Abstract. Salt deposits are commonly used as indicators of different paleoclimates and sedimentary environments, as well as being geological resources of great economic interest. Ordinarily, the presence of salt deposits is related to warm and arid environmental conditions, but there are salts, like mirabilite, that form by cooling and a concentration mechanism based on cooling and/or freezing. The diagenetic transformation of mirabilite into thenardite in the upper part of the lower Miocene unit of the Tajo basin (Spain) resulted in the largest reserves of this important industrial mineral in Europe. This unit was formed in a time period (~18.4 Ma) that, in other basins of the Iberian Peninsula, is characterized by the existence of particular mammal assemblages appropriate to a relatively cool and arid climate. Determining the origin of the thenardite deposits as related to the diagenetic alteration of a pre-existing mirabilite permits the establishment and characterization of the sedimentary environment where it was formed and also suggests use as a possible analog with comparable deposits from extreme conditions such as Antarctica or Mars.