Study On The Band-shift And Control Mechanisms Of Near-infrared Spectra In Chlorite Minerals

Near infrared light is defined as a kind of electromagnetic wave with wavelength located in the range of 350 nm to 2500 nm.It is the earliest band area of electromagnetic waves discovered by scientists.With the emergence of commercialized instruments and a lot of work done by predecessors,Near Infrared Spectroscopy（NIRS）has become an efficient and fast modern analytical technology.Especially in the field of hyperspectral remote sensing,most of the absorption bands of surface minerals are concentrated in the near infrared region,so all kinds of remote sensing sensors have set up important observation bands in the near infrared region.However,the potential of near infrared spectroscopy to detect the internal micro-information of altered minerals,especially clay minerals,has not been fully exploited and utilized.With the development of spectral instruments and imaging spectrometers,the traditional methods and theories of near-infrared mineral identification and hyperspectral mineral mapping are not adapted to the needs of deeper geological research.Therefore,the information of chemical composition and structure level reflected by near infrared spectroscopy of altered minerals is deeply studied,and the characteristic bands of near infrared spectroscopy caused by the change of molecular bond vibration behavior are systematically analyzed to characterize the variation characteristics.A set of methods for identifying the subclasses of altered minerals by using near infrared characteristic bands is preliminarily explored,which is helpful for deep mining of near infrared spectroscopy and hyperspectral image data.Hierarchical information and promoting remote sensing geological application of geology and mineral resources have important theoretical significance and practical value.Combining with the geological survey research project undertaken by the author,this paper takes the near infrared spectroscopy of the chlorite minerals as the research object.The samples were collected from different lithologies in Northwest China,including West Kunlun area in Xinjiang Province,Nachitai area of East Kunlun in Qinghai Province,Beishan area in Gansu Province and Qinling area in Shaanxi Province.Aiming at the phenomena of isomorphic substitution of metal cations in chlorite minerals,this paper systematically studies its proximity by combining with the characteristics of infrared spectrum band.The attribution of infrared spectra bands,the change model of crystal structure of chlorite minerals and the control mechanism of near infrared spectra bands under the change of metal cations caused by isomorphic substitution are proposed.This paper has made the following conclusions and innovative achievements:1.Attribution of near infrared spectra of chlorite minerals:The band position of infrared Raman spectroscopy is the same as the fundamental vibration frequency of molecule,while the band of near infrared spectroscopy is the overtone and combination frequency of the fundamental frequency vibration.In this paper,by collecting chlorite samples of different genetic lithologies（i.e.chlorite samples with different contents of Fe,Mg and Al）and analyzing the correlation bands between Raman spectra and near-infrared spectra,it is deduced that the near-infrared spectra bands of chlorite minerals are mainly produced by the combination frequency of fundamental vibration of hydroxyl（OH）in minerals.Among them,the hydroxyl peak near 4438 cm^-1 belongs to the combination frequency absorption band of（AlAl）O-OH stretching vibration near 3570 cm^-1 and（AlAl）O-OH bending vibration near870 cm^-1;the hydroxyl band near 4263 cm^-1 belongs to the combined frequency absorption peak of（SiAl）O-OH stretching vibration near 3442 cm^-1 and（SiAl）O-OH bending vibration near 760 cm^-1.The mean force constant of hydroxyl bond（O-H）in chlorite minerals is 6.87.2.The contents of isomorphic elements in chlorite minerals and their correlation studies show that:Mg-Fe substitution is common in chlorite minerals,followed by Al^IV ion and Si in tetrahedron.Al^VI ion is not substituted with Si in tetrahedron,so the correlation between Al₂O₃-SiO₂ contents are weak.In addition,the Si content ranges from 2.7 to 3.1,the interval length is 0.4,the Al^IV content ranges from 0.9 to 1.4,the interval length is 0.5,the Fe²⁺content ranges from 1.5 to 3.7,the interval length is 2.2,the Mg²⁺content ranges from 0.7 to 3.2,the interval length is 2.5,and the range of Si-Al^IV replacement is much smaller than that of Fe-Mg replacement.At the same time,Al^VI ion participates in the replacement of Mg²⁺in octahedron,but the relationship between Al^VI ion and Mg²⁺is very weak.In other words,the content of Si-Al in chlorite samples collected in this study is relatively stable,while the content of Fe-Mg changes significantly.The displacement of Al^IV ion and Si ion in silicon-oxygen tetrahedron mainly affects the vibration frequency of Si-O bond,while the vibration of O-H bond mainly affects the number of bands.The permutation of Fe-Mg ions in dioctahedron is the main factor affecting the O-H bond vibration frequency.3.The effect of isomorphic replacement in chlorite minerals on crystallization parameters:The correlation between Fe²⁺content and d₀₀₂ value of crystal plane spacing in chlorite minerals was analyzed by scatter point analysis.It was found that the iron content in chlorite was negatively correlated with d₀₀₂ value of crystal plane spacing（r²=0.4）,i.e.the crystal plane spacing value of iron-rich chlorite was lower.The content of Al^IV is positively correlated with the value of d₀₀₂（r²=0.17）,which indicates that the Al^IV-Si substitution with a small number of substitutions has little influence on the crystal structure of chlorite samples collected by our institute and can be treated as non-influence.4.Study on the mechanism of controlling the displacement of absorption band by the change of isomorphic element content and crystal structure of chlorite minerals:It is concluded that the controlling mechanism of the change of mineral crystal structure under the influence of Fe-Mg isomorphic replacement on the two near infrared characteristic bands is that the thickness of octahedral layer decreases with the increase of Fe content,i.e.the increase of Fe²⁺/(Fe²⁺+Mg²⁺)value,because the radius of Fe²⁺cation（0.061 nm）is smaller than that of Mg²⁺ion（0.072 nm）.The vibration frequency of hydroxyl bond decreases gradually with the increase of Fe content,resulting in the shift of the near infrared band near4438 cm^-1（2253 nm）from 4450.85 cm^-1（2246.76 nm）to 4420.69 cm^-1（2262.09 nm）,and the shift from 4298.22 cm^-1（2326.54 nm）to 495.43 cm^-1（2383.54 nm）near 4263 cm^-1（2345nm）.5.The ASTER multispectral data covered the Nachitai to Xidatan area of East Kunlun mountains in Qinghai province was selected for Mg-rich and Fe-rich chlorite mapping on basis of the Mg-rich and Fe-rich chlorite image spectral selection using the theory of the the chlorite band shift in near infrared region.The field verification work was conducted after the remote sensing mapping and showed a satisfactory result.This band shift theory was proved to be a practical approach in mapping finer mineral information.