Theoretical Study Of Ge/Si Partition Under Supergene Conditions

Under endogenetic conditions,due to the similar chemical properties and geochemical behavior of Ge and Si,Ge can be widely distributed in silicate minerals by replacing Si.However,Ge also has the characteristics of ferriophilic,thiophilic,organophilic,abbiotic availability,which leads to the differential geochemical behavior of Ge and Si under supergene conditions.Therefore,the Ge/Si ratio is expected to be a powerful index for quantitative tracing of silicate weathering.However,the lack of Ge/Si partition coefficients between mineral/organic matter and aqueous solutions severely limits the development of this beneficial tool.In this thesis,the first-principles calculation method based on density functional theory is used to geometrically optimize the main representative minerals under supergene enviroment,study the Ge/Si differentiation mechanism and quantify the degree in the process of the crystallization,organic chelation and surface adsorption of the secondary minerals under supergene conditions.The following conclusions are obtained:（1）The equilibrium partition coefficients ln K_D^EQ of exchange reaction between the representative mineral phase system（quartz-large,quarge-small,allophane,kaolinite,and ferrihydrite）and H₄Si O₄（H₄Ge O₄）aqueous solution are-9.39,-9.79,1.35,-6.48 and 8.36,respectively.In addition,we fit the linear relationship of ln K_D^EQ between quartz and aqueous solution in the range of 0-500℃as follows:ln K_D^EQ=-3093×T^-0.9843+2.05（T is the absolute temperature）Within the range of 0-130℃,the linear relationship of ln K_D^EQ between allophane and aqueous solution is shown as follows:ln K_D^EQ=567.4×T^-1.019-0.265（T is the absolute temperature）（2）The main adsorption state species:the equilibrium partition coefficients ln K_D^EQbetween the adsorbed species on kaolinite（010）face and iron oxyhydroxide and the aqueous solution are-2.0 and-0.58,respectively.The equilibrium partition coefficients ln K_D^EQ between the adsorbed species of kaolinite（010）face and monomethyl Ge（methyl Si）and dimethyl Ge（dimethyl Si）aqueous solutions are 1.84 and 1.39,respectively.（3）The equilibrium partition coefficient ln K_D^EQ of the different organic complexes and H₄Si O₄（H₄Ge O₄）aqueous solutions ranges from-4.59 to 83.07.In addition,with the increase of coordination number and the Si（Ge）-O average bond length,the ln K_D^EQ value gradually increases,with a positive correlation（r²=0.96）,which indicates that Ge tends to be enriched in organic complexes with high coordination number..With the appearance of Si（Ge）-C bond and the increase of Si（Ge）-C quantity,the ln K_D^EQ value will be higher,and Ge is more inclined to enter into organic compounds than Si.（4）Based on the transition state theory,the Ge/Si kinetic partition coefficient ln K _D^TST values of H4Si O4（H4Ge O4）adsorbed to the edge of kaolinite tetrahedron layer and the iron hydroxide surface are calculated,which are 13.05 and 15.22,respectively,indicating that Ge is highly enriched in the adsorption processes.（5）Finally,the ln K_D value of iron hydroxide and kaolinite is calculated with the change of kinetics,adsorption state and internal proportion of lattice as the crystallization reaction progresses.In nucleation,ln K_D=ln K_D^TST.As the crystal nucleus grows,when the masses of the substances formed by the surface adsorption process and the kinetic process are equivalent,the ln K_D values are 7.32 and 5.53,respectively.when the contributions of the kinetic process,surface adsorption,and equilibrium fractionation process processes are equal,the ln K_D values are 7.67 and 1.52,respectively.With the continuous exchange of elements between the lattice and the solution,the kinetic fractionation signal is continuously weakened until it disappears.The ln K_D values are 3.89 and-4.24 when the mass of the substances formed in interior re-equilibrium process and the surface adsorption process is equal.If the crystal has enough time to grow slowly,the complete Ge/Si exchange equilibrium between the mineral lattice and the solution is reached,and the ln K_D values are 8.36 and-6.48,respectively.These data have well unified the theoretical prediction and field observation results,revealing the internal mechanism of clay minerals and iron hydroxide enrichment of Ge.