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Mineralogical ClassificationIMA 2015-025 = penberthycroftite

30th Nov 2016 05:00 UTCMarco E. Ciriotti Manager

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.

30th Nov 2016 05:01 UTCMarco E. Ciriotti Manager

Reference:

▪ Grey, I.E., Brand, H.E.A., Betterton, J. (2016): Dehydration phase transitions in new aluminium arsenate minerals from the Penberthy Croft mine, Cornwall. Mineralogical Magazine, 80, 1205-1217.

Abstract:

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 occurred at a lower temperature of ~80°C. The DL phase was stable to a temperature of ~120°C. At higher temperatures, increased rotation of the columns was accompanied by a progressive amorphization of the sample. Bettertonite, penberthycroftite and the DL phase all exhibit negative thermal expansion (NTE) along all three axes with large NTE coefficients, of the order of -100 x 10-6 °C-1.

1st Dec 2016 13:40 UTCLuís Martins 🌟

Marco, can you send me the two papers?

1st Dec 2016 14:54 UTCMarco E. Ciriotti Manager

Done. Enjoy!

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