Abstract: Flowerlike magnesium carbonate pentahydrated was prepared by aquasolution method using magnesite as raw materials and potassium dihydrogen phosphate as additives. Microtopography and phase composition of the crystal were analyzed by scanning electron microscope(SEM)， Xray diffraction(XRD) and FTIR. The influence of thermal decomposition time on the crystal morphology was explored. The formation mechanism of products was studied， as well as crystal structure of magnesium carbonate pentahydrated. The results showed that flowerlike magnesium carbonate pentahydrated with 15~30 micrometers long and 100~500 nanometers wide， can be successfully synthesized under 30min of thermal decomposition time. For the magnesium carbonate pentahydrated crystal， Mg2+ presents two different octahedralcoordination， which constitutes Mg(H2O)2+6 and［Mg(H2O)4(CO2-3)2］2-， and its chemical formula is Mg(H2O)6·Mg(H2O)4(CO2-3)2. The adsorption of potassium dihydrogen phosphate on the surface of magnesium carbonate pentahydrated changes the surface energy and growth rate of individual crystal faces， promotes it to nucleate and gradually superimpose flowerlike bodies.

Key words: magnesite, preparation, magnesium carbonate pentahydrated, flowerlike, formation mechanism