广西凭祥英安岩的化学风化作用研究

Chemical weathering for dacite in Pingxiang, Guangxi

利用薄片观察、X射线粉晶衍射分析和化学分析方法研究了广西凭祥英安岩风化剖面的形成作用。风化作用初期,母岩中微量黄铁矿的氧化分解导致方解石与绿泥石的迅速分解;风化中期形成了大量的高岭石、伊利石、蒙脱石和蛭石;风化作用高级阶段以高岭石、石英和氧化铁矿物的富集为特征,但仍然存在少量蒙脱石、伊利石和蛭石。风化剖面的部分层段显示出与剖面其他部分明显不同的地球化学特征,即Na的富集和K的亏损。在Al2O3-(CaO+Na2O)-K2O三角图上,风化中期这些层段明显偏离了正常的风化趋势。矿物学和微形貌的研究表明,造成偏离的原因是古地下水引起的正长石的钠长石化作用。

Optical observation for thin sections, X ray diffraction analyses and chemical analyses are applied to study the formation of a dacite weathering profile in Pingxiang, Guangxi. In the initial stage of weathering, calcite and chlorite in parent rock broke down rapidly due to the oxidation of minor amount of pyrite. Kaolinite, illite, montmorillonite and vermiculite were formed in a large amount during the middle stage of weathering. When profile developed to advanced stage, the mineral assemblage is characterized by kaolinite, iron oxide minerals and quartz with a small amount of illite, montmorillonite and vermiculite reserved. Some layers in the weathering profile show different geochemical characteristics from the others, namely sodium enrichment and potassium depletion. In Al2O3 (CaO + Na2O) K2O triangular diagram, the weathering trends of these layers in the middle stage are remarkably deviated from normal ones. Both mineralogy and micromorphology of these layers indicate the deviation is formed by the albitization of orthoclase caused by fossil groundwater.