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Chapter 22

Chapter 22. Carbonates and other minerals with triangular anion groups. Sedimentary origins. Introduction. Triangular anion group: (XO 3 ) n- Carbonates: CO 3 2- Simple salts of carbonic acid H 2 CO 3 : calcite CaCO 3 ; dolomite CaMg(CO 3 ) 2

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Chapter 22

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  1. Chapter 22 Carbonates and other minerals with triangular anion groups. Sedimentary origins

  2. Introduction Triangular anion group: (XO3)n- Carbonates: CO32- Simple salts of carbonic acid H2CO3: calcite CaCO3; dolomite CaMg(CO3)2 With additional anions: malachite Cu2(CO3)(OH)2 Mixed compounds: sulfate-carbonates; phosphate-carbonates, etc. Nitrates: NO3- Borates: BO33- (some borates also tetrahedra BO45-) Study info for NB carbonates, nitrates and borates from Table 22.1

  3. Carbonates: Composition Forms by bonding of carbonic acid (H2CO3) to: Na, Ca, Mg, Fe, Mn, Zn, Sr, Ba low ionization potential large cations For Na+, H2O is incorporated into structure Bi2+, Cu2+and rare earth elements (REEs) only when OH-, F- or O2- present to weaken the CO32- complex

  4. Crystal chemistry • Rhombohedral (trigonal), orthorhombic or monoclinic • Calcite and dolomite: • CO32-assymetrical; occupies parallel planes in calcite and dolomite in same orientation • Therefore highly anisotropic – high birefringence: large difference between properties parallel and perpendicular to c • Structure analogy with NaCl • Replace Na+ with Ca2+; Cl- with CO32-; compress structure along body diagonal to give rhombohedral symmetry • Calcite: cubic close-packing of CO32- with Ca+ in interstices • Aragonite: hexagonal close-packing of CO32- with Ca+ in interstices, but with distorted structure • Dolomite: same structure as calcite but layers of Ca2+ are alternated with layers of Mg2+

  5. Crystal chemistry • Carbonates display polymorphism and isomorphism • Isomorphy: • Small cations: isomorphy within calcite structure (calcite, magnesite, siderite, rhodochrosite, smithsonite) • Large cations: isomorphy within aragonite structure (aragonite, witherite, strontianite, cerussite) • Polymorphy of CaCO3: High T; low P: calcite; Low T; high P: aragonite • Sheet and chain like structures common in carbonates • Polymerization of triangular groups common in borates: • BO33- isolated • Combined into B2O54- or B2O42- pairs, B4O84- rings or B2O54- chains

  6. Morphology Majority has rhombohedral symmetry Growth form: Commonly as cleavage rhombohedron Many other forms Symmetry decreases with increasing complexness of composition

  7. Physical and Optical Properties Colorless to white Mn: pink (rhodochrosite) Cu: green (malachite) or blue (azurite) Fe: yellow (ankerite, siderite) or brown Reaction with HCl 2HCl + Ca(Pb,Zn,Mg…)CO3 H2O + CO2 + Ca(Pb,Zn,Mg…)Cl2 Some only when concentrated acid or mineral heated High birefringence & high order interference colors A result of strong anisotropy

  8. Classification Associations: Al-Mg-(Na) association Magnesite Group Hydrotalcite Group Na(K)-Ca-Ba association Calcite-Aragonite Group Dolomite-Barytocalcite Group Bastnaesite Group Zn-Cu-Pb(U) association Smithsonite Group Malachite Group CerussiteGroup Structural type Calcite group Dolomite group Aragonite group Soda carbonate minerals Other carbonate minerals Nitrates Borates

  9. Often with minor Mg, Fe, Mn Limited at room T Very diverse habit Limestone: chemical or biological origin Usually high in Mg when precipitate from seawater Forms marble when metamorphosed Skarns and hydrothermal deposits Calcite Structural TypeCalciteCaCO3

  10. Endmember of magnesite-siderite isomorphic series Gray-white solid masses of granular texture Occur in: Dolostones affected by hydrothermal solutions – recrystallisation Veins formed during weathering of serpentinites Calcite Structural TypeMagnesiteMgCO3

  11. Calcite Structural TypeSideriteFeCO3 Brown crystals, highly vitreous luster Granular aggregates in hydrothermal veins Sedimentary rocks: hydrothermal alteration Chemical origin Fe ore Rusty color (white when nonoxidized); high density

  12. Calcite Structural TypeRhodochrositeMnCO3 Pale pink color (but also white, grey, green-grey) Hydrothermal deposits Sedimentary Mn deposits

  13. Calcite Structural TypeSmithsoniteZnCO3 Limited solid solution, with Zn replaced by: Fe, Mn, Ca, Pb, Mg, Cd, Co Occurs as: Colloidal, botryoidal, earthy masses OR White, green, brown crystals Oxidation product of sphalerite (ZnS) ores

  14. Dolomite GroupDolomiteCaMg(CO3)2 Double salt of Ca and Mg Calcite structure with alternating Mg and Ca layers Solid solution with calcite at high T Used as geotermometer when coexisting Solid solution with ankerite: CaFe(CO3)2 Hydrothermal deposits and sedimentary rocks In sedimentary rocks: usually result of secondary diagenetic transforming process

  15. Dolomite GroupAnkeriteCaFe(CO3)2 Double salt of Ca and Fe Calcite structure with alternating Feand Ca layers – therefore very similar to dolomite – solid solution Also in this group: Kutnahorite - CaMn(CO3)2

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