Carbonate Inhibits the Crystallization of Aluminum Hydroxide in Bauxite

Science  26 Jan 1979:
Vol. 203, Issue 4378, pp. 355-356
DOI: 10.1126/science.203.4378.355


Although the role of anions such as nitrate, chloride, and sulfate in inhibiting the crystallization of aluminum hydroxide in soils and geological deposits is generally accepted, the dramatic effect of the carbonate ion in maintaining aluminum hydroxide gel in the amorphous form has been overlooked by most earth scientists. Examination of bauxite occurrences suggests that the inhibiting effect of carbonate is quite apparent in the smaller grain size of gibbsite in karstic bauxite deposits as compared to that in lateritic bauxite. Scarbröite, a crystalline aluminum hydroxy carbonate, has been observed in rare instances. It is suggested that the highly reactive carbonate-containing aluminum hydroxide gels used as pharmaceutical antacids are amorphous or poorly crystalline forms of scarbröite. The effect of the sulfate anion is also apparent in certain bauxite deposits where dissolution of gibbsite by sulfuric acid has resulted from the oxidation of pyrite, with subsequent reprecipitation of gibbsite.