Chemical remanent magnetisation and phase transformations in iron oxide minerals
Chemical remanent magnetisation (CRM) occurs as the result of the acquisition of remanence by a rock through chemical change or mineral growth. Despite the important role CRM plays in rock magnetic and palaeomagnetic processes (as a mechanism by which rocks acquire a primary remanence or as a feature of post- depositional alteration) there are few studies of this process to date. This dissertation investigates the effect of phase and chemical change on the magnetic and remanence carrying properties of some geologically important iron oxide minerals. The mineral systems studied are: the goethite-hematite, the magnetite-maghemile-hematite and the lepidocrocite-maghemite-hematite systems. The mineralogy of these systems is studied using transmision electron microscopy, thermogravimetry and x-ray diffraction methods. A transformation mechanism for the dehydration reaction of goethite is presented. The cation distribution of maghemite and its intrinsic magnetic properties are investigated. The process of CRM is simulated in the systems goethite-hematite and lepidocrocite-maghemite-hematite and the results correlated with theoretical predictions of CRM intensity, blocking volume and their dependence on mineral growth rate. A self-reversal in the maghemite-hematite system is reproduced. The low temperature oxidation of magnetite to maghemite and hematite (for example in basalts) and the low temperature dehydration reactions of goethite and lepidocrocite (as occur, for example, in sediments) are important mineral transformations. The results and conclusions of this study are extrapolated to geological environments and time scales, in particular with reference to red bed palaeomagnetism.