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Ananda Coomaraswamy Memorial Oration

Mineralogy, petrography, geochemistry and economic potential of carbonate rocks of Sri Lanka

Author:

H.M.T.G.A. Pitawala

University of Peradeniya, Peradeniya, Sri Lanka, LK
About H.M.T.G.A.
Department of Geology
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Abstract

Sri Lanka comprises different paragenetic types of carbonate rocks such as Miocene limestone, crystalline limestone (marble), carbonatite and dyke like carbonate bodies. The purpose of this paper is to summarize documented studies on petrogenesis and economic potential of crystalline limestone, carbonatite and dyke-like carbonate bodies. Pure marbles, which are confined to the Highland Complex (HC) of the country are composed mainly of dolomite, while carbonatite occurrences at Eppawala consist mainly of calcite. Mono-mineralic calcite bodies with trace amounts impurities are found in the Southeastern part of the country. Detailed geochemical, stable isotope and petrological studies on marbles of HC have revealed that they have formed by granulite grade metamorphism of dolostones deposited in an open ocean. Pure marbles basically retain their primary isotopic signatures with only their δ18O values amenable to alteration due to 18O/16O exchange with percolating waters. Over a limited distance, geochemistry of contact zones of the marbles has been changed significantly due to metamorphic geochemical alterations.

Eppawala carbonatites are unique among the other carbonatites in the world in terms of field setting, isotope and elemental geochemistry and textural features of carbonate minerals. Trace and rare earth element composition as well as stable isotope ratios from the Eppawala carbonatites are not comparable with most of carbonatites in the world. But they are within the broader range of carbonatitic rocks and do not show any meta-limestone signatures. Textural features and chemical composition of carbonate minerals suggest that present carbonatite bodies have been dislocated from the emplaced positions in the crust due to deformations experienced by the country rocks. Geochemically and petrographically the dyke-like carbonate bodies differ significantly from both carbonatites and crystalline limestone. Concentrations of Sr and Rare Earth Elements (REEs) of dyke-like bodies lie between those of carbonatites and marbles. However, the concentrations are much lower than those of carbonatites. Carbon and oxygen isotope compositions are similar to those of most meta-sedimentary carbonates suggesting that the carbonate dykes were formed as melts from marbles at crustal levels due to shearing and thrusting of HC and Vijayan Complex (VC) plates.

Laboratory experiments carried-out in the recent past were able to synthesize value-added products such as Precipitated Calcium Carbonate (PCC), magnesium hydroxide [Mg(OH)2], magnesium oxide (MgO), their nanoparticles, their polymer-nanocomposites, Amorphous Calcium Carbonate (ACC) and poly(acrylate)(PA-)-encapsulated Mg(OH)2; Hollow Calcium Carbonate (HCC), hydrophobic PCC and hydrophobic Mg(OH)2 from carbonate rocks of Sri Lanka. Results of those studies suggest that there is a high possibility to synthesize nanoparticles and nanocomposites of PCC, Mg(OH)2 and MgO using Sri Lankan carbonate rocks with appropriate quality to meet industrial requirements and to fulfill the current demand.

How to Cite: Pitawala, H.M.T.G.A., 2019. Mineralogy, petrography, geochemistry and economic potential of carbonate rocks of Sri Lanka. Journal of the Geological Society of Sri Lanka, 20(1), pp.1–13. DOI: http://doi.org/10.4038/jgssl.v20i1.24
Published on 28 Feb 2019.

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