No abstract

The reactions leading to the formation of crystalline Mg3(OH),CI*4H20 (phase S), Mg2(0H),CI .4Hz0 (phase 3), and Mg(OH)* are compared for the systems Mg0-MgCIZ-H20 and NaOH-MgCIZ-H20. The crystalline phases were determined by X-ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system Mg0-MgCI2-Hz0, MgO serves only to increase the concentration of total magnesium and the pH of the MgClz solution.

The system BazSi04-Ca2Si04 was studied by heating mixtures of Ba2Si04 and Ca2Si04 at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases P-(Bao.osCal.es)SiOs and a'-(Bao.lsCal.8s)Si04 are isostructural with corresponding hightemperature modifications of CazSi04. The X phase (B~.,Cal.5zSi04) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Bal.31Cao.69Si04) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data.

The precipitation process of solid phases Mg3(OH),Cb4H20 (phase 5), Mg2(OH)3CI.4H20 (phase 3), and Mg(OH)2 was followed by the addition of NaOH water solution in MgClz water solutions of different concentrations (0.001 to 4.8 rnol dm-3) and characterized by chemical, potentiometric, coulometric, and X-ray diffraction analyses. The concentration range in which the precipitation of solid phases occurs was determined. The phase distributions relative to the pH of solution and concentrations of magnesium and chloride were defined by the equilibrium diagram. The approximate solubility products of stable solid phases formed at different ionic strengths and at 293 K were determined.