【作者】 唐艳；

【导师】 陈乾旺；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2007， 博士

【摘要】 含铁矿物在地球、火星上分布十分广泛，因此含铁矿物的实验室模拟合成以及稳定性等问题的研究对于阐明地球、火星早期的气候特征和演化过程有着重要的意义。在本论文中，我们选取了具有重要地质学意义的铁—硫体系矿物，研究了外加磁场对胶黄铁矿向热力学稳定相矿物转化的影响特征，发现磁场能加速亚稳相胶黄铁矿向热力学稳定相的转化，表明磁场对磁性矿物的生长有显著的影响。在此基础上，研究了外加磁场对磁铁矿和磁赤铁矿纳米粒子的成核、生长以及组装行为的影响。探索发现，火星表面存在大量的灰色赤铁矿，它被认为是火星曾经存在液态水的证据。合成化学的知识告诉我们，很多物质的形成条件并不是唯一的，人们已经发现火星大气和地球大气组成有很大区别，95％以上是CO₂，我们认为这可能是有机物彻底氧化的产物，那么可以进一步推测早期的火星曾经存在有机物的海洋。因地球曾经大量喷发过甲烷，这一过程在火星上也可能发生过，甲烷氧化可以生成甲醇、甲醛、甲酸，最后彻底氧化变成CO₂。在此分析基础上，我们在甲醇热液体系中开展了合成赤铁矿的研究，根据实验结果首次提出了“火星可能曾经存在有机物的海洋”的新观点。富含橄榄石的玄武岩的实验室模拟风化研究进一步验证了我们的观点。详细内容归纳如下：1．在FeCl₃—CH₄N₂S和FeCl₂—CH₄N₂S两种反应体系中研究了磁场对亚稳相胶黄铁矿向热力学稳定相矿物的转化过程的影响。实验结果表明0.2—0.4 T的外加磁场提高了转化速率，转化速率提高的程度依赖于所加磁场的磁场强度，随磁场强度的增加而增加。例如在FeCl₃—CH₄N₂S体系中反应进行12小时后，无磁、0.2T、0.4T外加磁场的反应速率分别为3.69×10^-4，4.22×10^-4，4.99×10^-4／S。Nd—Fe—B永磁铁产生的不均匀磁场能诱导溶液产生对流，从而促进了胶黄铁矿的溶解以及稳定相的结晶成核和生长的质量传输过程。基于实验的结果，我们推测地磁场可能对地球上原生磁性矿物的形成和演化产生过重要影响。2．以摩尔盐，柠檬酸三钠，PVP为原料，在温度为180℃时通过一个简单的低温水热方法制备出高产量的单晶磁铁矿纳米线。室温下测得样品的饱和磁化强度（Ms）和矫顽力（Hc）分别为69.6emu／g和97Oe，低于通常报道的块状样品的相应值，这可能与磁铁矿纳米线的形状各向异性和纳米尺寸有关。在单晶磁铁矿纳米线的形成过程中，柠檬酸盐起着非常重要的作用。一方面，柠檬酸盐和金属离子之间强的配位作用降低了溶液中自由Fe²⁺离子的浓度，使得反应能够更加缓慢的进行，晶粒得以取向生长。另一方面，柠檬酸盐吸附在晶体的表面，使得晶粒沿一个方向择优生长。这种方法不需要外加模板、前驱物、外加磁场的诱导，从而为大规模制备磁铁矿纳米线提供了一个简单、经济的方法。3．研究了0.2—0.4T外磁场诱导下新颖的多磁畴的磁赤铁矿核桃球超结构（由米粒形基本粒子构成）的成核、生长及自组装行为。0.2—0.3T的外磁场能诱导多磁畴的核桃球超结构在三维空间的组装，形成圆筒状或者长方体三维有序结构。实验结果表明米粒形基本粒子之间强的表面相互作用力促使粒子首先聚集成类似核桃一样的球，随后在磁场的诱导下组装成三维有序纳米结构。当磁场强度增加到0.4T时，核桃球超结构在没有完全形成之前就被磁场诱导沿磁力线方向排列，形成高度有序的一维链状结构。表明磁场强度达0.4T时，磁场与米粒形基本粒子之间的相互作用力要大于粒子之间的表面相互作用，阻止了核桃球形超结构的形成。磁场下磁性纳米粒子的组装有希望成为制备高度有序的纳米结构的新方法，具体的组装结构类型取决于参与组装粒子的磁畴结构和外磁场的大小。同时该实验结果也给我们提供了一个直观的途径来认识磁场与磁性纳米粒子相互作用的特征。4．现在的火星大气中含有95％的CO₂，我们猜想可能是有机物彻底氧化后形成的。研究报道甲烷有可能是地球早期的温室气体。在火星的早期，它与地球十分相似，那么早期的火星大气中也可能存在大量的甲烷。甲烷的氧化和光化学反应得到不同程度氧化的有机物，如甲醇、甲醛等，在特定的温度和压力条件下火星上可能存在有机物（如甲醇）的海洋。事实上，在火星的大气中已经检测到甲烷和甲醛的存在。基于上述分析，我们开展了在甲醇的热液体系中合成赤铁矿的研究。以无水FeCl₃为原料在甲醇环境中模拟合成了灰色的赤铁矿，反射谱也与所报道的灰色赤铁矿类似，通过深入分析有关结果和火星大气的演化历史，我们提出了“火星曾经存在有机物的海洋”的观点。为了进一步验证我们的观点，也为了探究火星表面探测到的大面积橄榄石的风化规律，在80—160℃温度条件下，我们继续在甲醇热液体系中开展了富含橄榄石的玄武岩的实验室模拟风化研究。研究结果表明在甲醇热液体系中橄榄石以物理风化作用为主，没有化学风化发生，而橄榄石在有水的环境中会被彻底的风化。该实验结果进一步验证了我们曾经提出的“火星表面曾经存在有机物的海洋”这个观点。预示着至少在橄榄石形成之后，火星的表面不再有过大量的液态水。更多还原

【Abstract】 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.更多还原