根据西安和宝鸡黄土中第5层红褐色古土壤的调查,在该层古土壤下部发现了红色铁质粘土结核。显微镜下和电子显微镜下的鉴定表明,铁质粘土结核是光性粘土胶膜聚集而成的,具有超微晶粒结构。铁质粘土结核主要由伊利石-蒙脱石混层粘土矿物组成,这表明西安与宝鸡地区第5层古土壤发育时的粘化作用为化学粘化或生物化学粘化。该地区第5层古土壤的铁质粘土结核迁出土壤粘化层底部的距离达1.0 m, 与江苏六合地区黄棕壤铁质粘土结核分布深度相同,指示该层古土壤发育时该区年均降水量可达1 000 mm左右。

According to the survey of 5th red-brown paleosol layer in the loess in Xi'an and Baoji areas, it can be discovered that red ferriferous clay nodules exist at the bottom of this paleosol and the top of underlying 6th loess. Most of the ferriferous clay nodules have the prismatic, tabular and ellipsoidal shapes mainly filling in the cracks and root holes. The minor-axis diameter of the nodules is about 2-4 mm, while the major-axis diameter can be accessibly about 10 mm.The material obtained by optical microscope indicates that these ferriferous clay nodules have the charatcter of red-brown optical clay films developed well. It can be known by electronic microscope that these ferriferous clay nodules is consist of ultramicro crystals. These ultramicro crystals apparently spread by orientation, which is the reason of possessing optical character just as the mineral crystal.X-ray diffraction reveals that in the ferriferous clay nodules, mixed structure mineral of illite-montmorillonite take 90% or so and quartz takes about 5%, plagioclaspe and sanguine and so on just a little. The mineral ingredient shows that weathering of early-middle period occurred for clay mineral forming. Laboratory analyses indicates that in these ferriferous clay nodutles, SiO₂ takes 42.13%-43.44%, Al₂O₃ 22.29%-23.46%, Fe₂O₃ 9.95%-10.36%. In the paleosol SiO₂ takes 65.91%, Fe₂O₃6.42%, Al₂O₃ 15.85%, which shows that SiO₂ content is about 12% lower in the ferriferous clay nodules than in paleosol, and Fe₂O₃ and Al₂O₃ are about 4.5% and 8% more in the nodules than in the paleosol respectively.In Xi'an and Baoji areas, the cracks and root holes developed well in the layer where the ferriferous clay nodules of 5th paleosol exist, and there is no water-resistance layer. Fe₂O₃ is high in content and there is no mineral formed in reduction condition. These shows that the ferriferous clay nodules of 5th paleosol formed not under the water resistance layer but by the ferriferous film migrating and gathering by the strong leaching.In Xi'an and Baoji areas, the ferriferous clay nodules lying in the parent loess are under the clay grouding layer of the paleosol bottom 1 m, which indicates that the affected depth of weathering was bigger than that of soil-formation band and the soil formation at that time is stronger than that of the contemporary drab soil. The distributive depth of the ferriferous clay nodules in 5th paleosol in these areas is primarily same with contemporary sub-tropical Yellow-brown Earth in Liuhe of Jiangsu, which indicates that subtropical climate migrated to these two areas at that time and the mean annual precipitation was about 1 000 mm.