Abstract
Modern algal nodules and encrustations on rock fragments occur in the beach and littoral environment of the Lawson's Bay, east coast of India. Mg-calcite is the predominant mineral form of the carbonates, with minor amounts of aragonite. Scanning electron microscopic observations have shown several polymorphs of Mg-calcite: micrite, peloids, columnar crystals with truncated terminations and bladed spar with rhombic terminations. Aragonite is confined to voids and occurs as acicular and fibrous needles arranged in a random fashion. The different crystallites of Mg-calcite ae considered represent an evolutionary sequence of carbonate cements. Some of the Mg-calcite rhombs show the diagenetic effect of fresh water periodically entering the bay.