Formation Of Magnetite, Siderite And Akaganeite: Implication For Biomineralization

Compared with abiogenic authigenic minerlas and diagenetic minerals, biogenetic minerals in nature usually possess unique crystallographic morphologies and sophisticated assembled architectures due to the regulation by the growth of orgamisms and the biomacromolecules in vivo. These features have been one of the criterions for the mineral biogenic origin. Therefore, biogenetic minerals can be used as the biosignatures for tracing the evolution of microbial ecosystem in geological environment, and provide useful information for exploring the terrestrial life origin and searching the extraterrestrial life. Iron is one of the most abundant elements within the Earth’s surface or near surface, and also one of the most important nutrient elements. Therefore, Fe-biomineralization has gained wide attentions of various fields including the mineralogy. In this dissertation, we used various synthetic methods under the conditions close to biomineralization to prepare different iron-bearing minerals’crystals and their assembled architectures, including the oriented chains of nanosized magnetite, the microsized magnetite octahedrons and their assembling flower-like architectures, the siderite microspherulites with different surface textures and the fiber-like akaganeite assembled by akaganeite nanoparticles. Meanwhile, the prepared siderite was further utilized as solid precursor to controllably synthesize magnetite for investigating the possible origin of the magnetite particles with non- thermodynamically stable morphologies in nature. Various characterization methods, such as X-ray diffraction, Raman spectroscopy, FT-IR spectroscopy, scanning electron microscopy, (high-resolution) transmission electron microscopy, Brunauer-Emmett-Teller (BET) gas sorptometry, vibrating sample magnetometer, were used to analyze the mineral phases of the products and to observe the morphologies of the crystals. Based on the investigation of the mineralization processes of different iron-bearing minerals, we proposed different mineralization mechanisms for the corresponding iron-bearing minerals. Our results may offer valuable information for better understanding the biomineralization process of iron-bearing minerals and the origins of some iron-bearing minerals with unique morphologies in natural environment. The details of dissertation are summerized as follows. 1. In the absence of biomolecules or organic additives, we designed a biomimetic experiment for synthesizing the oriented chains of magnetite (Fe3O4) nanoparticles by using akaganeite (β-FeOOH) and ferrous ions as the irons source to simulate the biomineralization process of magnetosomes in magnetotactic bacteria. Our results demonstrate that under the weak alkali environment (pH = 8.0), the obtained magnetite nanoparticles are of 35 nm in size and roughly cuboidal morphology, and can be self-assemblied into the oriented chains, which are similar to the magnetosome chains in the magnetotactic bacteria. The HRTEM observation shows that these cuboidal nanoparticles are connected, face to face, with {100} or {111} facets. Based on the intrinsic dipolar structures of magnetite crystal, we proposed that the magnetic dipole-dipole interaction drived the self-assembly of the magnetite particles into oriented chains. This implicates that in magnetotactic bacteria, except for the biological control, the dipolar interaction may be a potential candidate for the organization of magnetosomes into the oriented chain. Besides, the Lorentz force resulting from the dipolar interaction may be capable of inducing the elongated morphologies of magnetosome crystals during the crystal growth.Changing the pH values of the mineralization system, we found that a moderate alkali environment (pH = 8.0) was essential to generate the cuboidal magnetites, which resembled biogenic magnetosome in the magnetotactic bacteria, and the pH value of the alkali environment could be further determined at about 8.0. These results suggest that the environment within the vesicles of magnetotactic bacterias may be an alkali solution at the pH value of 8.0. The alkali environment originates from the decomposition of the water molecule in vesicles, followed by the H+ antitransportation out of the vesicles by some special proteins, leading to the OH- insides the vesicles increased. These insights can contribute to deepen our understanding about the entire circumstance of magnetosome formation and chain assembly, and may be useful to improve our knowledge concerning the functions of magnetoreceptive organelles in vivo.2. In the presence of biomolecule aspartic acid, we successfully fabricated the microsized magnetite octahedrons and their assembling flower-like architectures. The results show that after 24 h duration of the reaction in which aspartic acid was used as reductant and hydrated ferric oxides were used as raw materials, our product is pure magnetite, which is of 5μm in size and regular octahedral appearance with a smooth surface. A series of time-course experiments revealed that at the early stage of magnetite formation Fe3+-reduction and nanosized magnetite formation predominate, while at the later stage Ostwald ripening contributes to the growth of perfect octahedral magnetite. Meanwhile, we also investigated the magnetic properties and electrochemical performances of the magnetite octahedrons. The experimental data can be valuable for the potential industrial application of magnetite particles.Besides, with increasing the duration to 36 h at 200 oC, magnetite octahedrons can self-assemble into one-dimension rod-like structures. After 48 h, octahedrons can further assemble into three-dimension flower-like architectures. Meanwhile, we found that after 24 h duration at 240 oC, some magnetite octahedrons could directly self-assemble into flower-like architectures. These results reveal that magnetite octahedrons can assemble into the flower-like architectures through the subunits self-assembly. These magnetite rod-like structures and flower-like architectures are similar to the spear-like magnetite particles and their aggregates occuring at the clay sediment at the Paleocene-Eocene Thermal Maximum. Therefore, our results imply that some giant magnetite particles in nature may have diverse origins, which can be useful for geologists to investigate the ancient geological events or the ancient environment.3. Based on the above investigation, we successfully synthesized magnetite hollow microspheres and solid durian-like microspheres by using aspartic acid as the reductant, using triblock copolymer F127 as the assembly reagents in the aqueous system. The formation process of hollow microspheres can be described as follows. Firstly, the magnetite nanoparticles self-aggregate into the microspherulites under the modification of F127 molecules. Then, with hydrothermal reaction proceeding, F127 molecules capping to magnetite nanocrystals gradually decompose, leading to that the transient stabilization of the magnetite nanopartciles imposed by the capping F127 molecules would be broken. As a result, the magnetite within spherical aggregation could gradually diffuse to the outer layer by the dissolution-recrystallization. Finally, the hollow spherolites formed after long hydrothermal duration. Moreover, with increasing reaction durations, the hollow microspheres can evolve unique durian-like architectures with a compact surface and numerous octahedral vertexes. The SEM, TEM observations and the BET results indicate that the durian-like microspheres are solid. The evolution from magnetite hollow microspheres to durian-like solid microspheres is favorable to minimize total system energies. This aqueous method by using F127 as assembly reagent for synthesizing magnetite microspheres may be useful for magnetite to extend their applications in biomedical fields.4. Since Mckay et al. (1996) found the magnetite chain assembled by elongated magnetite particles in the carbonate globules of Mars meteorolite ALH84001, the origin of these magnetites with unique morphologies has gained wide attentions of various fields including mineralogy. It is considered that these magnetite particles could result from the decomposition of siderite. The developed habits of siderite could be preserved into the secondary mineral magnetite after the decarbonation of siderite, leading to the formation of non-thermodynamically stable morphologies of magnetite. For a deeper understanding of the transformation from siderite to magnetite, we designed a benign ascorbic acid-assisted synthetic strategy to obtain siderite (FeCO3) spherulites with different surface textures, and further fabricate magnetite throught an oxygen-limited thermolysis of siderite spherulites at 300 oC. The obtained siderite spherulites are of 10-30μm, and have different surface textures including polyhedrons, nanoprticles and triangular pyramids. The lower-temperature experimental results show that the formation of siderite microspheres is a successive multistep growth in the form of the rod-peanut-dumbbell-sphere transition, which is mainly driven by the intrinsic electric forces of siderite crystallites. With increasing ferrous concentrations in the reaction system, siderite nanoparticles or nano-sized triangular pyramids appear one after the other on the preformed spherical surfaces, resulting in different surface textures of the siderite microspheres. Based on the investigation of the tri-pyramid siderite structure obtained under the highest ferrous concentration and the direct observation of the broken siderite microspheres, we believe that the surface morphological mutations of siderite microspheres result from the secondary nucleation and overgrowth of siderite nanocrystals on the preformed spherical surface.Moreover, the calcination results show that the magnetite crystals can benignly inherit the original sizes, morphologies, surface and internal features of the precursor siderite microspheres, suggesting that the developed habits of the Fe-bearing carbonate minerals in nature could be preserved into the magnetite crystals through a change in oxygen fugacity of a coexisting fluid, a meteorite impact or a volcanic process, potentially leading to the enlongated or uncommon morphologies of some magnetite minerals in nature. This study can enrich our understanding concerning the origin of the magnetite particles with non-thermodynamical morphologies in nature.5. In nature, Gallionella Ferruginea can utilize ferric ions on its surrounding to synthesize fiber-like goethite (α-FeOOH). The formation of fiber-like goethite is closely involved with the extracellular polysaccharide. Nevertheless, the results of Carbon K-adge XANES spectrum show that the structure features of carbon atoms in the mineralized cellula filaments are similar to those of some biomacromolecules, such as serum albumin. This implies that except for the contribution of polysaccharide, the goethite mineralization may be regulated by some potential protein. In order to deepen our understanding about the biomineralization process of iron oxyhydroxides on the surface of Gallionella Ferruginea, we designed a biomimetic experiment at the low temperature to synthesize akaganeite by using bovine serum albumin as the modifier, using urea or ammonia water and the FeCl3·6H2O as the raw materials. The present results show that the obtained akaganeite (β-FeOOH) is of the fiber-like morphology, and 500-700 nm in size, similar to the biogenic goethite and the polysaccharide template-regulated akaganeite. These results suggest that except for the modification of the polysaccharide produced by bacteria, the biomineralization of the fiber-like akaganeite may be influenced by other extracellular polymer, such as protein. During the biomineralization process on the Gallionella Ferruginea surface, the synergistic operation of multiple biomacromolecules probably result in the unique fiber-like appearance of iron oxyhydroxide mineral. Our study can not only be helpful to further understand the formation mechanism of the iron oxyhydroxide mineral in vivo, but also deepen our knowledge concerning the metabolism and physiological activity related with the biomineralization process and the“mineral shields biology”mechanism.