The formation of alunite under hydrothermal conditions chemistry, unit-cell parameters and vibrational spectra

Rudolph, Wolfram W. (2001) The formation of alunite under hydrothermal conditions chemistry, unit-cell parameters and vibrational spectra. Masters thesis, Memorial University of Newfoundland.

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Abstract

Raman spectra have been measured for aqueous Al₂(SO₄)₃ solutions from 25°C to hydrothermal conditions at 184°C under steam saturation. The Raman spectrum at 184°C contained four polarized bands in the S-O stretching wavenumber range which suggest that a new sulfato complex, where sulfate acts as a bridging ligand, possibly bidentate of tridentate, is formed in solution in addition to a 1:1 aluminium(III) sulfato complex, where sulfate is monodentate, which is the only ion pair identified at room temperature. Under hydrothermal conditions it was possible to observe the hydrolysis of Al(III) aqua-ion by measuring the relative intensity of bands due to SO₄²⁻ and HSO₄⁻ according to the coupled equilibrium reaction [Al(OH₂)₆]³⁻ + SO₄²⁻ - [Al(OH₂)₅OH]²⁻ + HSO₄. The precipitate in equilibrium with the solution at 184°C could be characterized as hydronium alunite, (H₃O)Al₃(SO₄)₂(OH)₆. -- Alunites of the substitution series potassium alunite [KAl₃(SO₄)₂(OH)₆]- hydronium alunite [(H₃O)Al₃(SO₄)₂(OH)₆ have been synthesized from acidic solutions (pH values 1.5 to 3.0) between 100 to 205°C and between 1 and 18 bars in both sealed quartz and teflon lined bombs. The composition of the starting solutions ranged from 0 to 300% potassium with aluminium fixed at ca. 0.6 M and sulfate ranging from 0.16-0.18 M. -- For reasons of comparison a few end members of the alunite group have been synthesized such as sodium alunite [NaAl₃(SO₄)₂(OH)₆, ammonium alunite [(NH₄)Al₃(SO₄)₂(OH)₆], and rubidium alunite [RbAl₃(SO₄)₂(OH)₆].

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