Giant gypsum intergrowths from the Middle Miocene evaporates of southern Poland

Abstract

Sediments called the glassy gypsum, built of giant, up to 3.5. m high, juxtaposed skeletal cry-stalls occur in the Middle Miocene (Badenian) evaporites of southern Poland, the best exposed along the southern slopes of the Holy Cross Mts. The gypsum crystals form giant intergrowths similar to the contact 1-01 twins. These are not the true twins because any·crystallographic symmetry between the crystals does not exist. The intergrowths grew upward on the bottom of the evaporatic basin in competition for free space, and revealing unusual skeletal structures. The upper surface of the giant crystals is built of many parallel lens-shaped subcrytstals having their sharp edges pointed obliquely upward. The clay impurities, gathered in concavities of the crystals surface, were incorporated into the crystal bodies preferently along the boundaries between the lens-shaped subcrystals. The sedimentary structures in the glassy-gypsum layer, especially abraded and corroded gypsum crystals, load structures of the crystals·growing on the muddy bottom, indicate all the synsedimentary growth of the giant crystals in the glassy gypsum.