Selective dissolution of magnetic iron oxides in the acid-ammonium oxalate/ferrous iron extraction method-I. Synthetic samples

In soil magnetism, the magnetic parameters alone are not always sufficient to distinguish the lithogenic from the pedogenic magnetic fractions. Sequential extraction techniques have therefore been incorporated into magnetic studies to constrain the environmental interpretation. Here we report on the dissolution behaviour of magnetite and maghemite in the acid-ammonium oxalate method to see whether the method is suitable for specific dissolution of magnetic minerals from soils and sediments. To prevent changes in the extraction mechanism during the experiments (see Appendix A), we used an adapted version of the acid-ammonium oxalate (AAO) method, in which Fe2+ is added to the extraction solution prior to the experiment [the AAO-Fe(II) method]. The procedure was divided into several 30 min extraction steps to check the dissolution progress. Synthetic samples containing a quartz matrix with 0.1 wt per cent of iron oxides were extracted with the AAO-Fe(II) method. The iron oxides consisted of either magnetite or maghemite with grain sizes of <0.5 mm (fine grained or SD/PSD) and <5 mm (coarse grained or MD/PSD), or a 1 : 1 mixture of both minerals. Because only magnetite and maghemite were studied, the changes in magnetic characteristics could be monitored after each extraction step by analysis of the bulk susceptibility and hysteresis parameters measured at room temperature. The AAO-Fe(II) method preferentially dissolved the smaller iron oxides from the samples. For samples containing iron oxides with coarse grain size there is a preference for dissolving maghemite rather than magnetite. Extractions of the samples containing mixtures of two different grain sizes or with different mineralogy show that the method preferentially dissolves the smaller grains before attacking the coarse grains in the sample

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 investigate which magnetic minerals contribute to the environmental signal in the samples, chemical techniques such as the citrate-bicarbonate-dithionite (CBD) extraction method have been introduced in environ-mental magnetism studies. This technique is assumed to distinguish lithogenic (magnetite) from pedogenic (maghemite) mineral content in soils and sediments. Unfortunately, interpretation of the CBD extractions is not straightforward because the procedure is sometimes more suitable for distinction between grain size than for distinction between minerals. The procedure of the CBD extraction technique was investigated to determine the influence of extraction temperature and iron oxide concentration on the dissolution behaviour of the samples. Synthetic samples were extracted at three different temperatures (60º, 70º and 80ºC) at similar iron oxide concentration (5 wt%), and for three different concentrations (0.1 wt%, 1 wt% and 5 wt%) at the same temperature (60ºC). Our results show that a lower extraction temperature reduces the dissolution rate for all samples, while decrease in iron 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 are 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 of the procedure on the dissolution behaviour is taken into account. Ó 1999 Elsevier Science B.V. All rights reserved

Chemical distinction between lithogenic and pedogenic iron oxides in environmental magnetism : a search for the perfect solution

One of the goals in environmental magnetism is to understand the link between climate change and the characteristics of magnetic particles in rocks, soils and sediments. Rock-magnetic analyses sometimes are non-unique, which hinders an unambiguous identification of the magnetic particles that carry the climatic information. In this thesis, several complementary methods from chemistry and soil science are examined to explore whether they can assist in improving the determination of mineral-magnetic climate proxies. The first two parts of this thesis describe the results of tests with chemical extraction techniques (CBD and AAO-Fe2+ methods) on natural samples from a Czech loess-paleosol sequence as well as synthetic samples. The results indicate that both methods remove part of the pedogenic (climatically influenced) iron oxides, although in a varying degree. Neither method was able to remove the newly formed pedogenic particles with sufficient efficiency without affecting the lithogenic fraction as well. In combination with new mineral-magnetic techniques (FORC diagrams and automated IRM component analysis) the extraction methods did provide valid information about the composition of the samples which could not be retrieved from standard rock-magnetic data alone. A new technique in electrochemistry (voltammetry of microparticles) is discussed in the final part of this thesis. This technique was tested here for the first time on natural loess and paleosol samples to determine whether it is suitable for identifying and quantifying the magnetic mineral composition in natural samples. The results indicate that the technique is well suited for identifying as well as quantifying the weak magnetic minerals such as hematite, even when these are present in small amounts. With this thesis we have introduced and improved techniques for determining the magnetic mineral composition of natural samples, and the results have brought us a step closer to understanding the climate signal of magnetic particles in soils and sediments

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 investigate which magnetic minerals contribute to the environmental signal in the samples, chemical techniques such as the citrate-bicarbonate-dithionite (CBD) extraction method have been introduced in environ-mental magnetism studies. This technique is assumed to distinguish lithogenic (magnetite) from pedogenic (maghemite) mineral content in soils and sediments. Unfortunately, interpretation of the CBD extractions is not straightforward because the procedure is sometimes more suitable for distinction between grain size than for distinction between minerals. The procedure of the CBD extraction technique was investigated to determine the influence of extraction temperature and iron oxide concentration on the dissolution behaviour of the samples. Synthetic samples were extracted at three different temperatures (60º, 70º and 80ºC) at similar iron oxide concentration (5 wt%), and for three different concentrations (0.1 wt%, 1 wt% and 5 wt%) at the same temperature (60ºC). Our results show that a lower extraction temperature reduces the dissolution rate for all samples, while decrease in iron 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 are 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 of the procedure on the dissolution behaviour is taken into account. Ó 1999 Elsevier Science B.V. All rights reserved