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Foretite, 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. Foretite occurs as pale sky blue to aqua aggregates, up to 0.1 mm across, made up of minute plates no more than 20 m 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. Foretite 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 foretite crystals is 1.620(5). The empirical formula (based on 10 oxygen atoms per formula unit) is Cu1.94(Al1.96Fe0.04)2.00(As0.84S0.09Si0.04)0.97O10H5.19. Raman spectroscopy confirms the presence of OH and H2O in the structure. Foretite is triclinic, space group P1, with a = 6.969(9), b = 7.676(9), c = 8.591(11) A, = 82.01(9), = 71.68(8), = 102.68(8), V = 415(1) A3and 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, 010); 3.141, (24), (200, 200); 2.818, (24), (220, 220); 4.519, (23), (111); 2.343, (22), (131). The mineral is named in honour of Dr Jean-Paul Foret, who co-founded the project that turned the Cap Garonne mine into a protected site and museum. 2012 Mineralogical Society.

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Compilation and indexing terms, Copyright 2018 Elsevier Inc.<br/>20122815232197<br/>Acidic conditions<br/>arsenate<br/>Empirical formulas<br/>Formal Description<br/>France<br/>Languedoc-roussillon<br/>New mineral<br/>Space Groups