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.

The crystal structure of ceruleite, CuAl4[AsO4]2(OH)8(H2O)4, has been solved to an R1 of 0.0307, using the world's largest crystals from the Cap Garonne mine, France. Ceruleite crystallizes in space group P21/n, with the unit cell a = 7.2000(14), b = 11.345(2), c = 9.856(2) Å, β = 105.57(3)°, V = 775.6(3) Å3 and Z = 1. Ceruleite has a unique structure that consists of Al(O,OH)6 octahedra that are sharing edges to form rhombus-shaped tetramers. AsO4 tetrahedra share two corners with one such rhombus and the other two corners with each of two other rhombi, linking them into a very open mesoporous framework. Cu(OH)2(H2O)2 squares lie in the channels and link Al4 rhombi along || b. H2O molecules are also located in the channels.

Thérèsemagnanite was originally described from the Cap Garonne mine, Var, France. Its ideal formula was reported as (Co,Zn,Ni)6(SO4)(OH,Cl)10·8H2O; without crystal structure data, only the powder X-ray diffraction pattern was given. Revision of the holotype material revealed that thérèsemagnanite is identical to ‘cobaltogordaite’ (IMA2014-043), recently described from the Blue Lizard mine, Utah, USA. Thérèsemagnanite is thus redefined in accordance with the new data obtained for the neotype specimen from Blue Lizard (formerly the holotype specimen of ‘cobaltogordaite’) and ‘cobaltogordaite’ has been discredited by the International Mineralogical Association Commission on New Mineral Nomenclature and Classification (IMA CNMNC). Thérèsemagnanite has the ideal, end-member formula NaCo4(SO4)(OH)6Cl·6H2O. The empirical formulae of the holotype (Cap Garonne) and the neotype (Blue Lizard), both based on microprobe analyses and calculated on the basis of 17 O + Cl atoms per formula unit (with fixed 6 OH groups and 6 H2O molecules; H content is calculated by stoichiometry) are (Na0.64K0.09)Σ0.73(Co2.35Zn1.22Ni0.50)Σ4.07S1.02O3.98(OH)6Cl1.02·6H2O and Na1.01(Co1.90Zn1.37Ni0.48Cu0.15Mn0.05)Σ3.95S1.03O4.09(OH)6Cl0.91·6H2O, respectively. Thérèsemagnanite is trigonal, P$\overline 3 $, a = 8.349(3), c = 13.031(2) Å, V = 786.6(4) Å3 and Z = 2 (neotype). The strongest powder X-ray diffraction lines are [dobs in Å (hkl) (Irel)]: 13.10 (001)(100), 6.53 (002)(8), 4.173 (110)(4), 3.517 (112)(5), 2.975 (104, 10$\overline 4 $)(4), 2.676 (211)(5) and 2.520 (12$\bar 2$)(5) (neotype). Thérèsemagnanite is a cobalt analogue of gordaite, NaZn4(SO4)(OH)6Cl·6H2O. These minerals represent the gordaite group, accepted by the IMA CNMNC.