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鄂尔多斯地区的奥陶系主要由石灰岩和白云岩组成。在研究区东南缘的奥陶系马四组中发育一种特殊类型的白云岩，它主要由麦粒状白云石组成，其分布仅限于鄂尔多斯盆地的东南部，面积约２万平方公里。该区的相邻层位中均有膏岩及含膏岩类沉积。麦粒状白云石的晶体为一向延伸的菱面体，晶体的ｃ轴与延伸方向垂直，这与硬石膏晶体的光性特征类似。麦粒状白云石的有序度为０５２～０６０，平均为０５６；ＣａＣＯ３的摩尔含量是５１２２％～５２０５％，平均为５１６４％；δ１３Ｃ为－０１２‰～－０２０‰（ＰＤＢ），平均为－０１６‰；δ１８Ｏ为－５７５‰～－６２８‰，平均为－６０２‰；Ｆｅ２＋的含量为２９８０～３１４０（１０－６），平均为３０６３（１０－６）；Ｍｎ２＋的含量为８３～１０５（１０－６），平均为９４（１０－６）；Ｋ＋的含量为１２００～１４１０（１０－６），平均为１３０５（１０－６）；Ｎａ＋的含量为８４０～８５０（１０－６），平均为８４５（１０－６）。鄂尔多斯地区奥陶系马四组中的麦粒状白云石是由富含Ｍｇ２＋及ＳＯ２－４离子的卤水于埋藏环境中交代开阔海台地相石灰岩而形成。正是由于白云石化介质中ＳＯ２－４离子的影响，使得白?
The Ordovician in Ordos is composed mainly of limestone and dolomite. A special type of dolomite is developed in the Ordovician Ma IV Formation on the southeastern margin of the study area. It is mainly composed of barley dolomite. Its distribution is limited to the southeast of the Ordos Basin and covers an area of about 20,000 square kilometers. There are gypsum and gypsiferous sediments in adjacent layers in this area. The grain of dolomite is an elongated rhombohedron whose c-axis is perpendicular to the direction of extension, which is similar to that of anhydrite crystals. The order degree of wheat-grained dolomite was 052 ~ 060 with an average of 056. The molar content of CaCO3 was 5122% ~ 5205% with an average of 5164%. Δ13C was -05. 12 ‰ ~ -020 ‰ (PDB) with an average of -016 ‰; δ18O is -575 ‰ ~ -628 ‰ with an average of -602 ‰; and the content of Fe2 + is 2980 ~ 3140 10-6) with an average of 3063 (10-6); Mn2 + content of 83-105 (10-6) with an average of 94 (10-6); K + content of 1200-1410 Is 1305 (10-6); Na + content is 840 ~ 850 (10-6) with an average of 845 (10-6). The mallet dolomite in the Ordovician Ma IV Formation in the Ordos is formed by the confluence of open limestone in the open platform in buried environment with the brine containing Mg2 + and SO2-4 ions. It is because of dolomitic petrochemical media SO2-4 ions, making white?