Nanomagnetic and Micromagnetic Properties of Rocks Minerals and Sulphide Oxidation Products
Sammanfattning: Magnetic and related properties of rocks and products from oxidation of sulphide minerals have been investigated with magnetisation measurements (rocks and oxidation products), magnetic force microscopy (rocks), scanning electron microscopy (rocks and oxidation products), transmission electron microscopy (oxidation products), Mössbauer spectroscopy (oxidation products) and powder X-ray diffraction (oxidation products). The investigations have shown the following: During thermal oxidation of pyrite (cubic FeS2) and marcasite (orthorombic FeS2) in air, nano-crystals of a-Fe2O3 and g-Fe2O3 were formed. If smaller grains of FeS2 were used, then smaller crystals of both a-Fe2O3 and g-Fe2O3 were formed, and the relative amounts of g-Fe2O3 increased. The g-Fe2O3 was investigated with magnetisation measurements, showing decreasing crystal sizes when the temperature was increased above 500°C. The increased amount of g-Fe2O3 during oxidation of small FeS2 grains was due to the lower surface energy compared to a-Fe2O3. Due to the small crystal sizes, a large surface area will be accessible for adsorption of heavy metals and sulphate ions in oxidised FeS2. Therefore, the nano-crystalline nature must affect the geochemical behaviour of similarly formed industrial waste products. During oxidation of a FeSO4 solution with a bacterial suspension, the resultant phases were a function of starting pH of the solution, and they were investigated in the magnetically ordered state (below 80 K) with Mössbauer spectroscopy. It was argued that the amount of bound sulphate in schwertmannite increased with decreasing pH of the solution. During investigation of the rock samples, a new method to report demagnetisation properties was applied. It was based on calculation of the total sum of the lengths of the demagnetisation steps, and the determination of linearity parameters, unblocking data and calculation of the fraction of characteristic remanent magnetisations. Magnetic properties of two rock formations were determined, and results related all other methods of observation. They were the following rocks: (1) granulites from SW Sweden; and (2) sheeted dykes from the Seve Nappe Complex, N Sweden. In the granulites, it was possible two discern two magnetisation components and to quantitatively determine their relative contributions to the natural remanent magnetisation (NRM). There was a 930 Ma old high blocking component in exsolved hematite-ilmenite, and a Cretacceous - recent component in multidomain (MD) magnetite. The relative importance of the MD-component increased with increased metamorphic retrogression (due to changed rock composition), but the hematite-ilmenite component still gives rise to a negative aeromagnetic anomaly in the area. In the sheeted dyke samples, the properties were due to a mixture of SD and MD grains. The latter occurred as silicate-oxide intergrowths. These were better resolved with thermal than alternating field demagnetisation. The magnetic mineralogy of the dykes was directly related to different degrees of contact metamorphism, thereby allowing detailed characterisation of processes affecting magnetic properties of rocks in the continent - ocean transition. It is argued that reports of NRM unblocking data should be important in paleomagnetism and rock-magnetism.
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