Plate 166

Salt crystal casts

The samples were taken from the base of a siltstone bed in a Triassic dolomitic formation (Muschelkalk of central Europe). The marks on the bed surface reveal the shape of cubical objects, which have been replaced by silt sediment. These objects were crystals of halite (NaCl), precipitated from an evaporitic brine (hyperconcentrated solution) on the soft muddy bottom of a lagoon or a natural salina. Contrary to gypsum crystals, which nucleate within a host sediment, halite crystals form at the surface of the water body and fall to the bottom after reaching a certain size (their weight must win the surface tension of the brine). The crystals keep on growing on the bottom if the solution is supersaturated there. If a dilution occurs, the salt is dissolved but leaves its marks on the mud. Such a dilution may be caused by an influx of fresh water from rain or streams, or of marine water of normal salinity from the ocean.

Halite is one of the more soluble salts present in seawater and several lakes, and is extremely sensitive to salinity changes. At least 90% of water must evaporate to make halite separate from the solution.

One can hypothesize that the water of the Ancient lagoon evaporated completely and the bottom dried up. In that case, even a small amount of fresh water could have dissolved the salt. Is this hypothesis compatible with what you observe in the picture?

Photo: G. Piacentini 1970.