ALBERT

Description: Penberthycroftite, ideally [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]·8H 2 O, 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 〈 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/cm 3 . Optically, only the lower and upper refractive indices could be measured, 1.520(1) and 1.532(1) respectively. No pleochroism was observed. Electron microprobe analyses (average of 14) with H 2 O obtained from thermogravimetric analysis and analyses normalized to 100% gave Al 2 O 3 = 31.3, Fe 2 O 3 = 0.35, As 2 O 5 = 34.1, SO 3 = 2.15 and H 2 O = 32.1. The empirical formula, based on nine metal atoms and 26 framework anions is [Al 5.96 Fe 0.04 (As 0.97 Al 0.03 O 4 ) 3 (SO 4 ) 0.26 (OH) 8.30 (H 2 O) 5.44 ](H 2 O) 7.8 , corresponding to the ideal formula [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]·8H 2 O. Penberthycroftite is monoclinic, space group P 2 1 / 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 powder X-ray diffraction pattern are [ d obs 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)( 31). The structure of penberthycroftite was solved using synchrotron single-crystal diffraction data and refined to w R obs = 0.059 for 1639 observed ( I 〉 3( 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 decreased markedly (by ~10%) relative to that in bettertonite, with a corresponding reduction in the interlayer water content from 11 H 2 O per formula unit (pfu) in bettertonite to 8 H 2 O pfu in penberthycroftite.