| Summary: | International audience In the second half of the 18th century, private French collections furnished much precious material for the description and definition of mineral species, leading to the recognition of Mineralogy as a true science. The discovery of Fontainebleau calcite at this time played its part in the dynamics of the Enlightenment (“Siecle des Lumières”). The “BelleCroix” calcites, as they were then called, from the Fontainebleau forest in the Seine-et-Marne department of France figured among the first geological publications. Any self-respecting collection of minerals had to include a sample, and specimens of this “French super classic” are still to be seen in museums worldwide. Fontainebleau may be considered to be a sort of “type locality” for this type of calcite crystal with included sand grains. The aim of this article is to reexamine the historical and geological context of these sandy calcites with their highly individual aesthetics and their place in collections. Such crystallizations can still be found in the “Fontainebleau sandstones” of the Paris basin. Geologically, these sand calcite crystals and nodules in the unconsolidated Tertiary sand formations of the Paris basin are of Pleistocene age and correlate strongly with periods of glaciation. Their occurrence and distribution suggest deposition from palaeo-groundwaters under conditions of permafrost. The nodular sandy calcites formed at shallow depths where waters infiltrated features ascribed to themokarsts. Individual sand calcite crystals and groups formed in deeper parts of the profile where infiltration was slow. Sometimes their distribution in horizontal layers indicates the level of the palaeowater tables. Crystallisation of sand calcites was triggered by the degassing of cold carbonate-containing surface waters as they infiltrated warmer subsurface groundwater environments.From a palaeoenvironmental point of view, the sand calcites of the Paris Basin furnish important clues to the interactions between meteoric and ground waters ...
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