Study On Weathering Characteristics Of Black Shale And Oxidation Kinetics Of Pyrite Embedded In Rocks

Rock weathering always exist in the mutual conversion process of various geological masses in the development of geological history. Since last centuries, large quantities of issues related to geological disasters, engineering damages and environmental deteriorations caused by rock weathering have been reported by the geological researchers all over the world. The disaster investigations associatied with black shale weathering indicate that the weathering process of black shale are mainly manifested on the oxidation of sulfide minerals contained. The acid water and some expansive sulfate minearals produced by rock weathering could have damage effects on the black shale and other surrounding rocks, such as acidic corrosion, dissolution and expansion. Therefore, the chemical weathering of black shale, as the most significant path, influence the rock structures and engineering propertities as we as environmental safety. With the rapid development of basic construction in China, the human engineering activities has been increasing. The engineering geological problems caused by the chemical weathering of black shale under the natural conditions become more and more protrusive. However, the chemical weathering characteristics of black shale and the kinetic oxidation of the sulfide contained are still lack of deeper theory and experimental research.Black shale is widely distributed in China. The typical Low Cambrian black shale mainly distributes in the southern areas, such as Hunan province, Guizhou province and Yangzi platform. The other distribution areas such as Xinjiang, Zhejiang, Sichuan and Chongqing provinces, where the researches associated with the black shale were focused on the characteristics of geochemical elements. Therefore, studying on the chemical weathering behaviors of black shale and kinetic oxidation of pyrite contained have great significance for the evaluation and prediction for engineering and environment safety.Taking the Low Cambrian Qingxi Formation black shale in Sanjiang-County area in Guangxi province as research project, the petrological, mineralogical, morphological features and chemical weathering behaviors of the black shale and its weathering products are analysed. From the angle of chemical thermodynamics and kinetics theory, the thermodynamics stability of water-rock chemical interactions and kinetic oxidation rate of pyrite contained are systematically studied. The main achievements are summarized as follows:(1) Based on the field investigations and data collections, the sedimental environment and weathering intensity of black shale are analysed by the examination of mineralogical and geochemical components as well as morphological observation. The results indicat the black shale in study area sedimented in reducing epicontinental marine sedimentary environment. The chemical weathering parameters, such as chemical index of alteration (CIA) and A-CN-K chemical weathering trending diagram and so on, reveal that the Qingxi Formation black shale underwent medium chemical weathering. The denudation state of the original rock is relative stable, and some features of chemical constitutions are still preserved. (2) Based on the physical and chemical properties, distribution and formation conditions of the black shale weathering procducts, a half-immersion test was performed to explore the geochemical behaviors of the black shale chemical weathering under the joint effects of water and oxygen. The influences of external conditions on the types of weathering products and the temporal and spatial sequence of paragenetic sulfate minerals are mainly discussed.(3) Chemical thermodynamic theory and reaction models are applied to study the direction and intensity of water-rock interactions involved in the chemical weathering of black shale in acidic conditions using Gibbs free energy and equilibrium constant methord. The pe-pH diagrams are constructed to illustrate the thermodynamic stable areas and formation conditions of black shale weathering products. Based on the non-equilibrium thermodynamic theory, the irreversibility of black shale weathering and formation mechanism of some weathering phenomenon are explained.(4) A flow-through experiment is designed to study the kinetic behaviors of pyrite oxidation contained in black shale. Through the analysis of the impacts of the reaction between pyrite and dissolved oxygen on ion concentrations, rock’s permeability and morphological changes of rock surfaces before and after experiment, the saturation characteristics of pore water and major types of chemical reactions during water-rock interactions are obtained. Discussion is focused on the kinetic rates of pyrite oxidation and Si element releasing in acidic environment, and the rate equation of pyrite kinetic oxidation in experiment conditions is deduced.(5) By the employment of the geochemical numerical simulation software PHREEQC, the dissolution and precipitation processes of the experimental solutions and water samples obtained in field are simulated. What’s more, the kinetic behaviors of pyrite oxidized by dissolved oxygen and/or ferric ion as oxidants is simulated as well. The dissolution and precipitation characteristics of those weathering products, such as rozenite, copiapite and so on, are stated under the effect of continuous pyrite oxidation or water evaporation. The results illustrate the concentration changes of sulfate and iron ions caused by reactions between pyrite and dissolved oxygen as well as ferric ion.