Aluminium 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 room temperature and 4.2 K. A negative correlation between B-hf and Al substitution (R(2) = 0.751) was found by combining the data of all the samples, which was slightly improved (R(2) = 0.779) by taking the mean coherence length perpendicular to 111 (MCL(111)) into account. The effect of Al on lowering B-hf was, however, stronger for the tropical soil goethites than for those of the lake ores. This is parallelled by a corresponding difference in the unit-cell decrease per unit Al substitution. These differences are believed to result from the crystallization conditions in the two different environments.