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Abstract:

The Alcanadre glauberite deposits are interbedded within the Lerín Gypsum Formation (Oligocene to Lower Miocene) in the western part of the Tertiary Ebro Basin of Spain. Glauberite forms several beds associated with other evaporitic minerals such as halite, polyhalite, gypsum, anhydrite, magnesite and dolomite. This association formed by sedimentary and early diagenetic processes in a playa-lake complex. The precipitation sequence was: carbonate, gypsum-anhydrite, glauberite, halite-polyhalite.

Gypsum occurs as a laminated facies precipitated as fine crystals on the floor of a shallow saline lake. Anhydrite formed at the lake margin and grew as interstitial nodules and enterolithic layers, or replaced pre-existing gypsum. Glauberite occurs both in nodular and enterolithic facies, commonly with convolution-like structures. Massive and banded facies are also present. Glauberite has textures ranging from fine-grained to euhedral macrocrystalline aggregates (up to 3 cm) that formed either from interstitial primary growth or from gypsum (or anhydrite) replacement. Polyhalite replaced all former sulfate phases (mainly gypsum and glauberite) and displays laminated facies and spherulitic microstructures. The polyhalite is associated with halite, which precipitated as primary crystals on the floor of the saline lake or as interstitial cement. The high Mg2+ content of the brines probably favored early transformation of all original CaCO, phases into dolomite or magnesite, both of which show micritic textures.

Because of dissolution by less concentrated (meteoric) brines, some retrodiagenetic processes have also been observed; polyhalite was replaced by glauberite, and polyhalite and glauberite by anhydrite. Burial led to further transformation of gypsum to anhydrite, and during exposure all pre-existing sulfates (anhydrite, glauberite, polyhalite) were transformed into secondary gypsum near the surface.

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