In order to correctly recognize the genesis of calcium-magnesium carbonate sediments, the authors of this paper have carried out experimental studies on the system Ca2+ -Mg2+ -HCO31- -H2O at room temperature (6-35℃) and pressure. The conclusions obtained are given as follows:1. According to the rate of precipitation of materials in solid phase from solution and the kinetics of every experimental system, the mechanism of the formation of calcium-magnesium carbonate minerals has been studied. It has been found that, in the system with additional NaCl, the rate of precipitation of both magnesian-calcite and aragonite decreases rapidly with increase of the concentration of Mg2+ ions in the solution. It is apparent that the Mg2+ existing in solution inhibites not only the formation of magnesian-calcite but also the precipitation of aragonite, However, the inhibiting effect of Mg2+ on the precipitation of the magnesian-calcite is greater than that on the precipitation of aragonite. In the system with .13% additional NaCl, even though the molar ratio of Mg2+/Ca2+ in the solution reaches 40, the rate of precipitation of the magnesian-calcite is still greater than that of aragonite. In the system without additional NaCl, the precipitation of aragonite is also inhibited when the concentration of Mg2+ in the solution has been raised to a certain extent (molar ratio of Mg2+/Ca2+ =20).2. It is well known that increase in temperature of the solution favours the precipitation of CaCO3 in the form of aragonite. According to our experimental data, in the system without Mg2+ or with low concentration of Mg2+ this conclusion is correct. It is interesting to point out that, in the system with a higher molar ratio of Mg2+/Ca2+ (more than 10) and additional NaCl, increase in temperature in the range from 6℃ to 35 ℃ favours the precipitation of magnesian-calcite rather than that of aragonite. On the contrary, low temperature promotes the formation of aragonite, accompained by the precipitation of a lesser amount of monohydrocalcite (CaCO3·H2O). It is of theoretical importance to recognize the temperature conditions that favoured the formation of the ancient calcium-magnesium carbonate sediments.3. A series of magnesian-calcite with different content of Mg has been synthesized. The amount of Mg entering calcite lattice bears close relation with the temperature and the Mg2+/Ca2+ ratio in solution from which the magnesian-calcite has been precipitated. The amount of Mg entering calcite lattice increases with increase both in temperature and in molar ratio of Mg2+/Ca2+ in solution. In the system with or without additional NaCl, the amount of Mg entering calcite lattice was relatively slight. However, in the system with higher Mg2+/Ca2+ ratio, the addition of NaCl can significantly promote the formation of magnesian-calcite. Magnesian-calcite containing 66 mole % of MgCO3 has been synthesized by the authors.4. Recent marine carbonate sediments are composed principally of aragonite with a lesser amount of magnesian-calcite. On the other hand, the greater part of recent fresh-water carbonate sediments is formed by calcite. On the continent, aragonite deposits are formed only in regions adjacent to hot springs. According to the experimental data, reasonable interpretation for the geological occurrence of these carbonate minerals may be given. In the system lacking Mg2+ or with low Mg2+/Ca2+ ratio, without additional NaCl (analogous to fresh water on continent), only calcite or low-magnesian calcite was precipitated, in contrast, in the system with additional NaCl (analogous to sea water) when the molar ratio of Mg2+/Ca2+ in the system is equal to 5, the rate of the precipitation of aragonite is maximum. At present, the average value of the molar ratio of Mg2+/Ca2+ in sea water is 5.05.