Fluid inclusions and stable isotopes of giant transparent gypsum crystals in a huge geode recently discovered in Pulpí (SE Spain) are useful tools for explaining its geological formation. Fluid inclusions suggest a mixture of fluids, starting with freshwater and including seawater in more advanced stages of the gypsum crystal growth. The isotopic composition of the hydration water in the gypsum crystals agrees with a meteoric fluid. The gypsum is enriched in 34S, which denotes genetic links with marine sulphates via freshwater dissolution-recrystallization of earlier marine evaporites.

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