Carbonate Rocks PDF

Origin of carbonates  Carbonate rocks make up about 1/5th to 1/4th of all sedimentary rocks in the stratigraphic record.  Carbonate rocks occur in many Precambrian assemblages and in all geologic systems from the Cambrian to the Quaternary.  Limestones occur throughout the world in every ge...

Origin of carbonates  Carbonate rocks make up about 1/5th to 1/4th of all sedimentary rocks in the stratigraphic record.  Carbonate rocks occur in many Precambrian assemblages and in all geologic systems from the Cambrian to the Quaternary.  Limestones occur throughout the world in every geological period from the Cambrian onwards and reflect the changing treasures, through evolution and extinction, of invertebrates with carbonate skeletons.  In the Precambrian, carbonates are also abundant, but they are commonly dolomite and many contain stromatolites, produced largely by microbes, especially the cyanobacteria (‘blue–green algae’).  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 limestone rocks.  Biological and biochemical processes are dominant in the formation of carbonate sediments, although inorganic precipitation of CaCO3 from seawater also takes place.  The chemical and physical processes of diagenesis can considerably modify the carbonate sediment.  Carbonate rocks are primarily composed of carbonate minerals.  Carbonate rocks are so called because they are composed primarily of carbonate minerals.  Calcium carbonate (CaCO3) is the principal compound in limestones. Sedimentary rocks may also be made of carbonates of elements such as magnesium or iron, and there are also carbonates of dozens of elements occurring in nature (e.g. malachite and azurite are copper carbonates).  This group of sediments and rocks are collectively known as carbonates, and most carbonate rocks are sedimentary in origin. Exceptions to this are marble, is a carbonate rock recrystallized under metamorphic conditions, and carbonatite, an uncommon carbonate-rich lava.  Carbonate mineralogy:  Calcite (CaCO3) - most familiar and commonest carbonate mineral.  It is colorless or white, and in the field it could be mistaken for quartz.  There are two simple tests that can be used to distinguish calcite from quartz.  First, there is a difference in hardness: calcite has a hardness of 3 on Mohs’ scale, and hence it can easily be scratched with a pen-knife; quartz (hardness 7) is harder than a knife blade and will scratch the metal.  Second, calcite reacts with dilute (10%) hydrochloric acid (HCl), whereas silicate minerals do not.  Most common carbonate minerals (except dolomite) will react with the acid to produce bubbles of carbon dioxide gas, especially if the surface has been powdered first by scratching with a knife.  Magnesium ions can substitute for calcium in the crystal lattice of calcite.  Two forms of calcite are recognized in nature: low-magnesium calcite (low-Mg calcite)-contains less than 4% Mg, and high magnesium calcite (high-Mg calcite)- typically contains 11% to 19% Mg.  The hard parts of many marine organisms are made of high-Mg calcite, for example echinoderms, barnacles and foraminifers.  Aragonite  There is no chemical difference between calcite and aragonite.  Differ in their mineral form: calcite (trigonal crystal), aragonite (orthorhombic crystal form). Aragonite has a more densely packed lattice structure and is slightly denser than calcite.  Aragonite has specific gravity-2.95, and slightly harder (3.5–4 on Mohs’ scale).  Many invertebrates use aragonite to build their hard parts, including bivalves and corals.  Dolomite  Calcium magnesium carbonate (CaMg(CO3)2) is a common rock- forming mineral which is known as dolomite.  A rock made up of (CaMg(CO3)2) mineral is called dolomite.  The mineral is similar in appearance to calcite and aragonite, with a similar hardness to aragonite.  Dolomite can be distinguished in hand specimen by the use of the dilute HCl acid test.  There is usually little or no reaction between cold HCl and dolomite.  Dolomite rock is quite widespread, it does not seem to be forming in large quantities today, so large bodies of dolomite rock are considered to be diagenetic.  Siderite  Siderite is iron carbonate (FeCO3) with the same structure as calcite.  It is very difficult to distinguish between iron and calcium carbonates on mineralogical grounds.  It is rarely pure, often containing some magnesium or manganese substituted for iron in the lattice.  Siderite forms within sediments as an early diagenetic mineral.  Non-carbonate minerals in limestones include terrigenous quartz and clay, and pyrite, hematite, chert and phosphate of diagenetic origin.  Evaporite minerals, in particular gypsum–anhydrite, may be closely associated with limestones.  Components of limestones  Limestones are very varied in composition but broadly the components can be divided into three groups:  1. Allochem  (a) Non-skeletal grains (Ooids, Peloids, Oncoids, Pisoids, Aggregate grains, lithoclasts- Intraclasts & Extraclasts)  (b) Skeletal grains (Biotic fragments)  2. Micrite and  3. Cement (Spar) Non-skeletal grains & Sketal grains  Peloids  Peloids are spherical, ovoid, or rod-shaped, mainly silt-sized carbonate grains  Commonly lack definite internal structure  Generally dark gray to black owing to contained organic material  May or may not have a thin, dark outer rim  Size from about 0.05 to 0.20 mm  Peloids are composed mainly of fine micrite 2 to 5 microns in size, but larger crystals may be present.  Peloids are commonly well sorted and they may occur in clusters  Well-rounded, symmetrical shapes, are thought to be of fecal origin. These peloids are commonly called pellets.  Fecal pellets are produced by a variety of organisms that consume fine carbonate mud while feeding on organic-rich sediments.  Most pellets appear to be produced by organisms living in quiet, marine water with muddy bottoms. Thus, pellets in ancient limestones occur most commonly in muddy limestones (micrites), and their presence suggests deposition in low-energy environments.   Ooids  Ooids are small, more or less spherical to oval carbonate particles  Characterized by the presence of concentric laminae that coat a nucleus.  The nucleus may be a skeletal fragment, peloid, smaller ooid, or even a siliciclastic grain such as a quartz grain.  Ooids are sand- to silt-size particles and range in size from about 0.1 mm to more than 2 mm.  Carbonate grains that are structurally similar to ooids but are larger than about 2 mm are called pisoids  Agitated water ooids are white to cream in color and commonly have a pearly luster.  Quiet-water ooids may have a dull luster. Ooids are distinguished especially by the presence of concentric, accretionary layers or laminae.  Types of ooids:  True ooids: Thickness of rim is more than the nucleus.  Superficial ooids: rim thickness is less than nucleus.  Composite ooids: Contain more than one nucleus.  Agitated water ooids are white to cream in color and commonly have a pearly luster.  Quiet-water ooids may have a dull luster. Ooids are distinguished especially by the presence of concentric, accretionary layers or laminae.  Types of ooids:  True ooids: Thickness of rim is more than the nucleus.  Superficial ooids: rim thickness is less than nucleus.  Composite ooids: Contain more than one nucleus. Distribution of grains  MATRIX: (Micrite)  Interstitial material between grains  Micrite (microcrystalline carbonate sediment < 4µm)  Dark in transmitted light.  Lime or carbonate mud is that < 62µm  Formed under calm and quite environments  CEMENT: (Spar)  Chemical precipitates from aqueous solution.  Material between grains Crystals of aragonite or calcite  Vary in size and shape > 30 µm  Hyaline/transparent in transmitted light.  Formed under agitated water conditions