Laterites, bauxites and associated clays from western India

Abstract

Mode of occurrence, mineralogy and microtextures of laterites, bauxites and the associated clays from six bauxite deposits capping Deccan Trap basalts in western India were investigated. Light microscopy. X-ray diffraction and analytical electron microscopy were employed for the mineralogical and microtextural investigations.

Deccan Trap basalts at Ratnagiri, Srivardhan, Panhala Fort and Nangrataswadi in western Maharashtra, and those at Matanumad, western Gujarath, India progressively weather in situ to laterites and bauxites via an intermediate stage of saprolitic clays. Laterites and bauxites at Naredi, western Gujarath, India, however, represent transported and redeposited sediments.

The unweathered basalts are predominantly composed of a calcic plagioclase and a ferrous pyroxene, with minor proportions of magnetite, ilmenite, volcanic glass, iddingsite and palagonite. Dissolution etch pits form due to weathering on the surfaces of minerals in basalts. Weathered basalts are composed of various combinations of smectites, kaolinites, goethite, hematite and anatase.

Kaolinite is the principal mineral in the saprolitic clays overlying the basalts in the residual bauxite deposits. It commonly forms a framework of submicron sized, irregular shaped platelets, and occasionally occurs as booklets of hexagonal platelets that rarely form spheroidal aggregates.

Residual laterites overlying the saprolitic clays exhibit vesicular, spongy and pi soli tic macrotextures and are composed mainly of hematite and goethite with minor proportions of gibbsite, kaolinite and anatase. Bauxites, which occur as pockets and lenses within the laterites, display pisolitic , massive and nodular microtextures, and are predominantly composed of gibbsite with minor amounts of anatase, kaolinite, hematite and goethite. Laterites and bauxites are characterized by framework microtexture formed by three-dimensional packing of crystallites.

Natroalunite, a sodium-aluminum-sulfate-hydrate is the predominant mineral in the saprolitic clays at Matanumad. Natroalunite occurs as a replacement product of kaolinite in these saprolitic clays. Natroalunite occurs in subordinate amounts in laterites where hematite and goethite are the predominant minerals. Natroalunite alters to kaolinite and oibbsite in the bauxites.

Lateritic pebbly mudstone and bauxitic bouldery mudstone at Naredi represent debris flow deposits formed in two pulses. The residual laterites and bauxites capping the hills around Naredi were the sources rocks for these debris flow deposits.