Methods Lightly burned magnesium oxide was first mixed with water at a mass ratio of 1∶30 and hydrated in a water bath at 90 ℃ for 2 h to obtain Mg（OH）₂. Carbon dioxide gas was then introduced at a fixed rate for 1h to produce heavy magnesium water （Mg（HCO₃ ）₂）.The obtained water was subsequently pyrolyzed at 55 ℃ for 2 h. After suction filtration， the filter cake was dried at 50 ℃ for 20 h to obtain white powder nesquehonite （MgCO₃ ·3H₂O）.Finally， MgCO₃ ·3H₂O was mixed with deionized water at a mass ratio of 1∶30， and sodium citrate was added at concentrations ranging from 5%~50%. The mixture was stirred evenly， placed in a polytetrafluoroethylene liner， and reacted at temperatures within 110~190 ℃ for 3~13 h to obtain MgCO₃.

Results and Discussion When the hydrothermal time was extended to 13 h， prismatic anhydrous magnesium carbonate crystals with smooth surface， good dispersion， and an average diameter of 2 μm were produced. Increasing the hydrothermal temperature to 190 ℃ resulted in diamond-shaped anhydrous magnesium carbonate crystals with uniform morphology， consistent size，and average particle sizes of 3~5 μm. With the increase of sodium citrate concentration， the particle size of the obtained anhydrous magnesium carbonate decreased. With a hydrothermal temperature of 190 ℃， hydrothermal time of 13 h， and sodium citrate concentration of 10%~30%， diamond-shaped anhydrous magnesium carbonate crystals with uniform phase， shape， and average particle sizes of 3~5 μm could be obtained.During the hydrothermal reaction， strong electrostatic interactions between water molecules and metal ions created a barrier around the ions， forming hydrated metal ions. Water molecules easily integrated into the carbonate structure. Therefore， when carbonate ions combined with magnesium ions， the layer of water molecules around the magnesium ions remained. Under this condition， magnesium carbonate compounds containing crystalline water， such as hydrated magnesium carbonate （MgCO₃ ·3H₂O） and basic magnesium carbonate （4MgCO₃ ·Mg（OH）₂ ·4H₂O）， could be formed at room temperature. Among alkaline earth metal ions， the interaction between magnesium ions and water molecules was nearly the strongest， leading to basic magnesium carbonate often being an intermediate product in most hydrothermal systems. When sodium citrate was introduced into the hydrothermal system， the strong complexation reactions of citrate ions with Mg²⁺ weakened the binding effect of Mg²⁺ and H₂O，inhibiting the formation of basic magnesium carbonate. Additionally， since the ionic radius of Na⁺ was greater than that of Mg²⁺ ，the ionic hydration energy of Na⁺ was less than that of Mg²⁺ ， which further slowed down the hydration of Mg²⁺ and formed a new intermediate product， magnesium carbonate oxide （Mg₃O（CO₃ ）₂）.The Mg₃O（CO₃ ）₂unit cell is a body-centered cubic structure with amorphous morphology， while the basic magnesium carbonate unit cell is a monoclinic structure （P21/C） with a lamellar morphology. When the intermediate phase was basic magnesium carbonate， the anhydrous magnesium carbonate grains stacked on the sheet-like planes， growing in a stepped manner. Under the influence of sodium citrate， magnesium ions were complexed by citrate ions， forming a cubic structure of magnesium carbonate oxide. Without a plane similar to basic magnesium carbonate for support， the original cubic structure served as the matrix， growing in the fixed crystal directions， forming prismatic crystals.

Conclusion In the magnesium carbonate trihydrate-sodium citrate hydrothermal system， with a hydrothermal temperature of 190 ℃， hydrothermal time of 13 h， and sodium citrate concentration of 10%~30%， prismatic anhydrous magnesium carbonate crystals with uniform phase， shape， and average particle sizes of 3~5 μm can be obtained. During the hydrothermal reaction，the complexation of magnesium ions by citrate ions and the strong hydration of sodium ions inhibit the hydration ability of magnesium ions. This results in the intermediate product changing from flake-like basic magnesium carbonate crystals to bulk magnesium carbonate oxide. The different morphologies of the intermediate products correspond to different growth modes， transitioning from “stacked” two-dimensional growth to “stepped” three-dimensional growth， which causes the changes in morphology of the final product.