Hydrobasaluminite and basaluminite from Chickerell, Dorset

Abstract

SynopsisHydrobasaluminite and basaluminite, two hydrated basic aluminium sulphate minerals have been found in the weathering zone of the Oxford Clay at Crook Hill Brickyard, Chickerell, near Weymouth, Dorset. Hydrobasaluminite occurs as a reaction rim surrounding carbonate concretions, and is believed to have resulted from the neutralization of aluminium-bearing acid sulphate solutions formed by oxidation of pyrite and subsequent leaching of clay. Basaluminite is found only on concretions that have fallen to the floor of the pit, suggesting that it is formed as a dehydration product of hydrobasaluminite.Chemical analysis of hydrobasaluminite yields the composition 2Al2O3 · SO3 · 20H2O, although this almost certainly includes substantial amounts of adsorbed water. Chemical analysis of basaluminite gives the composition 2Al2O3 · SO3 · 9H2O, which is equivalent to a formula of Al4SO4(OH)10 · 4H2O if it assumed that water is present only as water molecules or hydroxyl ions. The sulphate ions are readily exchangeable. Electron-optical and X-ray powder diffraction data show the minerals to be monoclinic rather than hexagonal as previously reported. Indexed X-ray powder patterns give unit-cell parameters of a = 14.911(5) Å, b = 9.993(2) Å, c = 13.640(5) Å, β = 112.40(4)° for hydrobasaluminite and a = 14.857(3) Å, b = 10.011(3) Å, c = 11.086(7) Å, β = 122.28(3)° for basaluminite. The specific gravity of basaluminite is found to be 2.10 and Z = 4.Hydrobasaluminite dehydrates irreversibly to basaluminite under normal laboratory conditions, but can be preserved indefinitely at high relative humidity. A study of the dehydration of basaluminite using a diffractometer heating-stage shows the presence of three further distinct hydration states as well as interstratified intermediates. The dehydrations occur topotactically and involve major changes in the c* direction only. DTA and TGA curves can be interpreted in terms of progressive dehydration.It is suggested that the minerals possess a layer structure, probably containing gibbsite-like double-hydroxide layers with interlayer sulphate ions and water molecules. The data also seem to show a close structural relationship between basaluminite and the hydrated basic aluminium carbonate mineral, scarbroite.