Synthetic studies of protodolomite were carried out for understanding the conditions of protodolomite formation in sedimentary environments. Protodolomite was tried to be precipitated from concentrated sea water (by evaporating sea water) by adding 0.4M-Na2C03 solution drop by drop. The obtained minerals were amorphous carbonate, aragonite, Mg-poor calcite, Mg-rich calcite, protodolomite, huntite, monohy dromagnesite, monohydrocalcite and trihydrocalcite, depending on the concentrations of calcium and magnesium ions in the brine water, the added amount of sodium carbonate and reaction time.Protodolomite seems to be formed diagenetically from metastable minerals given above. The con centration of carbonate ions, Mg"/Ca 2+ ratio and temperature in a parent solution have an important influence on the formation of calcite type carbonate minerals and the Mg contents o f these minerals.
Abstract-Pleistocene calciferous dolomite occurs in association with, besides pyrite and carbonaceous matter, either (1) magnesian calcite, calcite and aragonite, or (2) calcite with or without aragonite, in fossiliferous wackestone of the Naha Limestone in south Okinawa. This difference in mineralogical assemblages depends on the degree of leaching through subaerial weathering, which was witnessed by the systematic difference in Cl , S04 and organic carbon and pyrite contents of the carbonate rocks and by a negative shift of S 13C in the associated calcite.
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