Thermomagnetic measurements of soil iron minerals: the role of organic carbon

Thermomagnetic measurements, particularly measurements of the Curie or Néel temperature, are often used to identify magnetic minerals in rocks or sediments. In many samples it is impossible to determine the Curie or Néel temperature as mineral changes occur during heating. Especially in soils or sediments, iron (hydr)oxides may be transformed to ferrimagnetic iron oxides. In this study we analysed the thermomagnetic behaviour of some natural and synthetic iron minerals: goethite, haematite, ferrihydrite, lepidocrocite and siderite. The change of magnetization M with temperature was determined with a magnetic translation balance. The sample was heated in air to a maximum temperature of 700°C and subsequently cooled back to room temperature. By adding organic carbon to synthetic samples and by destroying it in natural samples, the influence of organic substance on the reactions was determined. Goethite, ferrihydrite and haematite transform to a strongly magnetic phase only if organic carbon is present. Lepidocrocite and siderite transform without organic carbon, the reaction of siderite is even weakened if organic matter is added. The transformation starts below 400°C for ferrihydrite and lepidocrocite and around 450°C for goethite. We can conclude that, though these reactions impede the identification of the ferrimagnetic oxides in many soil and sediment samples, they can be used to distinguish between the most common soil iron hydroxides in these environments.