重庆地区紫色土和紫色泥岩物质组成与微结构研究

根据紫色土与紫色泥岩的X-射线矿物鉴定、粒度分析、电镜鉴定及野外观察,研究了紫色土与紫色泥岩的矿物组成、粒度组成及其易发生侵蚀的原因。结果表明,紫色土与紫色泥岩矿物成分与含量差异很小,均以石英和长石为主,黏土矿物中蒙脱石含量较其他矿物多;粒度成分也非常接近,以粉砂和黏粒为主。紫色土以碎屑粒状孔隙充填式胶结微结构为主,另有少量粒状接触胶结微结构和粒状块状胶结微结构,胶结物以黏土为主,仅含有3%左右的Fe2O3化学胶结物;紫色泥岩微结构的突出特点是微型片结构和微型块状结构占主要地位。蒙脱石的较高含量指示紫色泥岩与紫色土为膨胀类岩土。紫色泥岩与紫色土在湿润气候条件下的频繁胀缩是它们易于发生物理风化和易受侵蚀的主要原因,微型块状结构、微型片状结构和密集微型裂隙也是其抗蚀性差的重要原因。紫色泥岩的微型块状与微型片状结构和密集微型裂隙主要是在成岩过程中产生的,是原始沉积形成的大量球形、椭球形黏土形态受到成岩过程中挤压作用的结果。

Material composition and microstructure of purple soil and purple mudstone in Chongqing area

Based on X-ray mineral identification, particle-size analysis, electron microscopic identification and field observation of purple soil and purple mudstone, mineral composition and particle size composition as well as susceptibility to erosion were studied. Results show that little difference was observed in mineral composition and content between purple soil and purple mudstone, which are both mainly composed of quartz and feldspar, plus Smectite-dominated clay mineral. They are all very close in particle size and consisted dominantly of fine silt and clay. The micro-structure of purple soil appears to be mainly of clasts with pores cemented, and some of granulars cemented through contact and of granular cemented with lumps. Clay played a key role in cementing, of which only about 3% was attributed to Fe₂O₃, chemical agglutinate. The microstructure of purple mudstone is characterized predominantly by miniature laminate structure and micro-lump structure, which take up the main part. The high smectite content indicates that both purple soil and purple mudstone are dilatable soils. Purple soil and purple mudstone frequently swell and shrink when the weather is humid, making them susceptible to physical weathering and erosion and their miniature block structure, miniature laminate structure and dense micro-pores are also important causes of its poor erosion resistance. These structures of the purple mudstone are mainly formed during the diagenetic process, as a result of the compression during the diagenetic process of the large amounts of spheric and ellipsoidal clays formed during primitive sedimentation.