| Since discovery of dolomite in modern environments in 1960s, many dolomite models have been proposed but some long perplexed problems for the origin of dolomite formation has not been resolved. One of them is the inability to precipitate dolomite in the laboratory under conditions to replicate it that are found in modern natural environments. At the end of 20th century, Vasconcelos et al. simulated environmental conditions found in Lagoa Vermelha lagoon, Brazil, and successfully precipitated dolomite in the laboratory, which led to the formulation of the microbial model for dolomite formation. Many researchers have affirmed this model through researches on dolomite in South Australia, Arabian Gulf, outer shelf margin off western India, southern Italy and other places. The dolomite from Ordovician Majiagou Formation 5 Member in Ordos basin can be also considered as microbial origin. This microbial dolomite model may be significant in the research on mechanism of dolomite formation. As sulfate-reducing bacteria use the SO4 2- ion, they will utilize the accompanying Mg2+ ions inside their cells, because they form strong ion pairs. Uusing SO4 2- ions and release Mg2+ ions, microbes overcome the kinetic problems for their metabolism. The microbial factor converts sulfate ions from an inhibitor to a catalyst in chemical reactions of dolomite formation. Once dolomite is nucleated, the crystals continue to grow in the burial processes. Although the microbial dolomite model can not be used to explain every type of dolomite, both researches on modern dolomite environments and simulated experiments demonstrate that the model is convictive and viable. |
