A second world occurrence for bulachite, Al2(AsO4)(OH)3 · 3H2O, 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 Fe3+ oxyhydroxides. Bulachite crystals occur as very slender, curved fibres (about 250 nm in diameter and up to 150 μm 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) Å, b = 17.79(3) Å, c = 7.01(1) Å, V=1936(4) Å3.
Bulachite, a rare aluminium arsenate from Sardinia, Italy: the second world occurrence
FRAU, FRANCO;DA PELO, STEFANIA
2001-01-01
Abstract
A second world occurrence for bulachite, Al2(AsO4)(OH)3 · 3H2O, 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 Fe3+ oxyhydroxides. Bulachite crystals occur as very slender, curved fibres (about 250 nm in diameter and up to 150 μm 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) Å, b = 17.79(3) Å, c = 7.01(1) Å, V=1936(4) Å3.
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