U. Schwertmann et L. Carlson, ALUMINUM INFLUENCE ON IRON-OXIDES .17. UNIT-CELL PARAMETERS AND ALUMINUM SUBSTITUTION OF NATURAL GOETHITES, Soil Science Society of America journal, 58(1), 1994, pp. 256-261
Aluminum-iron substitution in goethites is a widely used parameter to
characterize soil goethites and to gain information on the pedogenic e
nvironment. 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 calibra
tion curves from natural goethites obtained from two contrasting genet
ic environments: 84 samples from tropical soils and 30 lake ore sample
s from Finland. Significant correlations between unit-cell parameters
and chemically determined Al (dithionite-citrate-bicarbonate-extractab
le Al minus citrate-bicarbonate-extractable Al) were obtained for both
groups. The negative slopes of the regressions for the tropical goeth
ites were significantly greater and closer to the slope for 106 synthe
tic goethites, and to the Vegard line, than those of goethites from la
ke 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 environm
ent. The younger age may also have had some influence on crystal perfe
ction.