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

9. Calcite Structural Type Calcite CaCO3
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

10. Calcite Structural Type Magnesite MgCO3
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

11. Calcite Structural Type Siderite FeCO3
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 Type Rhodochrosite MnCO3
Pale pink color (but also white, grey, green-grey)
Hydrothermal deposits
Sedimentary Mn deposits

13. Calcite Structural Type Smithsonite ZnCO3
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 Group Dolomite CaMg(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 Group Ankerite CaFe(CO3)2
Double salt of Ca and Fe
Calcite structure with alternating Fe and Ca layers – therefore very similar to dolomite – solid solution
Also in this group:
Kutnahorite - CaMn(CO3)2