Gypsum ooids from the Middle Miocene (Badenian) evaporites of southern Poland

Abstract

The gypsum ooids found in the Miocene evaporites of Poland were investigated in optic and scanning microscopy. Cortical fabric was recognized and interpreted in terms of environmental conditions. The cortices are built of porous and massive layers. The former are interpreted as originated in supersaturated in respect of gypsum and agitated brine, the latter – in supersaturated but relatively less agitated brine. The lateral disappearing of the porous layers, replaced by the massive gypsum, can result from the primary growth of radial massive polycrystals or from the local recrystallization. Rounded, mainly massive gypsum grains without clear concentric structure, exhibiting partly chaotic and partly quasi-radial arrangement of crystals, are interpreted as ooids or accretionary grains originated in relatively less agitated brine than the porous gypsum ooids. Cortical crystals are predominatly oriented radially with their 010 crystallographic planes tangential to the ooid surface. This orientation is explained as a result of: (i) competitive growth of the lens-shaped cortical crystals, (ii) initial accretion of crystals (nucleation or deposition according to the snow-ball mechanism) with the 010 tangential to the substrate. The latter possibility could be realized because supposed, produced by abrasion, microcrystalline fragments of gypsum, flattened along the 010 perfect cleavage plane, might become deposited or adsorbed on the ooid, with the largest 010 surface parallel to the substrate, in such a way creating the oriented nuclei of new cortical crystals.