Formation Condition of Smectite in Silica Scale: Role of Magnesium

Abstract

To elucidate the formation mechanism of smectite contained in silica scale formed in the pipeline at geothermal power plants, seven silica scale samples were characterized by chemical analysis, powder X-ray diffraction, and 27Al MAS NMR. Smectite was present in silica scale samples whose magnesium content was higher than 10 % as MgO, and aluminium content was lower than ca. 10 % as Al2O3. The peak assigned to 6-coordinated Al was observed in the 27Al MAS NMR spectra for magnesium-rich samples, while, no peak due to 6-coordinated Al appeared in magnesium-poor samples. From the comparison between the 27Al MAS NMR spectra for synthesized precipitates of Al2O3 and Al2O3-MgO, the half-width of peak for 6-coordinated Al of the Al2O3-MgO was narrower than that of Al2O3, suggesting that magnesium may stabilize the octahedral sheet (AlO6) in smectite.