We present reactions of size-selected free aluminum cluster cations, AlN+ (N = 1-14), exposed to a mixture of water and oxygen gases. It is featured that chemical species assignable to Al2O6H7+ and Al2O7H9+ were commonly produced as prominent reaction products from all of the sizes, except N = 1. These product ions were found to be produced via the formation of Al2O3+ in the initial stage of reactions with O2 and H2O, which was followed by successive hydrogenation and hydration. This reaction pathway was identified by examining reactivity of each intermediate product step by step. Structures of the product ions were analyzed by collision-induced dissociation experiments and DFT calculations; for example, coexistence of isomers, Al2O5H5(H2O)+ and Al2O4H3(H2O)2+, with one and two intact H2O molecules, respectively, was suggested for Al2O6H7+. The chemical compositions of the ions produced in the present reactions are expressed nominally as Al2O3(H2O)nH+, which is similar, except for the proton, to that of hydrated alumina, that is, forms of bulk aluminum abundant naturally. The present finding gives molecular-level insights into formation processes of aluminum minerals in a natural environment.
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