Dissolution behaviour of fine-grained magnetite and maghemite in the citrate-bicarbonate-dithionite extraction method

Mineral magnetic properties of soils and sediments are increasingly used as proxy parameters for environmental and palaeoclimate analysis. To investig ate which magnetic minerals contribute to the environmental signal in the s amples, chemical techniques such as the citrate-bicarbonate-dithionite (CBD ) extraction method have been introduced in environmental magnetism studies . This technique is assumed to distinguish lithogenic (magnetite) from pedo genic (maghemite) mineral content in soils and sediments. Unfortunately, in terpretation of the CBD extractions is not straightforward because the proc edure is sometimes more suitable for distinction between grain size than fo r distinction between minerals. The procedure of the CBD extraction techniq ue was investigated to determine the influence of extraction temperature an d iron oxide concentration on the dissolution behaviour of the samples. Syn thetic samples were extracted at three different temperatures (60 degrees, 70 degrees and 80 degrees C) at similar iron oxide concentration (5 wt%), a nd for three different concentrations (0.1 wt%, 1 wt% and 5 wt%) at the sam e temperature (60 degrees C). Our results show that a lower extraction temp erature reduces the dissolution rate for all samples, while decrease in iro n oxide concentration increases the dissolution rate. Thus, the parameters in the CBD procedure have a major influence on the dissolution behaviour of the samples. In practice this means that when natural samples of differing iron oxide concentration ape extracted with this technique, the results of the extractions cannot be compared. Therefore, the outcome of this type of extraction experiment can only be accurately interpreted when the effect o f the procedure on the dissolution behaviour is taken into account. (C) 199 9 Elsevier Science B.V. All rights reserved.