Laboratory-simulated Synthesis Of Magnetic Minerals And Cross-Discipline Research

Iron-bearing minerals are abundant on the surface of Earth and Mars. The formation mechanism and evolution processes of laboratory-simulated iron-bearing minerals may hold significant chemical clues to the early environments of Earth and Mars and subsequent evolution. In this dissertation, we investigate the influence of an external magnetic field on the conversion of greigite phase to stable iron disulfides due to the geological importance of these minerals. The conversion proceeds at a faster rate in the presence of an external magnetic field than that without an external magnetic field, clearly indicating that the applied magnetic field has a significant effect on the growth of magnetic minerals. Based on the result, we have investigated the nucleation, growth and directional aggregation of magnetic nanoparticles in the presence of an external magnetic field. The detection of large quantities of coarse-grained, gray crystalline hematite deposits on Mars has been used to argue for the presence of liquid water on Mars in the distant past. It is well known that the same materials can be formed under different conditions. There is difference in molecular composition between Earth and Mars. The presence of 95% carbon dioxide in the Martian atmosphere may be the final oxidation product of organic compounds. It is, therefore, reasonable to suggest that Early Mars may have had a methanol ocean. It has been proposed that methane is the greenhouse gas in early Earth, methane could be abundant in the atmosphere of early Mars due to the similarities between early Mars and Earth. Methane was oxidized to oxygenated organic species such as methanol and formaldehyde during some period of time in Martian history, and finally to CO₂. Based on the above analysis, synthesis of crystalline gray hematite in methanol-thermal system has been carried out. Based on the result we have proposed the hypothesis that early Mars may have had a methanol ocean for the first time. Laboratory-simulated weathering of olivine-rich basalt in methanol-thermal system has been performed to further verify the validity of hypothesis. The details are summarized as follows: 1. Both FeCl₃-CH₄N₂S experiments and FeCl₂-CH₄N₂S experiments have been carried out to study the influence of an external magnetic field on the transformation of metastable greigite phase to stable iron disulfides due to the geological importance of these minerals. The conversion proceeds at a faster rate in the presence of an external magnetic field than that without an external magnetic field. The rate of conversion depends on the intensity of a magnetic field applied, which increases with increase of the intensity of an applied magnetic field. For example, for FeCl₃-CH₄N₂S experiments in the presence of a zero, 0.2 T and0.4 T external magnetic field after 12h, the reaction rate is 3.69×10^-44, 4.22×10^-4, 4.99×10^-4/ S, respectively. It is suggested that an inhomogeneous magnetic field could magnetically induce convection so that the mass transport process during dissolution of precursor and precipitation of final product was promoted. Based on our experimental results, we could speculate that geomagnetic field could influence the alteration and evolution of the primary magnetic minerals on Earth.2. High yield of single crystalline magnetite nanowires have been synthesized by a simple low-temperature hydrothermal method using ferrous ammonium sulfate hexahydrate, trisodium citrate and PVP as starting materials at 180℃. The magnetic measurements show that saturation magnetization and coercivity of nanowires are lower than that of the corresponding bulk, which are explained from the viewpoint of the high shape anisotropy and small particle size of the magnetite nanowires. Citrate anion is proposed to be responsible for the formation of single crystalline magnetite nanowires. First, the formation of the complex [Fe(C₆H₅O₇)]^- could sharply reduce the free Fe²⁺ concentration in solution and thus result in a relatively slow reaction rate, which is favorable for the oriented growth of magnetite nanostructures along the easy magnetic axis. Second, citrate may preferentially bind to certain crystal faces of the magnetite crystals with the consequence that the crystal oriented growth along only one axis. Such an approach provides a direct and cost-effective method for large-scale production of magnetite nanowires without the need of additional template, precursor or exterior magnetic field induction. 3. The nucleation, growth and self-assembly of rice-like maghemite nanocrystals have been investigated under the induction of applied magnetic fields. It is observed that new three-dimensional architectures containing cylinder and cuboid is formed by the self-assembly of multidomain walnut superstructure composed of rice-like maghemite nanocrystals in the presence of a 0.2-0.3 T external magnetic field. In this case the surface interaction between the rice-like maghemite nanocrystals is stronger than both dipolar-dipolar interaction and magnetic interaction between the magnetic dipolar and the applied magnetic field. As a result, the walnut superstructures was formed firstly, and then assembled to 3D arrays of superstructures under the magnetic field induction. If the strength of an applied magnetic field increased from 0.3 T up to 0.4 T, highly aligned one-dimensional nanochains composed of maghemite nanocrystals instead of walnut superstructure occurred. The result shows that in the presence of a 0.4 T magnetic field, the interaction between magnetic dipole and the magnetic field is now stronger than the surface interaction between the rice-like maghemite nanocrystals, preventing the formation of walnut superstructures. The induction of an external magnetic field on magnetic nanoparticles is suggested to be a promising method for the preparation of highly aligned nanostructures. The formation of 3D or 1D architecture may depend on the magnetic domain structure of magnetic particles and the magnitude of an external magnetic field. On the other hand, the results provide direct visual insight into understanding the interaction between the applied magnetic field and magnetic particles.4. The atmosphere on Mars consists of 95% carbon dioxide, which may be the final oxidation product of organic compounds. It has been proposed that methane is the greenhouse gas in early Earth, methane could be abundant in the atmosphere of early Mars due to the similarities between early Mars and Earth. Process such as photochemical oxidation of methane could result in the formation of ocean or pool of organic compounds such as methanol. In effect, methane and formaldehyde have been detected in the Martian atmosphere. Based upon the above analysis, synthesis of crystalline gray hematite has been carried out by methanol-thermal treatment of anhydrous FeCl₃ at low temperatures of 70-160 ℃. On the basis of results we have proposed the hypothesis that early Mars may have had a methanol ocean, which provides an environment for the formation of large-scale hematite deposits on Mars. Laboratory-simulated weathering of olivine-rich basalt in methanol-thermal system at temperatures of 80-160 ℃ has been performed to further verify the validity of hypothesis. The experimental results demonstrate that physical weathering plays the dominant role, chemical weathering has not occurred. In the presence of water olivine is susceptible to chemical weathering. The results support our hypothesis that early Mars may have a methanol ocean. In addition, the observations have suggested that there is no substantial amount of liquid water on Mars after the formation of large deposits of olivine.