Micromorphological characteristics of the gypsic and petrogypsic horizons of some gypsisols from Syria

The aim the study is the comparison between gypsum forms from the gypsic and petrogypsic horizons of some soils corresponding to Gypsisols (F.A.O., 1988) or Gypsids (Keys to Soil Taxonomy, 1994).These soils - located in an arid region of Syria - are developed either on the weathering products from some stratified Neogenic evaporites, gypsic predominantly, or on the alluvial fans located at the lower part of an Euphrates terrace.Micromorphological features as form, size, arrangement of gypsum (related to precipitation conditions) are described.The lenticular habit is common in the studied soils. Prismatic and tabular crystals are rare and occur mainly in the C horizon of the Petric Gypsisols. The acicular habit is characteristic only for some microcrystals.Euhedral crystals are the most frequent in these soils. Subhedral crystals occur in the petrogypsic horizons, in crystal chambers and tubes and in the surface horizon of the irrigated soil (as effect of crystals corrosion) and less in the gypsic horizon. Anhedral crystals, specific to the gypsum deposit, occur rarely in the petrogypsic horizon and in some channels and microcrystal chambers.The large variation of crystals size observed in the petrogypsic horizon, in the surface of the irrigated soil and even in several gypsic horizon indicates different precipitation processes from different solutions. The sizes are approximately equal in the recent gypsic horizons.The cementation realised by the interpenetration of gypsum crystals is specific to the petrogypsic horizon; the concave-convex and sutured contacts between gypsum crystals are the result of the alternating phenomena of dissolution-precipitation and preferential circulation of soil solution into inter-crystalline space. There is no cementation or it occurs seldom in the gypsic horizons where characteristic are the tangential and linear contacts between gypsum crystals.