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Abstract: We studied the aluminum-induced early alteration of cultured diatom frustules in a series of laboratory experiments. Ashed frustules of the diatom Thalassiosira punctigera were exposed to natural seawater at varying dissolved aluminum (Al) concentrations (without addition, 100 nM and 500 nM Al) for periods of up to 6 months both in batch incubations and in flow-through chambers. These Al concentrations are representative of levels commonly found in pore waters in the marine environment. In general, the aluminum content of the frustules increased with time of exposure. A maximum increase of the aluminum to silicon mole ratio from 1.23×10−3 (day 0) to 3.14×10−3 was observed after 14 days of incubation at 500 nM Al. Time series with incubation times from 1 to 21 days at 100 nM Al confirm that the aluminum is rapidly associated with the diatom frustule. The uptake of Al during incubation goes along with a shift in pore structure towards smaller pores. The structural association of aluminum and silicon in biogenic silica was investigated using X-ray absorption spectroscopy at the Al–K-edge. Al–K-edge X-ray adsorbtion near edge spectra (XANES) of incubated diatom frustules demonstrate that most aluminum is present in tetrahedral coordination regardless of the Al concentration of the seawater and the duration of exposure. Al–K-edge EXAFS spectroscopy indicates the structural incorporation of aluminum into the Si framework. The results from X-ray absorption spectroscopy at the Al–K-edge, the textural changes of diatom frustules during incubation and wet chemical data are all consistent with the formation of an aluminosilicate phase on the surface of the diatom frustules. The precipitation this surface phase occurred under experimental conditions mimicking those prevailing at the sediment–water interface. We propose that the documented reaction pathway contributes to the rapid post-mortem aluminum enrichment of diatom frustules at the sediment–water interface. [Copyright &y& Elsevier]