Penberthycroftite, [Al₆(AsO₄)₃(OH)₉(H₂O)₅]·8H₂O, a second new hydrated aluminium arsenate mineral from the Penberthy Croft mine, St. Hilary, Cornwall, UK

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 < 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 pleochroism was observed. Electron microprobe analyses (average of 14) with H2O obtained from thermogravimetric analysis and analyses normalized to 100% gave Al2O3 = 31.3, Fe2O3 = 0.35, As2O5 = 34.1, SO3 = 2.15 and H2O = 32.1. The empirical formula, based on nine 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 powder X-ray 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)(1̄31). 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 decreased markedly (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.

Dehydration phase transitions in new aluminium arsenate minerals from the Penberthy Croft mine, Cornwall, UK

“…After removal of the visible associated minerals under a binocular microscope, quantities of very fine fragments of the impurities remained and were taken into account in Rietveld fitting of the diffraction patterns for heated bettertonite and penberthycroftite. Chemical analyses of both minerals are reported by Grey et al (2015aGrey et al ( , 2016. In addition to the main elements, small amounts of Fe and S (∼1 wt.% S) were analysed in both minerals.…”

“…TWO new secondary hydrated aluminium arsenate minerals have been described recently from the Penberthy Croft mine, located in the parish of St. Hilary, Cornwall, UK. They are bettertonite, [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]•11H 2 O (Grey et al, 2015a) and penberthycroftite, [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]•8H 2 O (Grey et al, 2016). The two minerals have related heteropolyhedral layer structures with similar monoclinic unit-cell parameters in the plane of the layers, a ≈ 7.8 Å, c ≈ 15.8 Å, but different interlayer spacings = 0.5b of 13.5 Å and 12.4 Å, respectively.…”

Bulachite, [Al6(AsO4)3(OH)9(H2O)4]⋅2H2O from Cap Garonne, France: Crystal structure and formation from a higher hydrate

“…The atomic coordinates were refined using soft distance restraints for the As−O and Al−O distances of 1.670(1) Å and 1.900(5) Å, respectively. The restrained distances used are the mean of As−O and Al−O distances obtained from single-crystal refinements of other aluminium arsenate minerals (Grey et al ., 2013, 2015, 2016 a ). Angle restraints were also used, for the AsO 4 tetrahedra, O−As−O = 109.47(2)°.…”

“…The refined structural parameters are reported in Table 2. The atom labels are the same as used for other hydrated Al arsenate minerals (Grey et al ., 2015, 2016a) where OH = hydroxyl, Ow = coordinated H 2 O and W = H-bonded H 2 O. Polyhedral bond distances, cation–cation distances and likely H-bonded pairs are given in Table 3.…”

“…As part of an ongoing characterisation of hydrated Al arsenate minerals (Grey et al ., 2013, 2014, 2015, 2016 a ,b ) we studied specimens of bulachite from Cap Garonne where the mineral forms patchy crusts of ultrathin lath-like crystals on yellow crystals of pharmacoalumite (Fig. 1).…”