EFFECT OF CITRATE BICARBONATE DITHIONITE TREATMENT ON FINE-GRAINED MAGNETITE AND MAGHEMITE

Mineral magnetic properties of soils and parent materials have been in terpreted in terms of paleoclimate and rates of soil formation but it is important to understand which minerals contribute to the mineral ma gnetic signal. Citrate-bicarbonate-dithionite (CBD) treatment has been used to determine the amounts of fine-grained, often pedogenic, ferri magnetic minerals relative to coarse-grained, often inherited, magneti c minerals. However, questions have been raised about the effect of pa rticle size on the efficacy of CBD in dissolving magnetite and maghemi te grains. In this paper we use magnetic susceptibility and its freque ncy dependence, and the low-temperature behavior of a saturation isoth ermal remanent magnetization, to track the dissolution of carefully si zed magnetite grains. We found that the standard CBD procedure dissolv es fine magnetite particles (ca. <1 mu m) but leaves larger particles (ca. >1 mu m) essentially intact. Thin oxidized coatings, presumably m aghemite, are also dissolved by the CBD procedure, These results suppo rt previous interpretations that the CBD procedure can be used to dist inguish between pedogenic and lithogenic magnetic grains, assuming tha t most pedogenic magnetic grains are sufficiently small (ca. <1 mu m) and most lithogenic magnetic grains are sufficiently large (ca. >1 mu m). These results also show that the standard procedure is too harsh t o differentiate between 1 mu m grains of magnetite and maghemite. A mo dified CBD extraction that uses half as much dithionite reduces the ma gnetic susceptibility of 1 mu m magnetite grains by only 10%. This met hod may be useful in distinguishing between magnetite and maghemite gr ains in this size range.