Genetic Characteristics Of Soil Developed By Gneiss Weathering Crust In Shenyang City

Gneiss is metamorphic rock,which is widely distributed all over the country.It is mainly composed of feldspar and quartz,and contains amphibole,biotite and other dark minerals,in which feldspar content is greater than quartz.Because of the gneiss gneissic structure and banded structure,the mineral distribution is uneven,which leads to the irregular weathering degree of the weathering crust.In recent years,many scholars have studied the discovery,geological characteristics and structural significance of gneiss,but few have studied the genetic characteristics of weathered gneiss in the temperate zone.In this study,the weathering crust of gneiss in Shenyang area is taken as the research object.According to the weathering degree of rocks and minerals,the size of weathering particles and the content of various minerals,the sections of weathering crust are sampled in layers.Through sieving,the experimental samples of seven grain size groups of more than 2 mm,2 1 mm,1 0.5 mm,0.5 0.1 mm,0.1 0.05 mm,0.05 0.02 mm and < 0.002 mm were obtained.The morphological,physical,chemical and mineralogical characteristics of the experimental samples were studied to reveal the genetic characteristics of the weathering crust of ancient gneiss in the temperate semi humid climate area during the process of soil formation.The results show that:?1?There are 16 layers in the profile of weathering crust under test,among which the depth of 530 700cm?the 12 th layer?is a large and fragile rock.In addition,with the increase of depth,the content of 1 0.05 mm particles increases gradually,0.05 0.02 mm and < 0.002 mm decreases gradually,and the content of > 2mm and 2 1mm increases first and then decreases.The content of particles larger than 2 mm is the least in the 11 th layer.With the deepening of soil depth,the root system and soil animals are reduced,the biological weathering is weakened,the content of sand is gradually increased,the content of silt and clay is gradually reduced,and the soil texture changes from clay to sand.?2?The weathering crust of gneiss in this study contains mainly plagioclase,K feldspar and pyroxene,as well as a small amount of Ca feldspar,Na feldspar,biotite,olivine and quartz.The content of Na feldspar,Ca feldspar and K feldspar at 0.33 nm decreased with the decrease of particle size.The mineral types with grain size larger than 1 mm in each layer are related to the content in each sampling layer.The more the grain size larger than 1 mm,the more feldspar,the less dark minerals.Dark minerals are mainly concentrated in 0.5 0.05 mm.With the increase of development degree,the morphology of feldspar gradually becomes fuzzy,the Fe and clay in the fracture gradually increase,and the particle size of the maximum feldspar content gradually increases.With the decrease of grain size,inclusions in feldspar decrease gradually,and pyroxene content decreases gradually.In the soil layer and completely weathered layer,biotite develops to vermiculite,and in the strongly weathered layer,biotite has two diffraction peaks,with low weathering degree and high content.Under the same depth,the mineral content of > 2 mm,2 1 mm and 1 0.5 mm grain size in the three layers with different development degree changes as follows: K feldspar > pyroxene > Na feldspar > Ca feldspar > biotite > quartz > olivine;the mineral content of 0.5 0.1 mm and 0.1 0.05 mm grain size changes as follows: K feldspar > pyroxene > Na feldspar > Ca feldspar ? biotite ? quartz ? olivine.?3?The secondary minerals in the weathering crust are vermiculite,illite and kaolinite.The secondary minerals of pyroxene and amphibole are chlorite,but chlorite is not found in the X-ray diffraction pattern,which indicates that the development degree of pyroxene and amphibole is small.In the section with strong weathering degree,there are more vermiculite and illite,good crystallinity,indicating that the weathering degree of biotite and plagioclase is strong;the content of kaolinite is less,poor crystallinity,indicating that the development degree of orthoclase is weak.In the section with weak weathering degree,the content of the three secondary minerals is less and the crystallinity is poor,which indicates that the development degree of pyroxene,hornblende and feldspar is poor in the strong weathering layer.?4?The contents of SiO₂,Fe₂O₃,Na₂O and K2 O are higher in fine soil,SiO₂,Na₂O are positively correlated with K2 O,and TiO₂ content increases with the decrease of particle size.Al₂O₃ and Fe₂O₃ increase with the decrease of particle size,and the law of change in different depths is soil layer > completely weathered layer > strongly weathered layer,which shows that Al₂O₃ and Fe₂O₃ are enriched with the deepening of weathering.Na₂O and K2 O showed a significant positive correlation in each particle class,and SiO₂ and Fe₂O₃ showed a significant negative correlation in four particle classes of > 2 mm,1 0.5 mm,0.5 0.1 mm and 0.05 0.002 mm.Among the three particle sizes of 1 0.5 mm,0.5 0.1 mm and 0.1 0.05 mm,TiO₂ has a very significant or significant positive correlation with MnO and MgO.