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Limestone and Dolomites Balrambhadu ONGC, Dehradun Limestone and Dolomites BalramBhadu ONGC, Dehradun 1.0 Introduction Understanding of earth’s history is core business of geology and to understand it in totality requires inputs from other associated disciplines. Study of geology involves origin, composition and structure of the earth. Among the various disciplines of earth system sciences, sedimentology is an integral part to reveal earth’s history. Sedimentology involves study of sedimentary rocks and their formation through various geological processes covering weathering, erosion, transportation and deposition by geological agents like water, air, ice etc. Sedimentology also involves post depositional changes (diagenesis) which take place after deposition of sediments upto the stage of initiation of metamorphism. Sedimentology deals with all type of sediments whether recent or ancient through the geological periods deposited in wide range of depositional environments. By volume percentage, sedimentary rocks may be lesser in amount out of total rocks of the earth but they provide much of the world’s iron, potash, salt, building material and energy resources. Sedimentary rocks include- a. Siliciclastic sedimentary rocks (epiclastic, detrital/ terrigenous): Formed by physical and chemical disintegration of pre-existing rocks like conglomerate, breccias, sandstone, siltstone, shale, and mudstone. These are also known as clastic sedimentary rocks. b. Biogenic, biochemical/ organic sedimentary rocks: These types of rocks are formed precipitation of minerals by the organisms like carbonates, chert, phosphates etc. These are also known as non-clastic sedimentary rocks. c. Chemical sedimentary rocks: Formed by precipitation of dissolved components in water like evaporites.These are also part of non-clastic sedimentary rocks. Among the non-clastic sedimentary rocks, carbonates are the most abundant rocks constituting about 10% of total sedimentary rocks. These rocks are formed chiefly by chemical and biochemical processes. Moreover, biological and biochemical processes dominant the formation of carbonates 1 but inorganic precipitation of CaCO3 from sea water also known. After deposition, physical and chemical processes of diagenesis considerably modify the carbonate sediments (Tucker, 1991). The carbonate rocks include the limestones made of calcium carbonate (CaCO3), and dolomites composed of the mineral dolomite (CaCO3.MgCO3). Some workers prefer to use the term ‘dolomite’ for the mineral and refer the rock as ‘dolostone’. Carbonates are important aquifers and hydrocarbon reservoirs owing of the high porosity which they sometimes contain (Selley, 2000). Carbonate rocks account for about 60% of hydrocarbon reserves of the world and notable hydrocarbon reservoirs in the Middle East (Saudi Arabia and Kazakhstan) and India (Mumbai Offshore) are found in carbonates. Limestones also host epigenetic lead and zinc sulphide deposits across the globe. Limestone is also important source of lime to make cement, and is hence a component of all concrete, brick and stone buildings and other large civil engineering structures such as bridges and dams. Limestones are most common and widespread rocks that form the peaks of mountains in the Himalayas and also form characteristic karst landscapes and many spectacular gorges throughout the world. Limestone deposits are common through much of the stratigraphic record and include some very characteristic rock units, such as the Late Cretaceous Chalk, a relatively soft limestone that is found in many parts of the world. The origins of these rocks lie in a range of sedimentary environments: some form in continental settings, but the vast majority are the products of processes in shallow marine environments, where organisms play an important role in creating the sediment that ultimately forms limestones (Nichols, 2009). Difference between siliciclastic and carbonate sediments is tabulated here for basic understanding (Table 1). Table 1: Difference between siliciclastic and carbonate sediments Carbonate Sediments Siliciclastic Sediments Mainly marine sediments Both marine and non-marine are common Grain size of sediments is generally controlled by size Reflects hydraulic energy in the of organism skeletons and calcified hard parts environment Presence of lime mud indicates rich growth of Presence of mud is an indicator of settling organisms through suspension medium Shallow water bodies reflect localized biological and Shallow water sand bodies are result of physiochemical changes interaction of current and waves Localized build up alter the character of surrounding Change in sedimentary environment sedimentary environment reflect change in hydraulic regime Main deposition in shallow tropical environment Sedimentation occur in all type of climate and environment Sedimentary facies features are obliterated even Sedimentary facies features are not during low grade metamorphism obliterated in low grade metamorphism 2 2.0 Carbonate Mineralogy and Chemistry About 60 natural carbonate minerals are known and most common are calcite, aragonite and dolomite. Calcium carbonate (CaCO3) is the dominant constituent of modern carbonates and ancient limestones. It occurs as two minerals, aragonite and calcite. Aragonite crystallizes in the orthorhombic crystal system while calcite is rhombohedral. Calcite forms an isomorphous series with magnesite (MgCO3). Ancient limestones are composed largely of low magnesium calcite, while modern carbonate sediments are made mainly of aragonite and high-magnesium calcite. Aragonite is found in many algae, lamellibranches, and bryozoa. highmagnesian calcite occurs in echinoids, crinoids, many foraminifera, and some algae, lamellibranches, and gastropods. Skeletal aragonite and calcite also contain minor amounts of strontium, iron, and other trace elements. The relationship between carbonate secreting organisms, mineralogy of their shells and trace elements are important because their variation and distribution play role in the early diagenesis of skeletal sands. Dolomite is another important carbonate mineral, giving its name also to the rock. Dolomite is calcium magnesium carbonate (CaMg(CO3)2). Isomorphous substitution of some magnesium for iron is found in the mineral termed ferroan dolomite or ankeriteCa(MgFe)(CO3)2. Unlike calcite and aragonite, dolomite does not originate as skeletal material. Dolomite is generally found either crystalline, as secondary replacement of other carbonates or as a primary or penecontemporaneous replacement mineral in cryptocrystalline form. Siderite,iron carbonate (FeCO3), is one of the rarer carbonate minerals. It occurs, apparently as a primary precipitate, in ooliths. These "spherosiderites," as they are termed, are found in rare restricted marine and freshwater environments. Spherosiderite is oftenassociated with the hydrated ferrous aluminosilicate, chamosite, in sedimentary iron ores. Siderite also occurs as thin bands and horizons of concretions inargillaceous deposits, especially in deltaic deposits. Siderite clasts are also found in intraformational conglomerates. These facts suggest that siderite forms diagenetic duringearly burial while the host sediment is still uncompacted. Its formation is favoured by alkaline reducing conditions. Figure1 shows different carbonate minerals stable at earth’s surface temperature and pressure (Selley, 2000). 3 Fig. 1: Composition triangle of carbonate minerals stable at surface temperature and pressure Sedimentary rocks may also be made ofcarbonates of elements such as magnesium or iron,and there are also carbonates of elementsoccurring in nature (e.g. malachite and azurite arecopper carbonates). This group of sediments and rocksare collectively known as carbonates to sedimentarygeologists, and most carbonate rocks are sedimentaryin origin. Exceptions to this are marble, which is acarbonate rock recrystallised under metamorphicconditions, and carbonatite, an uncommon carbonate-rich lava. Calcite The most familiar and commonest carbonate minerals calcite (CaCO3). As a pure mineral it is colourless orwhite, and in the field it could be mistaken for quartz. It can be differentiated simply by hardness (3 onMohs’ scale) and reaction with dilute hydrochloric acid (HCl). Although calcite sometimesoccurs in its simple mineral form, it most commonly has a biogenic origin. A wide variety of organisms usecalcium carbonate to form skeletal structures and shells and calcareous sediments/ rocksare formed of material made in this way.Magnesium ions can substitute for calcium in thecrystal lattice of calcite, and two forms of calcite arerecognised in nature: low- magnesium calcite (low-Mgcalcite), which contains less than 4% Mg, and highmagnesiumcalcite (high-Mg calcite), which typicallycontains 11% to 19% Mg. The hard parts of manymarine 4 organisms are made of high-Mg calcite like echinoderms, foraminifers etc. Strontium may substitute for calcium in the lattice. It is in smallquantities (less than 1%) but important for strontium isotopestudies for dating of rocks. Aragonite There is no chemical difference between calcite andaragonite, but the two minerals differ in theirmineral form: whereas calcite has a trigonal crystalform, aragonite has an orthorhombic crystal form.Aragonite has a more densely packed lattice structureand is slightly denser than calcite (specific gravity of2.95, as opposed to 2.72–2.94 for calcite),and is slightly harder (3.5–4 on Mohs’ scale). Many invertebratesuse aragonite to build their hard parts, includingbivalves and corals. Dolomite
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