Clay Mineralogy In The Tongcheng Granite-Derived Soil And Evolution Of Typical Minerals Of Granite During Weathering Under Subtropical Climate

Weathering is the mechanism for redistributing mass at the Earth’s surface,playing an important role in the nutrient cycling,climate regulation and mineral deposits formation.It is closely related to the Earth’s livable environment the sustainable development of human society.Compared with other climatic conditions,the chemical weathering in subtropical climate zone is intense and relatively fast,and the weathering profiles are well developed,which is conducive to revealing the alteration process and mechanism of minerals during weathering.In this thesis,the weathering profile developed on biotite monzogranite in Tongcheng,Hubei Province was analyzed using X-ray powder diffraction（XRD）,Fourier infrared spectrometer（FTIR）,differential thermal-thermogravimetric analysis（DSC-TG）,Field emission scanning electron microscopy（FESEM）and focused ion beam-transmission electron microscopy（FIB-TEM）.The alteration process and mechanism of biotite and K-feldspar were studied based on the alteraction characteristics of secondary phases,lattice structure changes and other aspects.The influence of crystal structure of precursor minerals on the alteration process and mechanism was explored.Combined with the mineralogy,element geochemistry and Sr isotope geochemistry of weathering profile,the development processes of weathering profile in subtropical climate were illustrated.The main insights are as follows:（1）Biotite transforms simultaneously into biotite/vermiculite mixed layer and halloysite,and the vermiculite layer in the mixed layer is interhydroxyl vermiculite phase.Biotite/vermiculite mixed layer exists in the whole regolith,and the content of vermiculite layer is 5%,17%,18%and 4%in saprolite,C horizon,B horizon and A horizon,respectively.The biotite/vermiculite mixed layer in the upper part of the profile is partially transformed into discrete vermiculite,which is caused by intense weathering.Vermiculitization of biotite involved leaching of K⁺and accumulation of hydrated Mg²⁺and Fe³⁺in the interlayer space of biotite precursor and thus occurred in a solid-solid state transformation.The neoformed halloysite is dehydrated phase.Halloysite occurs in tubular morphology at the edge of biotite or in well-oriented lath-like clusters on the basal surface of biotite crystals.The former crystallized from solutions,and the lattice structure of biotite controls the orientation of halloysite.The latter formed from local leaching of Si–O sheets of biotite and subsequently in situ recrystallization.The parallel lath-like halloysite clusters occur in certain extending directions with angles of～30°and～60°respectively,resulting from the size misfit between the tetrahedron and octahedron of the crystal structure of biotite precursor.Thus,the formation of halloysite can be considered as a dissolution–precipitation process.（2）In the process of weathering,the development of goethite in biotite grains can be divided into three stages:In the early stage,a small amount of crystallized goethite precipitates in the interlamellar pores of biotite,and the crystal lattice of biotite on both sides does not change significantly;Further weathering results in the dissolution of biotite lattice,forming amorphous Fe-Al-Si phase,from which a large amount of goethite crystallizes,and biotite transforms into altered biotite（10.5?）,halloysite（7?）,and vermiculite（15?）in the meantime.In the final stage,the biotite lamellars continue to separate,and the tubular halloysite is formed.The weathering sequence of biotite in the regolith derived from biotite monzogranite in Tongcheng in subtropical climate is:biotite→halloysite+biotite/vermiculite mixed layer+goethite（hematite）→vermiculite（interhydroxyl vermiculite）.（3）The weathering causes the dissolution of K-feldsapr surface,and produces a string of etched pits which arranged along their diagonal,long axis and short axis on the K-feldspar surface.Further dissolution makes the etched pits connect with each other to form cracks,indicating that the weathering process of K-feldspar is mainly controlled by a surface reaction-controlled dissolution mechanism.The dissolution cracks are mainly along the cleavage plane and composition face of K-feldspar.The amorphous Si-rich phases are produced in the K-feldspar grains during the dissolution.The TEM images show the nm-sharp structural interface between K-feldspar and amorphous phase,which may be caused by the precipitation after the dissolution of K-feldspar.Therefore,the dissolution mechanism of K-feldspar is interfacial dissolution-reprecipitation mechanism.The bonding force of cleavage plane and composition face is weak in the crystal structure.The amorphous Si-rich precursor formed by dissolution crystallizes into kaolinite lamellae,and the lamellae gradually develops stacking,finally forming a booklike kaolinite aggregate.The weathering sequence of K-feldspar minerals is:K-feldspar→kaolinite.（4）According to the variation characteristics of ⁸⁷Sr/⁸⁶Sr isotope ratio,the profile is divided into three stages:In stage I（400-240 cm）,⁸⁷Sr/⁸⁶Sr ratios are relatively small,ranging from 0.7176 to 0.7202;In stage II（240-120 cm）,⁸⁷Sr/⁸⁶Sr ratios fluctuate greatly in a wide range（0.7728-0.7352）.In stage III（120-0 cm）,Sr isotope ratios gradually decrease,and the variation range become smaller（0.7244-0.7279）.Sr abundances are negatively correlated with CIA,and the ⁸⁷Sr/⁸⁶Sr ratios increase when the samples have CIA values<93 and decrease when their CIA values>93.It indicates that more Sr with low ⁸⁷Sr/⁸⁶Sr ratio is released from residual primary minerals with lower chemical weathering intensities（CIA<93）and more radiogenic Sr associated with absorption in clay mineral is leached with extreme high chemical weathering intensities（CIA>93）.（5）The weathering process involves a strong depletion of Ca,Na,K and Si and an enrichment in Fe,indicating the profile undergoes intense chemical weathering.The weathering profile exhibits three profile patterns of trace elements as revealed by mass transfer coeffificients（τNb,j）:（ⅰ）enrichment（e.g.,Zr,Hf）,（ⅱ）depletion（e.g.,Li,Be,Co,Rb）,（ⅲ）depletion-enrichment（e.g.,U,Sc,Ba,Cs）.The profile shows the pattern of light rare earth element（LREE）accumulation and heavy rare earth element（HREE）.However,in the upper part of the profile,there is a trend of relative enrichment of HREE,which leads to higher LREE/HREE fractionation.K-feldspar,quartz and kaolin control the content of K,Si and Al in the profile,respectively.In the upper section（120-0 cm）,the content of Al,mass transfer coefficient of Al（τNb,Al）and Fe（τNb,Fe）are between 16.21-21.85 wt%,-0.59～0 and 0.96-2.40,respectively,which are significantly lower than those in the lower layer.It shows that the fine particles of kaolin and iron minerals in the upper layer of the profile migrate to the lower section with the infiltration of soil water due to the hydrodynamic force.（6）The clay mineral assemblage of granite weathering profile in subtropic climate is kaolinite（halloysite）+illite+interhydroxyl vermiculite+vermiculite/illite mixed layer.The content of kaolinite is the highest,ranging from 60 to 90%,and interhydroxyl vermiculite mainly develops in the surface layer of soil profile.In addition,the main iron oxide minerals in the weathering profile are goethite and hematite.The content of goethite is greater than that of hematite in the lower layer of the profile,while the content of hematite turns to be larger than that of goethite in the upper layer of the profile.