Hematitic sylvitite from the Eocene of Spain. (3.35 cm across at its widest)

The attractive rock shown above is sylvitite, a scarce evaporite. Evaporites are crystalline-textured, chemical sedimentary rocks that form by the evaporation of water (usually seawater) and the precipitation of dissolved minerals. The most common evaporites are rock salt (halitite), which is composed of the mineral halite (NaCl - sodium chloride), and rock gypsum (gyprock), which is composed of the mineral gypsum (CaSO4·H2O - hydrous calcium sulfate). Scarce evaporites include rock anhydrite and sylvitite, which is shown here. Sylvitite is composed of the mineral sylvite, which is KCl - potassium chloride. Halite has a salty taste, but sylvite (and sylvitite) have a strongly bitter salty taste. The reddish coloration of this sample is due to abundant hematite inclusions (hematite is Fe2O3 - iron oxide).

This sylvitite is part of an evaporite succession in northeastern Spain called the Cardona Saline Formation (Upper Eocene). This unit has rock anhydrite and rock salt (halitite) and sylvitite. The latter two lithologies do not compact when deeply buried - they famously "bloop" upward as diapirs, or salt domes. Salt domes are moderately common in the Gulf of Mexico. They are also known in northeastern Spain. This rock comes from the Cardona Diapir. Diapirism occurred in the Early Oligocene, during Pyrenean orogenesis and regional deformation. The Pyrenees Mountains are a collision zone mountain belt between Spain and France.

Stratigraphy: Upper Evaporitic Member, Cardona Saline Formation, Priabonian Stage, upper Upper Eocene