Principles of Sedimentology - Fall 2024 Lecture Notes PDF

Principles of Sedimentology PG207 Fall 2024 - Part 2 Carbonate Sediments and Sedimentary Rocks Lecture 1 Introduction & Carbonate Minerals Dr. Mahmoud Thabet Geology department, Faculty of Science, Alexandria Univ.  Sedimenta...

Principles of Sedimentology PG207 Fall 2024 - Part 2 Carbonate Sediments and Sedimentary Rocks Lecture 1 Introduction & Carbonate Minerals Dr. Mahmoud Thabet Geology department, Faculty of Science, Alexandria Univ.  Sedimentary Rocks Sedimentary rocks are those rocks which form at or near the earth's surface at relatively low temperatures and pressures primarily by either: deposition by water, wind or ice precipitation from solution biologically mediated and /or growth in position by organic processes (e.g., carbonate reefs )  What Are Carbonates? They are sediments that precipitate either inorganic (chemically) or in-organic (biochemically)  Carbonates versus Siliciclastics Similarities: ◼ 1. Both composed of grains ◼ 2. Both contain matrix material ‐ mud‐ sized material ◼ 3. Both contain mineral cements ◼ 4. Both display sedimentary structures  Carbonates versus Siliciclastics Differences Between Siliciclastic and Carbonate Sediments. Siliciclastics Carbonates Mineralogy Mainly silicates (Quartz – Carbonates (Calcite – Aragonite – Feldspars – Rock Fragments – Dolomite ………. ) clay minerals.) Origin - mainly extrabasinal, -intrabasinal, - Inorganic -mostly organic - terrestrial & marine - mostly marine, rare terrestrial Affect of - little effected - strongly effected diagenesis - late cementation - early & late cementation -rarely recrystallized - easily recrystallized  Importance of Carbonate Rocks Carbonate rocks produce about 50% of all oil and gas, and include many of the reservoirs in the World and the huge reservoirs in the Middle East. Carbonate deposits host many ore deposits (Pb-Zn, Mn-deposits,..) As industrial "mineral“ ( stone, cement) Most carbon on earth is stored in carbonate sediments, therefore plays a major role in the global carbon cycle it is important to reconstruct the paleogeography, paleo- environment, relative age dating from the fossil contents.  When is a sedimentary rock termed “Carbonate”? Carbonate sediments and sedimentary rocks are made up mainly of carbonate minerals (calcite, aragonite, and dolomite) Modern carbonate sediments are mainly made up of aragonite and calcite Ancient carbonates are predominantly calcite and dolomite  Whether Chemically or Biochemically How Carbonate Sediments Are Precipitated? Precipitation CO2 (gas) + H2O = H2CO3 H2CO3 = H+ + HCO3- HCO3 = H+ + CO3-2 Ca 2+ + CO3-2 = CaCO3 Dissolution CaCO 3 + H 2CO 3  2HCO -3 + Ca 2+  What is a CaCO3 Supersaturated Solution? The equilibrium constant and the saturation index of the reaction forming CaCO3 are expressed by the following equations: [Ca++] [CO3--] [Ca++] [CO3--] Ksp = Ω = Ksp [CaCO3] Ω = 1 saturated solution, Ω > 1 supersaturated solution Ω < 1 undersaturated solution  In order to precipitate CaCO3 You have to have 1. A certain degree of solid supersaturation 2. High activity of calcium & carbonate ions 3. No kinetic (e.g. ion pair, hydration behavior,..) nor thermodynamic (Ca/Mg ratio, temperature,.. ) barriers for crystal nucleation 4. Free surface for crystal nucleation to start 5. Crystal growth in open void space Carbonate Minerals There are Three Groups of Carbonate Minerals Calcite Group Trigonal Aragonite Group Orthorhombic Dolomite Group Trigonal  Calcium “Ca” is an atom in the second column of the Periodic Table. All of the atoms in this column readily form 2+ cations. Consequently, these atoms can bond with the carbonate ion: MgCO3 is magnesite In addition, several transition elements (long rows in the middle of the Periodic Table) can be 2+ ions, and they can also bond with carbonate: FeCO3 Siderite,  Carbonate Mineralogy The mineralogy of carbonate rocks is simple Carbonate minerals: Trigonal Trigonal  Isomorphs & Ionic Substitution The crystal structures of these minerals are identical, but their chemical formulas vary. Such minerals are called isomorphs ("iso" for "same"; "morph" for "shape"). Isomorphs have the same crystal structure but different chemical formulas. (Calcite & Dolomite) Sometimes ionic substitution within a specific crystal structure occurs with just a few of the ions. For example, in every calcite crystal, there are a few magnesium ions where calcium ions should be.  Calcite & Aragonite are not isomorphs Polymorphs BUT They are two CaCO3 polymorphs (i.e. they are different crystal forms for the same chemical composition)  Calcite Group: Trigonal Calcite: CaCO3 minor Mg, Mn, Fe, Zn, Co Magnesite: MgCO3 [Mg, Fe] CO3 Siderite: FeCO3 [Fe, Mg] CO3 minor Ca,Mn  Types of Calcite Pure calcite (< 1 mole% MgCO3) Low Mg Calcite: (2-3 mole% MgCO3) Intermediate Mg Calcite ( 4-11 mole% MgCO3) High Mg Calcite ( 12-19 mole% MgCO3)  Aragonite Group: Orthorhombic Aragonite: CaCO3 minor Pb, Sr, Ba, K Witherite: BaCO3 minor Sr Strontianite: SrCO3 minor Ca Skeletal Mineralogy Dolomite Group: Trigonal Dolomite: CaMg(CO3)2 minor Mn, Co, Zn Ankerite: Ca[MgFe](CO3)2 transition series to dolomite  Dolomite Crystal Structure Important Ordering & Stoichiometry of Dolomites So, We have a series of “dolomite” minerals based on the difference in ordering & stoichiometry Ideal dolomite (ordered & stoichiometric) Ca Mg (CO3)2 0.5 0.5 1 Non-stoichiometric dolomite 0.6 0.4 1 0.7 0.3 1 0.4 0.6 1  Carbonate Mineral Stability Ideal Dolomite is the most stable carbonate mineral Ordered & Stoichiometric Non ordered, non stoichiometric Calcite Pure Calcite Stability Low-Mg Calcite (LMC) Intermediate-Mg Calcite decrease High-Mg Calcite, Aragonite Identification of Carbonate Minerals Simple Acid Test Polarizer Microscope Staining Tests Cathodoluminescence X-Ray Diffraction  Simple Tests For Calcite Or Dolomite Field test with HCl: ◼ Limestone (CaCO3) bubbles visibly releasing CO2 gas ◼ Dolostone CaMg(CO3)2 will not react unless powdered ◼ Dolostones commonly weather with a yellowish colour because iron (Fe) often substitutes for Mg  Identification of Carbonate Minerals Using Polarizer Microscope Carbonate Minerals are: moderate to high relief usually colourless to buff in plane polarized light perfect cleavage High Twinkling twin lamellae common, extreme birefringence gives creamy high-order interference colors, commonly white Carbonate Staining Tests Alizarin Red S Potassium Ferricyanide  Alizarine Red S Calcite is stained red, dolomite not stained the blue color is a vug  Potassium Ferricyanide Fe-rich Calcite Cathodoluminescence Petrography CL Intensity: Role of Mn and Fe CL Activators & Quenchers Activators: Mn, REE, Pb, Quenchers: Fe, Ni, Co, CL Imaging of Dolomite Phases Middle Permian San Andres Formation, subsurface, West Texas Cathodoluminescence Plane light Dolomite 1 Dolomite 3 Dolomite 2 From Ruppel and Cander (1988)  Identification of Carbonate Minerals Using X-Ray Diffraction Uses of X-Ray Powder Diffraction Identification of single-phase materials – minerals, chemical compounds or other engineered materials. Identification of multiple phases in microcrystalline mixtures (i.e., rocks) Determination of crystal structure of identified materials Identification and structural analysis of clay minerals Diffractometer Components and Geometry Our Scintag Diffractometer (Pt. 1) Scintag Diffractometer Geometry  Identification of Carbonate Minerals Using X-Ray Diffraction How to identify Minerals?? Determine d-spacings and intensities from experimental data (film or strip chart) Use paper indexes to determine likely phases Compare likely phases with database of standard minerals by matching peaks  Identification of Carbonate Minerals Using X-Ray Diffraction How to identify Minerals?? Determine d-spacings and intensities from experimental data (film or strip chart) Use paper indexes to determine likely phases Compare likely phases with database of standard minerals by matching peaks Identification of Carbonate Minerals Using X-Ray Diffraction Calcite Identification of Carbonate Minerals Using X-Ray Diffraction Dolomite Identification of Carbonate Minerals Using X-Ray Diffraction Aragonite Identification of Carbonate Minerals Using X-Ray Diffraction  Identification of Carbonate Minerals Using X-Ray Diffraction Mixture Calcite Dolomite Quartz  Identification of Carbonate Minerals Using X-Ray Diffraction Mixture Calcite Dolomite Anhydrite  Identification of Carbonate Minerals Using X-Ray Diffraction Mixture Anhydrite Dolomite Automated Search/Match: Results Display Search/Match: Printout of Results (background restored) Search/Match: Printout of Results (background removed)