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Huang Cuifen;Zhang Guangrong;Wang Yinghua Institute of Geology, State Bureau of Building Materials Northwest,Geological Company, State Bureau of Building Materials Department of Geology,Peking University  
Mineralogical studies have been made on different types of dolomite from vario-us horizons in some areas of China, together with determinations of their oxygenand carbon isotopic compositions and consideration of their modes of occurrence inthe field. A conclusion has been arrived at that the ratio between major elementsof dolomite and the perfection of its lattice structure are controlled by the timeand environmeut of rock formation. Generally speaking, dolomite formed affer theTertiary possesses the following features. It contains more Ca than Mg, with theformer making up 55% and the latter 45% its unit cell is much larger than thatof ideal dolomite, which is ascribed to a higher content of Ca and a low degree ofordering. These features are quite shnilar to those of synthetic dolomite. All of the-se show that the mineral generally has a rather low degree of ordering and evenis disordered during the initial stage of its formation, and disordered dolomite beco-mes ordered with the change of rock-forming environments in the lengthy geologictime. In other words, highly ordered dolomite has generally once gone through alow ordered or even disordered stage. Dolomite formed before the Tertiary is char-acterized by a chemical composition and unit-cell size Somewhat close to those ofideal one and a high degree of ordering, which do not show a linear relation withthe formation time but have much to do with the rock types and genesis. Fine-tocoarse-crystalline dolomite is better developed than micritic dolemite. Regarding the genesis,diagenetic dolomite and epigenetic dolomite formed by backflow binfiltra-tion metasomatism are better than that of penecontemporaneous metasomatic dolo-mite. The most favourable condition for its formation is changing salinity causedby mixture of fresh water and saline water. Determination of oxygen and carbon isotopic compositions has revealed thatthe Z values of most dolomite lie in the high and medium-value fields, implyingthat the majority of dolomite is formed in a condition of highly saline brines, withno connection with changing salinity and salinization as well as evaporative pump-ing and backflow infiltration metasomatism, and the rock-forming temperature ishigher for diagenetic dolomite and epigenic metasomatic dolomite than that ofpenecontemporaneous metasomatic dolomite.
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