ALUMINUM INFLUENCE ON IRON-OXIDES .18. THE EFFECT OF AL SUBSTITUTION AND CRYSTAL SIZE MAGNETIC HYPERFINE FIELDS OF NATURAL GOETHITES

Two sets of natural Al-substituted goethites from contrasting surface environments (24 tropical and subtropical soils vs. ten lake iron ores from Finland) were characterized by Mossbauer spectra obtained at roo m temperature and 4.2 K. A negative correlation between B-hf and Al su bstitution (R(2) = 0.751) was found by combining the data of all the s amples, which was slightly improved (R(2) = 0.779) by taking the mean coherence length perpendicular to 111 (MCL(111)) into account. The eff ect of Al on lowering B-hf was, however, stronger for the tropical soi l goethites than for those of the lake ores. This is parallelled by a corresponding difference in the unit-cell decrease per unit Al substit ution. These differences are believed to result from the crystallizati on conditions in the two different environments.