Bulachite, a rare aluminium arsenate from Sardinia, Italy: the second world occurrence

摘要

A second world occurrence for bulachite, Al_2(AsO_4)(OH)_3 centre dot 3H_2O, has been found in a few specimens collected on a mine dump from old workings in an arsenopyrite lens in south Sardinia (Italy). The mineral occurs as sparse, whitish, satiny, crumbly polycrystalline aggregates on mansfieldite and goethite encrusting quartzite clasts. Bulachite is intimately mixed with kaolinite, illite and an amorphous allophane-like phase, and with minor amounts of quartz and Fe~(3+) oxyhydroxides. Bulachite crystals occur as very slender, curved fibres (about 250 nm in diameter and up to 150 #mu#m long) aggregated into radial or sub-parallel groups. A least-squares unit-cell refinement of 34 reflections from X-ray powder diffraction analysis gave the following results for an orthorhombic cell: a = 15.53(3) A, b = 17.79(3)A, c=7.01(1)A, V= 1936(4)A~3.