Decomposition of Primary Minerals and the Process of Bauxitisation in Shevaroy and Kolli Hills, Salem District, Tamil Nadu

The bauxite deposits of Shevaroy and Kolli hills have been investigated to understand the processes involved in decomposition of the primary minerals leading to bauxitisation. The charnockitic suite of rocks underlying the bauxite deposits consists of quartz, plagioclase and potash feldspars, ortho- and cline-pyroxenes, garnet and secondary hornblende. During weathering, feldspars directly give rise to gibbsite and sometimes through kaolinite as an intermediate product. Under similar drainage conditions plagioclase feldspars in particular and mafic rocks in general are found to be more favourable for direct bauxitisation than potash feldspars and felsic rocks. The secondary product from pyroxenes (and hornblendes) is usually goethite but rarely clay minerals of nontronite group. Gibbsite in minor amounts is believed to be another secondary mineral in addition to goethite in the case of garnet alteration. Quartz is subject to dissolution and is replaced by gibbsite and goethite towards the final stages of bauxitisation. Order of mineral stability for weathering is, feldspars=pyroxenes < garnet < quartz.

Secondary enrichment bauxite and/or laterite as a result of migration and reprecipitation of aluminium and iron hydroxides has been observed. Hematite and boehmite are believed to have been formed in the advanced stages of weathering under an acidic environment during these secondary processes.