Characterization of the submarine disposal of a Bayer effluent (Gardanne alumina plant, southern France): II. Chemical composition of the clarified effluent and mineralogical composition of the concretions formed by its discharge in the Mediterranean Sea

•The Bayer effluent of the Gardanne alumina plant is a Na–Al–OH solution at pH 12.5 and a total dissolved load of 1.5 g/L.•Its discharge into the Mediterranean Sea leads to the massive formation of submarine concretions upon mixing with seawater.•The main component of the concretions is a Ca- and S-containing layered double hydroxide of the hydrotalcite supergroup, as such likely belonging to the wermlandite group.•Because hydrotalcite is not stable in the marine environment, it will dissolve in the long term and release its components to the environment.