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Carbonate Mineralogy and Chemistry

This PPT presentation helps users understand mineralogy and chemistry of carbonate rocks.

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Carbonate Mineralogy and Chemistry

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  1. Carbonate Composition: • Mineralogy and Chemistry ShibiruMerga Dec, 2016

  2. Outlines 1. Introduction 2. Mineralogy of Carbonates 2.1. Principal Carbonate Groups 2.2. Ion Substitution in Carbonates 2.3. Non-Carbonate minerals 2.4. Identification of Carbonate minerals 2.5. Mineralogy of Carbonate secreting Organisms 3. Chemical and Isotope Composition of Carbonates 3.1. Elemental Composition 3.2. Stable Isotope Composition 3.2.1. Oxygen Isotope 3.2.2. Carbon Isotope 3.2.3. Stable-Isotope composition of carbonate sediments and fossils 3.3. Radiogenic isotopes in carbonate rocks

  3. 1. Introduction Carbonate rocks make up about one-fifth to one-quarter of allsedimentary rocks in the stratigraphic record. They occur in many Precambrian assemblagesand in all geologic systems from the Cambrian to the Quaternary Both limestone and dolomiteare well represented in the stratigraphic record. Dolomite is the dominant carbonate rock in Precambrian and Paleozoic sequences, whereas limestone is dominant in carbonate units of Mesozoic and Cenozoic age (Ronov, 1983). They contain much of the fossil record of past life forms, and they are replete with structures and textures that provide invaluable insight into environmental conditions of the past.

  4. 2. Mineralogy of Carbonates 2.1.Principal carbonate groups Carbonate rocks are so called because they are composed primarily of carbonate minerals. These minerals, in turn, derive their identity from the carbonate anion (CO3-2), which is afundamental part of their structure. • The common carbonate minerals fall into three main groups: • Calcite group, • Dolomite group, and • Aragonite group

  5. In calcite minerals, the atoms are arranged such that layers containing Ca, Mg, Mn, Fe, or Zn atoms alternate with layers of carbonate atoms. • In dolomite minerals, the cation layers alternate; that is, layers of Ca+2 ions alternate with layers of Mg+2(or Fe2+) ions. • Orthorhombic carbonates have aninefold coordination (three cations for each oxygen). • In aragonite-group carbonates, the CO3-2groups do not lie midway between Ca (or Pb, Sr, Ba) layers. • Further, they are rotated30 degrees to right or left so that each oxygen atom has three neighboring Ca (or other cation)atoms.

  6. Because the common cations in carbonate minerals have the same charge and similar ionic radii, substitution of cations is common. • Substitution of Mg+2(ionic radius 0.072 nm) for Ca+2(ionic rad. 0.100nm) is particularly common. • On the other hand, the larger Ca+2 ion does not readily substitute for Mg+2 • Magnesian calcite or high-magnesian calcite (Mg-calcite): Calcite containing more than about 4 mol% MgCO3 (5 mol% according to some authors)

  7. Low-magnesian calcite or simply calcite: Calcite with less than about 4 mol% MgCO3 • ferroan calcite: Less commonly, Fe2+can substitute for Ca+2or Mg+2in calcite • Aragonite in skeletal grains transforms to calcite with time and, as indicated,high-magnesian calcite may either lose Mg and alter to low-magnesian calcite or gain Mg toform dolomite.

  8. 2.3. Non-carbonate components • Non-carbonate minerals may include common silicate minerals suchas: • quartz, • chalcedony or microquartz, • feldspars, • micas, • clay minerals, and • heavy minerals. • Other minerals reported incarbonate rocks include • fluorite, • celestite, • zeolites, • iron oxides, • barite, • gypsum, • anhydrite, and • pyrite

  9. Most non-carbonate minerals in limestones and dolomites are probably of detrital origin; However, some minerals such as chalcedony, pyrite, iron oxides, and anhydrite may form during carbonate diagenesis. Carbonate rocks may also contain fine-size plant or animal organic matter. The meanorganic content of carbonate rocks is only about 0.2 percent (Hunt, 1979)

  10. 2.4. Identification of carbonate minerals Identification can be greatly aided by staining and etching techniques. For example,aragonite is stained black with Fiegl’s solution (Ag2SO4 + MnSO4), whereas calciteremains unstained. Calcite is stained red in a solution of Alizarin red and dilute HCl, whereas dolomite remains unstained. Dolomiteand high-magnesiancalcite can be stained yellowin an alkaline solution of Titan yellow. Etchingrequires polishing the surface of the sample, then immersing the polished surface in dilute HCl so that the acid dissolves some of the carbonates leaving the impurities behind. Then, the etched surface is examined under microcope. The carbonate minerals can be differentiated also by X-ray diffraction methods.

  11. 2.5. Mineralogy of carbonate-secreting organisms The skeletal remains of calcium carbonate-secreting organisms are volumetrically important components of many limestones. These skeletal remains may consist of aragonite, calcite, or high-magnesiancalcite containing as much as 30 mole percent MgCO3. For example, most molluscsare composed of aragonite, although some (e.g. some gastropods) are composed of low-magnesiancalcite. Echinoderms are composed of high-magnesian calcite, and foraminifers are composed of low- or high-magnesian calcite. Note that the mineral composition of calcareous organisms may change with burial diagenesis. Aragonite in skeletal grains transforms to calcite with time and, as indicated, high-magnesian calcite may either lose Mg and alter to low-magnesian calcite or gain Mg to form dolomite.

  12. 3. Chemical and isotope composition 3.1. Elemental composition Limestones contain one major cation,Ca+2, and one minor cation, Mg+2 . Numerous other cationsmay be present in limestones in trace amounts. The most abundant of these trace cationsare silicon, aluminum, iron, potassium, manganese, strontium, sodium, phosphorus, titanium, and boron. The major anion in carbonate rocks isCO32- , but significant amounts of SO42- , OH- ,F- , and Cl-may also be present. The major element composition of carbonate rocks is a function of the kinds and amounts of carbonate minerals, fossils, and non-carbonate constituentspresent in the rocks. On the other hand, trace elements such as B, P, Mg, Ni, Cu, Fe, Zn, Mn, V, Na, U, Sr, Pb, K, and Ba are particularly concentrated in skeletal material.

  13. Cations with large ionic radii, such as strontium and, to a lesser extent, barium, lead, and uranium, commonly show higher concentrations in aragonite than in calcite. Elements with small ionic radii, such as magnesium, manganese, iron, nickel, and phosphorus tend to be concentrated in calcite (Milliman, 1974) Magnesium appears to interfere with precipitation of aragonite; therefore, the magnesium content of most aragonites is reported to be less than about 0.5 percent. The magnesium content of calcite may be low (< ∼4mol%MgCO3) or high (up to 30 or more mol% MgCO3). Many trace elements, such as Si, Al, Na, K, Ti, Mn, Fe, and Ba, probably owe their presence in carbonate rocks mainly to the content of non-carbonate constituents in these rocks.

  14. 3.2. Stable-isotope composition The most important stable isotopes in carbonate rocks are isotopes of oxygen and carbon. Oxygen has threestable isotopes16O, 17O, 18O; however, 18Oand16Oare most abundant and are the principal oxygen isotopes used in isotope studies. Carbon has only two stable isotope, 13Cand 12C, but it has an additional radioactive isotope (14C). 18O

  15. The carbon isotope composition of carbonates is expressed as the ratio of 12C/13C with reference to a standard in the same manner that oxygen isotopes are expressed. The standard for both oxygen and carbon isotopes was originally the University of Chicago PDBStandard, which is the isotope composition of a fossil belemnite from the Cretaceous Pee Dee Formation of South Carolina. Oxygen isotope abundances are also reported relative to a sample of ocean water called SMOW(Standard Mean Ocean Water). This standard is also referred to as V-SMOW, where V stands for Vienna, the location of the ocean-water sample.

  16. 3.2.1. Oxygen isotopes The oxygen isotope composition of carbonate minerals is a function primarily of the isotope composition and temperature of the water in which the minerals are precipitated. Because most carbonates are precipitated in warm regions, calcites precipitated in equilibrium with meteoric waters typically have δ18Ovalues of about −4‰ (Anderson and Arthur, 1983). The equilibrium fractionation of oxygen isotopes is strongly affected by temperature. The relationship between ocean temperature and oxygen isotope composition was determined by Epstein et al. (1953) to be T(o C )= 16.9 – 4.2(c - w) + 0.13 (c - w )2 where δc = equilibrium oxygen isotope composition of calcite and δw = oxygen isotope composition of the water from which the calcite was precipitated.

  17. 3.2.1. Carbon isotopes The carbon isotope composition of carbonate sediments is influenced primarily by the isotope composition of the water from which carbonate minerals precipitate and to a lesser extent by temperature and biogenic fractionation. The δ13C values in carbonates reflect the 13C/12Cratio of CO2 dissolved in water. Values of δ13Cin the total dissolved carbon of ocean water are commonly higher than those in meteoric waters, with average values of approximately +1‰. Oxidation of this low-δ 13Corganic matter leads to production of low-δ 13Cdissolved bicarbonate (HCO3-), which is then used by organisms to build shells.

  18. The carbon isotope composition of carbonates is affected also by temperature in a manner similar to that in which oxygen isotopes are affected. The temperature fractionation effect is not nearly so strong, however, and carbon isotopes are not commonly used in paleo-temperature studies. Thus, because fractionation during carbonate precipitation is relatively insensitive to temperature, δ13Cvalues of ancient marine carbonates reflect the δ13C value of dissolved inorganic carbon in the past. 3.2.3. Stable-isotope composition of carbonate sediments and fossils Freshwater carbonates tend to have negative δ13Cand δ18Ovalues whereas marine carbonates tend to have positive values. The distribution of δ13Cand δ18Ovalues in various kinds of Quaternary carbonates is summarized in figure below.

  19. +6 Green algae Shallow water marine limestone +8 -10 • 18O(%o) Deep sea limestones Hermatypic corals Shallow water Molluscs And Foraminifers -10 Figure 1Distribution of δ18O and δ13C values in various types of marine carbonates. (After Milliman, J. D., 1974, Marine Carbonates) Freshwater limestone -20 13C (%o)

  20. 3.3. Radiogenic isotopes in carbonate rocks In addition to the stable isotopes of oxygen and carbon, carbonates may contain several radiogenic isotopes, including carbon-14, thorium-230, protactinium-231, and strontium-87. Carbon-14, thorium-230, and protactinium-231 isotopes, in particular, have proven to be useful for direct determination of the ages of young carbonate sediments.

  21. THANK YOU !!!

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