长江三角洲第一硬土层中微结核的矿物化学特征及其成因

长江三角洲第一硬土层是古土壤层,是长江河漫滩型沉积物,其形成经历多次沉积和成土过程.硬土层中的铁锰结核以菱铁矿、菱锰矿和褐铁矿为主,通过铁锰元素离子和矿物Eh-pH稳定关系分析,最有利于菱锰矿和菱铁矿结核生成的条件是:Eh＜0且pH＞8.成土期处于末次冰期低海面、气候寒冷干燥期,土壤的化学过程是以氧化作用为主,是褐铁矿等氧化性矿物形成时期;冰后期海相层的沉积覆盖使土壤层进入成岩阶段,土壤层的化学过程以还原作用为主,成岩过程是铁锰碳酸盐结核的主要生成期.

CHARACTERISTICS OF MINERAL CHEMISTRY AND FORMATION OF THE MICRO-NODULES IN THE FIRST STIFF CLAY LAYER IN THE YANGTZE RIVER DELTA

The 1st stiff clay layer in the Yangtze River delta is buried paleosol in the flood land of the Yangtze River and its formation experienced many sedimentary and pedogenic processes.The main iron-manganese nodules in the stiff clay layer are rhodochrosite,siderite and limonite.Based on the graph of Eh-pH relationship of iron-manganese,the best advantage for the formation of nodules of rhodochrosite and limonite is Eh<0 and pH>8.The time of the pedogenic process was in the last ice age,therefore this area was colder and drier in climate and lower in sea level than today.The main chemical process in the soil was oxidation,a period of the formation of oxide minerals such as limonite.With the deposition and covering of the marine facies layer in the postglacial period,the soil turned into diagenetic process.Consequently the chemical process in the soil was dominated by reduction and this process was the main development period for nodules of iron-manganese carbonate to form.