The local structural environments in a series of natural and synthetic alunite samples [ideally AAl3(SO4)2(OH)6, A = H3O+, D3O+, Na+, and K+] have been probed by solid-state 1H, 2H, 23Na, 27Al, and 39K NMR spectroscopy. The natural alunite [KAl3(SO4)2(OH)6] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on 27Al NMR. A new 27Al local environment (AlD) was observed and assigned to Al with one Al vacancy in the first cation sphere. Three different proton environments, Al2-OH, Al-OH2, and H3O+ are detected by 1H and 2H MAS NMR. The hydronium ion (H3O+) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in 1H and 2H NMR spectra of alunite and natroalunite despite K (Na) occupancies of significantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al3+ vacancy are compensated by adding an H3O+ and 3 H+ (creating 3 Al-OH2 groups), respectively, is too simplistic. Instead, a significant fraction of the Al3+ vacancies are compensated for by 4 H+ ions, resulting in 4 Al-OH2 groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H3O+ ion present on the A site. The resultant H2O molecule is unnecessary for charge balance, accounting for the A-site deficiency often observed. The presence of Al3+ and A+ vacancies appears closely correlated based on NMR.

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