Rollandite, Cu3 (AsO4)2-4H2O, was found in the old copper mines of Roua (Alpes-Maritimes, France), associated with olivenite, conichalcite, clinotyrolite, cornubite, kolfanite, pharmacosiderite, gerhardtite, atacamite, gilmarite, wallkilldellite-Fe, cuprite, domeykite, algodonite and native copper. It forms aggregates (1mm diameter) consisting of perfect crystals (0.5 × 0.15 × 0.1 mm maximum dimension). The bottle-green crystals are elongated along a, slightly flattened on {010}, and have a good cleavage on {001}. They are not twinned. The mineral is orthorhombic, Pnma, a = 5.6906(4), b = 17.061(1), c = 9.732(1) Å, V = 944.9(1) Å3, Z = 4, Dmeas = 3.9(1) and Dcalc = 3.84 g/cm3. The strongest lines in the X-ray powder diffraction pattern (dobs in Å, (hkl), Ivis) are: 8.52, (020), 100; 3.721, (131), 60; 3.221, (141) (051), 90; 3.102, (132), 40; 2.817, (103)(033), 35; 2.795, (142), 35; 2.350, (143) (053), 25; 2.133, (080) (251), 25. Mohs' hardness is 4-4.5. Luster is vitreous transparent, streak is very light green; crystals are biaxial (-) with α = 1.745(5), β = 1.755(5), γ = 1.760(5) at 590 nm; 2Vobs = 71(2)°, 2Vcalc = 70.1(4)°. Non pleochroic. The crystal structure was solved by direct methods (MoKα radiation) and refined using 1407 observed unique reflections to R = 0.086, Rw = 0.046. The structure of rollandite is formed by (020) sheets of CulO6 octahedra and [100] chains of Cu2O5 square pyramids linked by AsO4 tetrahedra. The water molecule O6H2 is bonded by hydrogen bonds in the channels running along [100].