Abstract
Mg-bearing silicate precursor gels have been prepared by mixing 0.74 molL sodium metasilicate and 1.48 molL magnesium nitrate solutions. Caustic soda solution of 1.0 molL oncentration was introduced to regulate pH. The magnesium nitrate solution was added dropwise to the sodium silicate solution in equi-volume at various pH values. Raw and heat-treated gels were characterized by XRF,DTA,XRD and FE-SEM.As a result,gel compositions were dependent on pH values of mixing solutions.The pH value yielding stoichiometric forsterite composition,MgOSiO2=2 was determined to be pH 9.6 from the variation curve. In addition,this value was pH 8.4 for stoichiometric enstatite composition,MgOSiO2=1.With decreasing pH from 9.6,the ratio became less than 2 and forsterite and enstatite precipitated by heating the gels.With increasing pH from 9.6,the ratio became more than 2 and forsterite and periclase precipitated by heating the gels. DTA curves showed a characteristic exothermic peak centered at 700-900,indicating relatively low temperature formation of crystalline phases due to the presence of polycondensed frame works of silicates in the precursor gels.
