Reference:
▪ Grey, I.E., Betterton, J., Kampf, A.R., Macrae, C.M., Shanks, F.L., Price, J.R. (2016): Penberthycroftite, [Al6(AsO4)3(OH)9(H2O)5]·8H2O, a second new hydrated aluminium arsenate mineral from the Penberthy Croft mine, St. Hilary, Cornwall. Mineralogical Magazine, 80, 1149-1160.
Abstract:
Penberthycroftite, ideally [Al6(AsO4)3(OH)9(H2O)5]·8H2O, is a new secondary aluminium arsenate 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 less than 20 μm. The laths are flattened on {010} and elongated on [100]. The mineral is associated with arsenopyrite, bettertonite, bulachite, cassiterite, chalcopyrite, chamosite, goethite, liskeardite, pharmacoalumite-pharmacosiderite and quartz. Penberthycroftite is translucent with a white streak and a vitreous to pearly lustre. The calculated density is 2.18 g/cm3. Optically, only the lower and upper refractive indices could be measured, 1.520(1) and 1.532(1) respectively. No observable pleochroism. Electron microprobe analyses (average of 14) with H2O obtained from thermogravimetric analysis and analyses normalised to 100% gave Al2O3=31.3, Fe2O3= 0.35, As2O5=34.1, SO3=2.15, H2O=32.1. The empirical formula, based on 9 metal atoms and 26 framework anions is [Al5.96Fe0.04(As0.97Al0.03O4)3(SO4)0.26 (OH)8.30(H2O)5.44](H2O)7.8, corresponding to the ideal formula [Al6(AsO4)3(OH)9(H2O)5]·8H2O. Penberthycroftite is monoclinic, space group P21/c with unit cell dimensions (100 K): a = 7.753(2) Å, b = 24.679(5) Å, c = 15.679(3) Å and β= 94.19(3)°. The strongest lines in the X-ray powder diffraction pattern are [dobs in Å(I) (hkl)] 13.264(46)(011); 12.402(16)(020); 9.732(100)(021); 7.420(28)(110); 5.670(8)(130); 5.423(6)(-131). The structure of penberthycroftite was solved using synchrotron single crystal diffraction data and refined to wRobs = 0.059 for 1639 observed (I>3σ(I)) reflections. Penberthycroftite 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 layers are identical to those in bettertonite, but they are displaced relative to one another along [001] and [010] such that the interlayer volume is markedly decreased (by ~10%) relative to that in bettertonite, with a corresponding reduction in the interlayer water content from 11 H2O per formula unit (pfu) in bettertonite to 8 H2O pfu in penberthycroftite.
▪ Grey, I.E., Betterton, J., Kampf, A.R., Macrae, C.M., Shanks, F.L., Price, J.R. (2016): Penberthycroftite, [Al6(AsO4)3(OH)9(H2O)5]·8H2O, a second new hydrated aluminium arsenate mineral from the Penberthy Croft mine, St. Hilary, Cornwall. Mineralogical Magazine, 80, 1149-1160.
Abstract:
Penberthycroftite, ideally [Al6(AsO4)3(OH)9(H2O)5]·8H2O, is a new secondary aluminium arsenate 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 less than 20 μm. The laths are flattened on {010} and elongated on [100]. The mineral is associated with arsenopyrite, bettertonite, bulachite, cassiterite, chalcopyrite, chamosite, goethite, liskeardite, pharmacoalumite-pharmacosiderite and quartz. Penberthycroftite is translucent with a white streak and a vitreous to pearly lustre. The calculated density is 2.18 g/cm3. Optically, only the lower and upper refractive indices could be measured, 1.520(1) and 1.532(1) respectively. No observable pleochroism. Electron microprobe analyses (average of 14) with H2O obtained from thermogravimetric analysis and analyses normalised to 100% gave Al2O3=31.3, Fe2O3= 0.35, As2O5=34.1, SO3=2.15, H2O=32.1. The empirical formula, based on 9 metal atoms and 26 framework anions is [Al5.96Fe0.04(As0.97Al0.03O4)3(SO4)0.26 (OH)8.30(H2O)5.44](H2O)7.8, corresponding to the ideal formula [Al6(AsO4)3(OH)9(H2O)5]·8H2O. Penberthycroftite is monoclinic, space group P21/c with unit cell dimensions (100 K): a = 7.753(2) Å, b = 24.679(5) Å, c = 15.679(3) Å and β= 94.19(3)°. The strongest lines in the X-ray powder diffraction pattern are [dobs in Å(I) (hkl)] 13.264(46)(011); 12.402(16)(020); 9.732(100)(021); 7.420(28)(110); 5.670(8)(130); 5.423(6)(-131). The structure of penberthycroftite was solved using synchrotron single crystal diffraction data and refined to wRobs = 0.059 for 1639 observed (I>3σ(I)) reflections. Penberthycroftite 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 layers are identical to those in bettertonite, but they are displaced relative to one another along [001] and [010] such that the interlayer volume is markedly decreased (by ~10%) relative to that in bettertonite, with a corresponding reduction in the interlayer water content from 11 H2O per formula unit (pfu) in bettertonite to 8 H2O pfu in penberthycroftite.