Efflorescences in the geothermal field of SE Melos, Greece, contain significant amounts of hydrated Al sulphate, alunogen, which could represent the Melian alumen exploited in Roman times and commended by Pliny. The efflorescences at subaerial fumaroles are explained as follows: Sulphur crystallizes on oxidation of H2S emanating from depth. Weathering produces sulphuric acid enhancing groundwater alteration of volcanic rocks. The high geothermal gradient and arid climate stimulate efflorescences. Salts are recycled during wet and dry weather leading to Al-enrichment on loss of Fe(II,III) and other cations. δ34S‰V-CDT values for sulphur in fumarole sublimates, solfatara soils and ‘veins’ range from −0.3 to 6.4‰, mean 3.8‰ (n = 8) while Al, Ca and Mg-sulphates in diverse settings range from −4.1 to 6.8‰ (n = 16). The values for sulphur indicate that the initial H2S had an igneous source and the signature is largely inherited by the sulphates.

This study aims to underpin research into the exploitation of industrial minerals in the Roman period. When searching for early alumen workings, areas with evidence of acid sulphate alteration (white rocks) and sulphurous fumarole activity should be investigated.

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