Spatial Differentiation And Evolution Of Iron Oxides In Tropical Volcanic Rock Region Based On Spectrum And Magnetism

Iron oxides,widely distributed on the surfaces of Earth and Mars,are the ultimate products formed by the weathering of iron-bearing minerals under ambient conditions.The surface of primary and secondary minerals is usually covered with nanometer-sized iron oxides,which have an important impact on the optical and magnetic properties of soil and sediment,and is widely used in soil classification,environmental reconstruction and ecological function assessment.Normally,the weathering process of tropical volcanic rocks is accompanied by the destruction of different lithogenic iron oxide phases and the enrichment of pedogenic iron oxide phases.However,there is still uncertainty regarding the environmental indicative significance of optical and magnetic properties related to iron oxides.In this paper,we first prepared soil simulation samples containing iron oxides with synthetic minerals,and then extracted the color index,first and second derivative spectra,and continuum removal spectra of 176 simulation samples through diffuse reflectance spectroscopy.The effects of hematite particle size change and associated maghemite on traditional spectral quantification of iron oxide were systematically explored.Meanwhile,the spectral parameters that can effectively shield hematite particle size and maghemite interference were selected to establish quantitative models of iron oxide.In addition,a total of 201 soil samples were collected from the tropical Nandu River Basin and Golden Rooster Volcano in China,and their soil basic physicochemical properties,iron oxide spectral quantification and magnetism were analyzed.The spatial interpolation method is combined to help explore the influence of parent rock and terrain on the spatial differentiation of iron oxides and their magnetic particles.The main results are shown as follows:(1)In the soil simulation samples with single particle size hematite,with the increase of hematite content,the change rate of purple and redness in particle size follows the following order: 100 nm > 200 nm > 300 nm > 700 nm.Meanwhile,with the increase of hematite particle size,the redness and violet of the sample showed an upward and downward trend,respectively.The height of characteristic peaks of the first derivative spectrum(570 nm),the second derivative spectrum(560 nm and 580 nm)and the continuum removal spectrum(550 nm)gradually decreased,and the position of the characteristic peak shifts towards the direction of shorter wavelength in second derivative spectrum.In addition,the spectral characteristics of the soil simulation sample with maghemite are the closest to that of the sample containing hematite with a particle size of700 nm.(2)In the simulation samples mixed with hematite of different particle sizes(100 nm and 700 nm),coarse hematite was added to the sample containing fine hematite.As a result,the redness of the samples has an upward trend,while the violet has a downward trend.Moreover,the height of characteristic peaks of the derivative and continuum removal spectrum also tends to decrease,while the peak position of the second derivative spectrum moves to the direction of a shorter wavelength.The content of fine hematite in the mixed samples can be estimated by the redness,the peak height between 560 nm and580 nm of the second derivative spectrum and the peak height at 550 nm of the continuum removal spectrum.The content of coarse hematite can be estimated by the peak height at900 nm of the continuum removal spectrum,and the total content of hematite can be estimated by the brightness value.(3)In the simulation samples mixed with hematite(100 nm)and maghemite,when magnetite was added to the sample containing fine hematite,resulting in a decrease in redness and an increase in violet,because the spectral characteristics of maghemite are similar to those of coarse hematite.And the height of the first derivative spectral characteristic peaks(570 nm,800 nm,and 950 nm)and the second derivative spectral characteristic peaks(560 nm and 600 nm)decrease synchronously.And the heights of characteristic peaks of the first derivative spectrum(570 nm,800 nm,and 950 nm)and the second derivative spectrum(560 nm and 600 nm)decrease synchronously.(4)The iron oxides of tropical volcanic rocks and surrounding watershed soils exhibit significant differences in different parent rocks,which are related to the type of parent rock and its age of eruption.The total iron,free iron,hematite,goethite and magnetic components in volcanic rocks are much higher than those in sedimentary rock.The content of crystalline iron oxide hematite is higher in volcanic rocks,especially in old volcanic rock areas,while the content of goethite is higher in sedimentary rock.The single-domain ferrimagnetic particles are more enriched in young volcanic areas,while finer superparamagnetic particles are more enriched in old volcanic areas.(5)The iron oxides in volcanic soils in tropical monsoon regions exhibit significant elevation and slope differentiation,which is related to the redistribution of water and heat regulated by terrain.Crystalline components are differentiated along the leeward and windward slopes,while amorphous components are mainly differentiated along the shady and sunny slopes.The coarse hematite and fine hematite are enriched on the top of the volcano and leeward slope,respectively,while goethite is more concentrated on the windward slopes.The multidomain ferrimagnetic particles are mostly concentrated on the top of the volcano.The single-domain ferrimagnetic particles are concentrated on the leeward slopes,while the finer superparamagnetic particles are more concentrated on the windward slopes.