedoc-Roussillon, France. Although it was identified as a potentially new mineral in 1993, a formal description has only been possible as a result of a new find in a chamber called Annex S located near the entrance of the Cap Garonne mine. Forêtite occurs as pale sky blue to aqua aggregates, up to ∼0.1 mm across, made up of minute plates no more than 20 μm in length. It has a very pale blue streak. Individual crystals have a vitreous lustre and are transparent, whereas clusters appear translucent. The calculated density is 3.286 g cm–3. The crystals are brittle, with an irregular fracture and have a hardness of ∼3–4 on Moh's scale. Forêtite is found in direct association with bariopharmacoalumite, cyanotrichite, parnauite, chalcophyllite and mansfieldite in an Al-rich assemblage which is presumed to have formed under acidic conditions. It is biaxial; the average refractive index measured in white light on aggregates of forêtite crystals is 1.620(5). The empirical formula (based on 10 oxygen atoms per formula unit) 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 structure. Forêtite is triclinic, space group P, with 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 pattern [d in A ˚ , (I), (hkl)] are as follows: 7.307, (100), (010, 00); 3.141, (24), (200, 00); 2.818, (24), (20, 20); 4.519, (23), (111); 2.343, (22), (11). 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.

Keywords

ForÊtitenew mineralCap Garonne mineFrancearsenatesecondary mineral
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 3 , June 2012 , pp. 769 - 775
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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