Redistribution of elements and mineralogical changes during chemical weathering of Rajmahal Basalt Jharkhand

Abstract

Elemental mobilization and variation of mineralogy during chemical weathering of Rajmahal basalt were studied on the weathering profiles developed in the Pakur district of Jharkhand, India. Samples were collected from four different weathering profile, two rindlet system and surface water from river draining the Rajmahal Basalt and adjoining area. In this study, powdered XRD analysis of 96 samples, elemental analysis of 42 sample from weathering profile and 22 river water samples were investigated to understand the behavior and redistribution of elements during chemical weathering and associated mineralogical changes. Extent of chemical weathering reaches up to latritization as lateritic cap observed in few locations during field work. Unaltered basalt is composed of plagioclase phenocrysts with groundmass consist of clinopyroxene and plagioclase, zeolite, glass and opaque minerals. Saprolith sample indicates alteration of primary minerals, fractured coarse grain of pyroxene and feldspar, precipitated with iron oxy-hydroxide in red coloration along their grain boundary and fractures. Semi- quantitative analysis based on XRD result of Pakuria profile sample shows 0 90% of plagioclase and 0 39% of pyroxene. Other minerals like ilmenite, goethite, clay and secondary minerals along with quartz are also observed. Similar mineralogical assemblages were also observed at Bengal mine profile. newlineiii newlineConcentration of mobile major oxides like CaO, MgO, Na2O and P2O5 decreases whereas, Fe2O3(T), MnO increases towards upper layers of the profile and form Fe-oxy-hydroxides nodules. Minor decrease of SiO2 concentration is also observed towards upper layer and K2O shows variation within the profile. In general, trace elements like Li, Rb and Cs shows similar behavior as K2O, whereas high field strength element (HFSE) like Zr, Hf, Nb and Ta behaves similar to TiO2. Increase in HFSE and Rare Earth Element (REE) is observed towards upper layers. Weathering indices indicate extent of weathering in the upper layers is high, which is