Bettertonite, 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 Al5.86Fe0.26(AsO4)2.65(SO4)0.23(OH)9.82Cl0.13(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 [dobs 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.

Bettertonite, [Al6(AsO4)3(OH)9(H2O)5]•11H2O and penberthycroftite, [Al6(AsO4)3(OH)9(H2O)5].8H2O, two new minerals from the Penberthy Croft mine, Cornwall, have flexible layer structures based on corner-connected heteropolyhedral columns. Their response to dehydration on heating was studied using in situ synchrotron powder X-ray diffraction at temperatures in the range -53 to 157°C. The bettertonite sample transforms to penberthycroftite in a narrow temperature range of 67 to 97°C with a large (8%) contraction of the layer separation and a 6 Å sliding of adjacent layers relative to each other. Above 100°C a second phase transition occurs to a DL (displaced layer) phase, involving another 8% inter-layer contraction combined with a rotation of the columns. On heating the penberthycroftite sample the phase transition to the DL phase occurs at a lower temperature of ∼80°C. The DL phase is stable to a temperature of ∼120°C. At higher temperatures, increased rotation of the columns is accompanied by a progressive amorphization of the sample. Bettertonite, penberthycroftite and the DL phase exhibit negative thermal expansion (NTE) along all three axes with large NTE coefficients, of the order of-100 x 10 -6 °C-1.