The Characateristic Of Soil Clay Minerals In Mountain And Basin Of North Slope Of Tianshan Mountain And Their Responses For Soil-forming Environments

Clay minerals widely exist in soil,weathering crust,various sediments and rocks in land and sea.Weathering process is the most important way to form clay minerals.The formation of clay minerals in supergene environment are affected by many factors,such as parent rock,climate,soil,vegetation,geomorphology and medium environment.This research chose soils of mountain and basin of North Slope of Tianshan Mountain in Xinjiang to study clay mineral assemblage and morphology characteristics by using X-ray diffraction?XRD?,electron microscopy analysis?TEM?.By analyzing stability isotope¹⁸O/¹⁶O and D/H ratio of the clay mineral,crystallinity of illite,?¹³C and?¹⁸O of soil secondary carbonate and?¹³C of soil organic matter to interpret information about climate and environment change during soil forming-process and reveal the evolution characteristics of clay minerals in mountain and basin.It provides a proof and reference for exploring the formation and transformation of soil clay minerals in arid areas and enriching the theory of soil genesis and evolution.The following conclusions were obtained by this study:1.In the mountain and basin system formed by the North Slope of Ttianshan Mountain andjunggar basin in Xinjiang,elevation from 1800m to 270m,there are successivelydistributed chernozem soil,chestnut soil,brown calcium soil,gray desert soil,saline-alkalisoil and aeolian soil.There are salinized soil of different degrees widely distributed in thebasin.In addition to swamp soil,the thickness of organic matter layer in chernozem,chestnut calcium soil,brown calcium soil,gray desert soil,saline soil and aeolian sandsoil is getting thinner and thinner,and the content of organic matter is gradually reduced.Meanwhile,the layer of calcium deposit in soil is getting higher and higher,and the soilsalt content is gradually increased.2.In the mountain and basin system,the main clay minerals are smectite,illite,vermiculite,chlorite,and kaolinite.Clay minerals in chernozem are dominated by smectite with a smallamount of vermiculite,illite and kaolinite.The content of smectite in chestnut soil andbrownstone soil decreased gradually,while the content of illite increased.So it is believedthat smectite these soils are products of illite transformation and vermiculite is theintermediate stage of this transformation.The underground water level of the swamp soilis high,and the soil contains a small amount of salt,and the pH value is between 7.36 and8.07,smectite in swamp are products of illite transformation.3.In the inner basin,illite dominates in the grey desert soil and saline soil,followed bysmectite and chlorite,with a small amount of kaolinite and no vermiculite.In soil layerswith relatively high water content below 10cm,there is a significant positive correlationbetween soil salt content and smectite content?r=0.61,n=45,p<0.01?,which shows thepresence of salt is conducive to the formation of smectite because the soil layers with salthave the water conditions and chemical components for smectite precipitation:?1?pH>8.5,which promotes the dissolution of Si⁴⁺in the primary mineral?2?there are a large amountof Ca²⁺,Na⁺and Mg²⁺in the soil solution.4.In chernozem,chestnut soil,brown soil,gray desert soil,saline soil and aeolian sand soil,layers of carbonate are higher and higher,especially carbonate content in the soil surfaceof saline soil is the highest,which phenomenon explains eluviation be weak.Carbon andoxygen isotopes in soil carbonate and carbon isotopes in soil organic matter indicate thatvegetation types are successional from C₃ plants to C₄ plants from chernozem to aeoliansoil,with rainfall decrease,temperature increase and gradually dry climate5.The illite crystallinity value in the chernozem soil is the lowest,indicating that there is anobvious transformation process of illite in the soil,and vermiculite is the intermediatestage of this transformation.With the decrease of rainfall,IC value gradually increased,indicating that the transformation of illite in Chestnut soil and brown calcium soil wasgradually weakened.From chernozem to gray desert soil,the values of D and ¹⁸O of soilclay minerals increased,indicating that the climate was gradually dry,soil moisturedecreased,and evaporation increased.At the same time,the positive correlation between¹⁸O and smectite content in clay minerals is extremely significant?n=18,r=0.81,p<0.01?,which also indicates that the increase of soil moisture in the study area is conducive to theformation of smectite.In addition,the negative correlation between ¹⁸O and illite contentin the clay minerals is significant?n=18,r=0.76,p<0.01?,indicating that the humidity in the study area decreases,which is conducive to the preservation of illite,and illite also has a very good response to the humidity in the soil-forming environment.Climatic conditions determine that the chemical weathering process is more intense in the distribution area of chernozem and chestnut soil,while the climate in the distribution area of brownite gray desert soil and saline soil is relatively dry and weathering is weak.Therefore,the illite petrochemical index?ICI?in chernozem and chestnut soil>0.5.To sum up,the soil clay minerals on the north slope of Tianshan Mountain in Xinjiang mainly include illite,chlorite,smectite,vermiculite and kaolinite.From the distribution area of chernozem to aeolian soil,the climate is gradually arid,with reduced rainfall,increased evaporation,reduced soil leaching and weaker transformation of illite to smectite and vermiculite.Affected by the salt content,the higher the salt content is in the soil profile of the basin,the higher the smectite content is,indicating that climatic conditions affect the transformation of illite to smectite and vermiculite in the mountain soil,while the salt content in the basin affects the precipitation of smectite.Under the influence of groundwater,the transformation process of illite to smectite also exists in the swamp soil.