ALUMINUM INFLUENCE ON IRON-OXIDES .17. UNIT-CELL PARAMETERS AND ALUMINUM SUBSTITUTION OF NATURAL GOETHITES

Aluminum-iron substitution in goethites is a widely used parameter to characterize soil goethites and to gain information on the pedogenic environment. The extent of Al substitution is obtained from calibration curves consisting of linear relationships between the unit-cell edge length c and the Al content of synthetic goethites. We derived calibration curves from natural goethites obtained from two contrasting genetic environments: 84 samples from tropical soils and 30 lake ore samples from Finland. Significant correlations between unit-cell parameters and chemically determined Al (dithionite-citrate-bicarbonate-extractable Al minus citrate-bicarbonate-extractable Al) were obtained for both groups. The negative slopes of the regressions for the tropical goethites were significantly greater and closer to the slope for 106 synthetic goethites, and to the Vegard line, than those of goethites from lake ores. This is tentatively attributed to the formation environment: the temperature is lower while the rate of formation and the presence of crystallization inhibitors are probably higher in the lake environment. The younger age may also have had some influence on crystal perfection.