Accumulation of carbonatic components in arid and semiarid regions is one of the most significant parameters in evaluating the degrees of soil development, soil age, soil classification and subsequently land use. Although the distinction of carbonatic features is usually simple in noncalcareous parent material, but due to carbonatic parent material of many arid and semiarid soils, the distinction between primary and secondary carbonates in calcareous soils is not always an easy task. Therefore, to make sure the origin and probably the mechanism of the formation of these features, usually the use of more detailed laboratory techniques is indispensable. The present study is one of the first investigations about micromorphological properties of carbonates and their most probable mechanism of formation in arid and semiarid soils around Urmia Lake with calcareous and noncalcareous parent material. The properties of these features were investigated on thin sections by use of a petrographic microscope. Based on micromorphological properties, carbonate accumulations were placed into five categories: 1) Needle fiber calcite that are present in large pores and as coatings on skeleton grains and aggregates. These needles are accumulated as MA type. The MA type needles are long and smooth, composed of calcite fibers. 2) Micritic calcium carbonate coatings (Calcans) on peds and around skeleton grains with different thicknesses are subdivided into 3 types. The first type is recognized as Carbonate rinds. The rinds uniformly cover the entire skeleton grains and are single layered. The second type are also coatings on skeleton grains but with more than one layer, and the third type are coatings that occur on the underside of coarse skeleton grains, where downward growths of calcium carbonate rich water have been restricted by contact with the soil matrix. This type is named Calcium carbonate coats. 3) A third form of calcium carbonate accumulations are nodules, composed of micritic and microsparitic and in some cases sparitic carbonates. The observed nodules are subdivided into 3 types, Typic nodules, Nucleic nodules and Geodic nodules. 4) A forth form of calcium carbonate accumulations are carbonatic pendants that have formed as stalactite-like masses on bottoms of calcium carbonate coats or directly on bottoms of coarse fragments. 5) The last and the fifth form of carbonates are Carbonatic hypocoatings. These features have been formed by impregnation of soil matrix in edges of voids with micritic calcite. In addition to these five categories, carbonate depleted zones were also observed in thin sections, in relation with other features.
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