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The synthesis of true cation-ordered dolomite at low temperature has been tried many times in the past without success. However, the natural formation of early diagenetic dolomite in various marine and nonmarine sedimentary environments is frequently observed when magnesium-calcium solutions of molar ratio greater than seven react with preexisting calcium carbonate. We have been able to synthesize protodolomite via an intermediate, amorphous Ca-Mg-carbonate phase from CaCO3 and MgCO3 solutions with a ratio of Mg/Ca = 1. Our experiments were as follows.
Reagent grade CaCO3 and MgCO3 were dissolved (0.01 molar) in distilled water with the following Mg/Ca ratios: 0.2, 0.5, 0.8, 1.0, 1.7, 3.0, 5.3, 25. Freeze drying these solutions produced X-ray amorphous Ca-Mg-carbonate phases (gels). These substances were then moistened with distilled water and maintained in sealed tubes at 55°C for a week.
A definite relation was observed between the final crystalline product and the Mg/Ca ratio of the initial solution. Solutions with a Mg/Ca ratio up to 0.8 formed only low-magnesian calcites with varying magnesium content. At a ratio Mg/Ca = 1, protodolomite was obtained within the composition range of Ca0.65 Mg0.35 to Ca0.54 Mg0.46. At a ratio Mg/Ca >1.7, high-magnesian calcite with a maximum of 19 mole MgCO3, as well as hydromagnesite and nesquehonite, developed.
The synthesized protodolomite consisted of spherules (diameter 1 and 5 µm) and spherular aggregates. The X-ray patterns, differential-thermal-analysis curves, and the congruent solubility of the material clearly distinguish it from other low-temperature magnesian calcite and prove that protodolomite (according to its original meaning) was synthesized.
There is some evidence of protodolomite formation resulting from this mechanism in nature. We have found protodolomite with a radiocarbon age of only a few decades in speleothems of the Eibengrotte (a small cave in West Germany), and C. C. von der Borch and J. B. Jones have described modern spherular dolomite from sediments of the Coorong area in South Australia.
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