Tertiary selenites of Europe

The term selenite is traditionally used for large transparent gypsum crystals and comes from the Greek name of the moon (σελήνη) because the appearance of sun- light turns into that of moonlight passing through thecrystals or reflecting their surfaces. In the Russian literature the name of seienite refers also to fibrous gypsum (satinspar). Since the nineteen-seventiesthisterm has been widely used in western sedimentological literature for large (centimetric to metric) gypsum crystals grown at the bottom of evaporite basins. The present authors accept this last genetic meaning of selenite for designation of a particular sedimentary rock.Selenite deposits are typical of many marine salinas and salt lakes where during solar evaporation, under permanent cover of Ca-sulphate saturated brine, they crystallize at the bottom forming rigid crusts. Selenites are also known from ancient evapqrite basins, howeyer because of the common and easy transformation of gypsum into anhydrite during early diagenesis or burial, seienite facies are rarely preserved. Ancient selenites cover areas of many hundreds kilometres in size and reach hundreds of metres in thickness. The most widespread in Europe are the Miocene selenites: the Messinian of the Mediterranean and the Badenian of the Carpathian Foredeep Basin. The best outcrops exist in Spain, Italy, Cyprus, Ukraine and Poland. The Miocene selenites are famous because of the giant size of their crystals (decimetric to metric). Such crystals show a great variety of morphological forms and commonly create unusual complex crystalline constructions.Selenite crystals are frequently twinned or aggregated. The most common are (100) twins occurring in many morphological forms. The (^-101) twins are relatively rare. They occur in Eocene (Lutetian) selenites in the environs of Paris (represented by famous twins from Montmartre). In the Ukraine, Poland and the Czech Republic, giant gypsum intergrowths resembling the (^-101) contact twins are a real natural curiosity. They display unique primary skeletal structures. The (100) twins commonly form narrow (several centimetres to 20 cm) and long (up to several metres) vertical crystal ‘swords’. Strata built of such crowded narrow twins occur in Sicily, in the Vena del Gesso Basin of Northern Apennines, and in Cyprus. In Spain, in the emrirons of Alicante, decimetric (100) twins are composed of broad chevron-like segments with the re-entrant angle opened up. The other variety of segmented (100) twins with chevrons opened downwand occurs in the environs of Madrid.Single selenite crystals, twins or intergrowths frequently form rows arranged into palisade-like strata of various thickness (centimetric to metric). A single generation of seienite crystals grown from one common substrate is termed the grass-like structure. The selenites form more complex structures when crystals grew ‘nucleating’ one on another.The selenites commonly create radial aggregates and variable domal forms. Some seienite domes are of centimetric sizes, others attains several metres in height and a dozen or so metres in diameter. The huge domes reveal variable intemal structures (both radial and concentric) and commnly are juxtaposed. Thousands of such domes occur within single strata traceable on distances of many kilometres in Sicily and the Ukraine. In Sicily and Vena del Gesso the domes are composed of quasi-radially arranged narrow (100) twins. In the Ukraine, the huge domes are built of long sabre crystals curved upward, which initially grew as smali intergrowths similarto (^-101) twins. In Poland the domes are built of horizontally curved sabre selenites which started to grow from tiny (100) twins. The most spectacular three-dimensional exposures of these domes exist in the famous Podolian caves in the Ukraine, which are the largest gypsum caves in the world.Selenite crystals clustered into separate fan-like aggregates (up to several metres in size) can be seen in the emrirons of Sorbas in Spain and are known as seienite nucleation cones. Similar conical forms are recognized in Sicily, in the Eocene evaporites of N. Spain and the Badenian gypsum of Poland and the Czech Republic.