Clay mineralogy of Visakhapatnam shelf sediments, east coast of India: comment

Abstract

In Murty and Rao's recent study of the clay mineralogy of the shelf sediments off Visakhapatnam (Marine Geology, 88:153-165) I have noticed contradictory statements, analytical drawbacks, peculiar results which disagree with other studies from the same area, and inferences not supported by the data. On p. 155, in the section Earlier Studies it was stated that 'the easterly flowing rivers of the peninsula contribute detritus from humid tropical weathering to the western part of the bay. Under humid tropical conditions, chemical weathering results in high leaching of alkalies and alkaline earths and the dominant clay mineral that forms is the kaolinite. The peninsular rivers mentioned by the authors contribute abundant illite, ~;hlorite and smectite-dominated clays. In the X-ray diffraction techniques, the procedures adopted to identify and confirm the presence of chlorite and kaolinite were not mentioned. Further, the method used to calculate the chlorite percentage was not given. The procedure for semiquantitative estimation refers only to the three clay minerals, but the percentages of chlorite were presented in table 3. In the results section, the statement that illite is the dominant clay mineral in the shelf sediments is not supported by the data presented in table I. Indeed, no mineral has a characteristically high abundance. Critical observation of the table shows that more than 30% of the surface samp!es, 35% between 10 and 20 cut, 60% between 30 and 40 em