Holocene phosphorites of the western continental margin of India

R.R. Nair


Seventeen samples collected during different cruises of RV Gaveshani, were analysed by X-Ray diffractometer for apatite. These samples consists of algal and oolitic limestones and were recovered from depths ranging from 70 to 150 m on the western continental shelf from Saurashtra to Kerala. Prominent apatite peak was present only in the algal limestones off Goa. These occur as nodules and are similar in appearence to rhodoliths. Minerals present, in order of abundance, are high-magnesium calcite, francolite (carbonate flourapatite), quartz, feldspar and aragonite. P2 O5 concentration varies between 0.8% and 10.8% indicating a wide variation in chemical composition with the area. Age determined by the radiocarbon method is 11,040 years. It appears that these phosphorites were formed by diagenesis and the enrichment of phosphate in these limestones might be related to the physico-chemical activity of microorganism that built the algal mat


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