AbstractBettertonite, ideally [Al₆(AsO₄)₃(OH)₉(H₂O)₅]·11H₂O, is a new mineral from the Penberthy Croft mine, St. Hilary, Cornwall, England, UK. It occurs as tufts of white, ultrathin (sub-micrometre) rectangular laths, with lateral dimensions generally <20 μm. The laths are flattened on {010} and exhibit the forms {010}, {100} and {001}. The mineral is associated closely with arsenopyrite, chamosite, liskeardite, pharmacoalumite, pharmacosiderite and quartz. Bettertonite is translucent with a white streak and a vitreous to pearly, somewhat silky lustre. The calculated density is 2.02 g/cm³. Optically, bettertonite is biaxial positive with α = 1.511(1), β = 1.517(1), γ = 1.523(1) (in white light). The optical orientation is X = c, Y= b, Z = a. Pleochroism was not observed. Electron microprobe analyses (average of 4) with H2O calculated on structural grounds and analyses normalized to 100% gave Al₂O₃ = 29.5, Fe₂O₃ = 2.0, As₂O₅= 30.1, SO₃ = 1.8, Cl = 0.5, H₂O = 36.2. The empirical formula, based on 9 metal atoms is Al_5.86Fe_0.26(AsO₄)_2.65(SO₄)_0.23(OH)_9.82Cl_0.13(H₂O)_15.5. Bettertoniteis monoclinic, space group P2₁/c with unit-cell dimensions (100 K): a = 7.773(2), b = 26.991(5), c = 15.867(3) Å, β = 94.22(3)°. The strongest lines in the powder X-ray diffraction pattern are [d_obs in Å(I)(hkl)] 13.648(100)(011); 13.505(50) (020); 7.805(50)(031); 7.461(30)(110); 5.880(20)(130); 3.589(20)(02); 2.857(14)(182). The structure of bettertonite was solved and refined to R₁ = 0.083 for 2164 observed (I > 2σ(I)) reflections to a resolutionof 1 Å. Bettertonite has a heteropolyhedral layer structure, with the layers parallel to (010). The layers are strongly undulating and their stacking produces large channels along [100] that are filled with water molecules. The basic building block in the layers is a hexagonal ring ofedge-shared octahedra with an AsO₄ tetrahedron attached to one side of the ring by corner-sharing. These polyoxometalate clusters, of composition [AsAl₆O₁₁(OH)₉(H₂O)₅]^8–, are interconnected along [100] and [001]by corner-sharing with other AsO₄ tetrahedra.