Mineralogical Framework of Alluvial Soils Developed on the Aravalli Sediments

The study area consists of the alluvial plains of Kothari River, a tributary of Banas and situated in Eastern Rajasthan Uplands. The area is surrounded by Aravalli hills. Sand constituted the major part of the soils of Kothari river plains. Quartz was the dominant mineral followed by feldspars and mica in all the plains. There was a mixed mineralogical makeup of clay fraction. Mixed mineralogy of alluvial plains revealed that the soils of the area were formed by mafic and felsic igneous rocks under moderate precipitation with higher base status. Higher cation content or base saturation with low rainfall (<1000 mm) favour the formation of smectites. Decreasing order of minerals (estimated semi-quantitatively) in the soils of alluvial plains could be depicted as: Illite/mica > smectite > kaolinite > feldspars > quartz > vermiculite = chlorite. Smectite content was highest in the soils of lower plains and least in upper rolling plains while illite/mica was highest in the soils of middle sloping plains. The ratio of 001 and 002 basal reflections of mica was more than unity, suggesting the presence of both biotite and muscovite mica minerals. Other minerals were found randomly distributed without any specific pattern. Youthful nature of the soils of upper rolling plains was indicated due to the presence of talc in their clay fraction. Mineralogy of silt size fraction was very similar to clay mineral framework except the higher content of quartz and feldspars and an equivalent reduction in 2:1 or 1:1 minerals. Smectite, mica, kaolinite, quartz, feldspars with trace amount of chlorite and vermiculite were found in silt size fraction.