Weathering Sequence Of Clay Minerals And Structure Of Intergrade Minerals In Red Earth Sediments In The Mid-Lower Reaches Of Yangtze River

Red earth is distributed widely in the middle and lower and reaches of the Yangtze River (25°N-31°N) in subtropical China. The sediments are continuous and occur widely in the areas of hills, terraces, coalesced alluvial pans, and parts of the piedmont belts. In recent years, special attention has been paid to investigate geological setting, geochemical composition, paleoclimatic records, and the origin and provenance of red earth sediments. They have been considered as the consequence of increasing aridity of the dust source area in the north, coupled with intensified East Asian winter monsoon activity in association with uplift of the Tibetan Plateau and the consequent southward displacement of Northern Hemisphere westerlies. They are accretionary in nature, syndepositional pedogenesis proceeded during weathering under humid tropical to subtropical climate conditions since the mid-Pleistocene age. During the pedogenic process and the conversion of the pre-existing clay minerals into stable phases by weathering, a sequence of intermediate interstratified clays could have developed, and intergrade minerals such as hydroxy-interlayered vermiculite (HIV) could arise from previous clay species that occur commonly in the sediments. The alteration is controlled by the nature of parent minerals, chemical composition of solutions and climatic conditions. The syndepositional pedogenesis processes would have caused clay transformation in response to the weathering conditions, and thus intermediate clays and evidence of clay transformation may be preserved in the soils. Although in recent years the sediments have attracted the interest of many pedologists and geologists for the purpose of reconstructing palaeoenvironments and palaeoclimates in south China, interpretations of the weathering sequence of the clay minerals, transformation mechanisms and structure of intergrade clay mineral are still unclear under the specific climate. Therefore, X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM) were used to provide evidence of clay mineral alteration in red earth sediments and illustrate the mechanism of transformation and weathering sequence of clay minerals. Thermodynamic calculations and quantum chemistry calculations were used to illustrate the stability relationships among clay phases in the red earth sediments and the formation and structure of the intergrade minerals. This is of great importance to clay mineralogy, soil mineralogy, and even the global change research and so on.The study area lies in the monsoon climatic zone of subtropical China, where red earth sediments are widely distributed. The studied red earth profile is located at Nanhu village (116°00’13"E,29°42’40"N), Jiujiang city, Jiangxi province, in the zone of the middle to lower reaches of the Yangtze River. The area is warm and humid in summer and relatively cool and dry in winter, with a mean annual temperature of16-18℃and a mean annual precipitation of1000to1600mm. The Quaternary sediments usually occur in Tertiary peneplains and terraces along the Yangtze River and its tributaries. The Jiujiang section is situated on the second terrace of the Yangtze River, with an elevation of41m above sea level. The profile is-14m thick. It can be divided into three pedostratigraphical units:the upper portion (0to-3.2m), the middle portion (-3.2to6.9m) and the lower portion (-6.9to14m). These portions have distinct colors and show the occurrence of white net-like veins.The clay mineral components were invested using XRD with various treatments (Air, Mg-25, Mg-Gy, Mg-400, Mg-600, K-25, K-400and K-600). The characteristic (001) reflections of clay phases are14,10-14,10, and7A, respectively. The identification of clay minerals was made according to the position of these peaks under different treatments.In the upper portion, after Mg-glycerol saturation, the14A peak did not expand, indicating the absence of smectite. When the Mg-glycerol saturated samples were heated to400℃, the14A peak shifted to13.8A, and when heated to600℃, the14A peak shifted to12A with no apparent residual reflection at14A peak, suggesting that the vermiculite species is hydroxy-interlayered and there was no chlorite in the samples. The hydroxy ions in the interlayer sites, could improve the thermal stability of vermiculite. The randomly interstratified I-HIV clays were characterized by a broad peak ranging from10-14A in air-dried conditions, which did not change in position after Mg-glycerol saturation, but collapsed to10A after being heated to600℃. I11ite was identified by the10,4.99and3.33A peaks. Kaolinite was indicated by7.15and3.57A, which are attributed to its (001) and (002) peak reflections respectively, and which disappeared upon heating to600℃. The small peak of8.16between7A and10A indicated that HIV-K clays might be present, the weak peak-intensity suggested that only trace amounts of HIV-K were present in the samples. After K saturation, the14A peak remained but collapsed to12A, which also indicated the vermiculite is hydroxy-interlayered. In the middle portion, after Mg-glycerol saturation, the14A peak did not expand but collapsed to10A after400℃,600℃and K saturation, suggesting vermiculite minerals and absence of smectite and chlorite. Interstratified I-V clays were identified by peaks ranging from10-14A in air-dried conditions, which did not change in position after Mg-glycerol saturation, but collapsed to10A after K saturation. Illite was identified by the10,4.99and3.33A peaks. Kaolinite was indicated by7.15and3.57A which disappeared upon heating to600℃. Also, small amounts of V-K mixed-layer were indicated by peaks between7-10A which disappeared upon heating to600℃. In the lower portion, the clay mineral component was similar to the middle portion, contained mainly vermiculite, illite, kaolinite, mixed-layer I-V and V-K, but the relative contents were different. Kaolinite was much abundant.A variety of lattice fringe spacings of10A,12A and7A layers was observed under HRTEM observation. In the HRTEM images, no irregular lattice fringe spacings were observed, so the epoxy used in the sample preparation may not potentially intercalate with clay minerals. Illite showed straight and regular lattice fringes, with a single layer spacing of10A. HIV or vermiculite were unstable under the electron beam; the14A lattice fringe spacing collapsed to12A due to loss of water in the interlayer and the layers were relatively more curved than those of illite. Kaolinite is easily beam-damaged, so its7A lattice fringes were only occasionally observed, and they were much more blurry. During the alteration process, clay minerals usually formed mixed-layer species which represented the intermediate phases of clay mineral transformation, as revealed in this research. In some particles, the12A layers were interstratified with10A layers; the former may be interpreted as HIV or vermiculite layers, while the latter are illite layers. In the I-HIV or I-V mixed-layer clay minerals, the I-HIV or I-V sequences were disordered. During the transformation of2:1to1:1layered clay mineral, HIV or vermiculite can partially alter to kaolinite to form an intergrade V-K. In the HRTEM images of the upper portion, it was also found that illite and kaolinite formed mixed-layer I-K, most of the layers had a lattice fringe spacing of10A, although white fringes vanished over the long range due to the electron beam damage, the7A kaolinite layer was clearly found interstratified with the10A illite layers.The interstratified phases may provide direct evidence of clay transformation. In the upper portion, HIV layers were interstratified with illite layers, the I-HIV mixed-layer showed a random nature and was dominated by illite. The transition of two illite layers into a HIV layer was observed, which suggested that illite transformed into HIV. A gradual change of the12A HIV layer to7A kaolinite layer could be attributed to partial weathering of one HIV layer into kaolinite. Two kaolinite layers terminated to one illite layer, which provided direct evidence for the mechanism of illite-to-kaolinite transformation. In the middle portion and lower portion, the12A lattice represented vermiculite layers, and the transformation mechanism was also found in the mixed-layer clays. Two illite layers altered into a vermiculite layer, and a vermiculite layer gradually changed into a kaolinite layer illustrated the transformation of illite to vermiculite and vermiculite to kaolinite.Based on XRD and HRTEM, the weathering sequences of upper portion in Jiujiang soils are proposed as:illiteâ†'I-HIVâ†'HIVâ†'HIV-Kâ†'kaolinite, while illite could transform simultaneously to kaolinite directly. In the middle portion and lower portion, the weathering sequences are proposed as:illiteâ†'I-Vâ†'Vâ†'V-Kâ†'kaolinite.Dissolution-precipitation (DP) and solid-state transformation (SST) are two main mechanisms proposed for clay mineral transformation. DP mechanism requires dissolution of the entire clay particles, and the dissolved particles pass into the bulk of the solution to form new minerals. Solid-state transformation is a reaction process that involves rearrangement of a group of atoms, the rearrangements are sequential and may spread within the lateral layers, and are usually coupled with a loss of ions that diffuse in and out of the crystal through the interlayer space. SST mechanism favors gradual structure changes, while DP mechanism requires abrupt transformation between the parent and daughter crystals. Clay mineral transformation in Jiujiang red earth sediments occurred at a layer scale, such as two illite layers into one HIV or vermiculite layer, one HIV or vermiculite layer to one kaolinite layer, and one illite layer to two kaolinite layers, the parent and product clay minerals exhibited topotactic relationship with gradual structural changes instead of full dissolution of the parent clays. The mechanism is most likely to be a SST transformation process.Thermodynamic calculations may help to determine which phase is dissolving or crystallizing to illustrate the nano-mineralogy observations. Therefore, chemical thermodynamics was employed here to discuss the illite-HIV-kaolinite stability relationships to study the intergrade minerals formation. The Al2O3-SiO2-H2O system at25℃and1atm was chosen in our calculation, as it has been deployed in other publications for clay mineral stability diagrams. pH4SiO4was been chosen as the independent variable with pH-1/3pAl as the dependent variable. The dissolution of illite was represented as:pH-1/3pAl=0.38pH4SiO4+2.03, HIV solubility was represented as:pH-1/3pAl=0.29pH4SiO4+1.6and kaolinite solubility:pH-1/3pAl=1/3pH4SiO4+1.34. At a Si concentration of10-2.6mol/L (pH4SiO4=2.6), which is the saturation solubility of amorphous silica, illite will become unstable below a pH-1/3pAl value of3.02and may transform into HIV or kaolinite. Below a pH-1/3pAl value of2.35, HIV begins to dissolve and alters into kaolinite. In the range of pH-1/3pAl values from2.21to2.35, illite alters to kaolinite with a possible vermiculitization stage. At a Si concentration of10-4.1mol/L (pH4SiO4=4.1), which is the saturation solubility of quartz, illite weathers into HIV or kaolinite below a pH-1/3pAl value of3.59, while HIV transforms to kaolinite below a pH-1/3pAl value of2.79as weathering proceeds. In the range of pH-1/3pAl values from2.70to2.79, illite transforms to kaolinite. Illite alters to kaolinite in a much smaller region than HIV, so only minor amounts of illite-kaolinite mixed-layer may be present in the samples, which is consistent with our XRD and HRTEM findings.Structure of hydroxy-interlayered vermiculite was investigated by quantum chemistry calculations. We employed the CASTEP code imbedded in material studio6.0, with a plane wave basis set within the density functional theory (DFT) formalism. The energy cutoff was340eV and the Brillouin zone k-points2×2×1. We used the generalized gradient approximation (GGA) density functional, specifically Perdew, Burke, and Ernzerhof (PBE). Further convergence details per BFGS iteration are as follows:geometry optimization, energy change2.0e-5eV/atom,|F|max0.05eV/A. The optimizing structure parameters were:a=5.12A, b=8.94A, c=14.03A; a=92.15°, β=91.65°, γ=90.05°. There are two basic structural features within the individual2:1layer-bond distances in the octahedral sheet and bond distances in the tetrahedral sheet. The results suggested that the2:1layer was consistent with experimental data. We observed two types of hydrogen bonds in the interlay region-one type formed between hydroxy aluminium complex and the2:1layer and another formed between hydroxy aluminium complexes themselves. These hydrogen bonds may result in the improved thermal stability of HIV.