Lecture 15: Carbonate Minerals

You will work in groups on the structure and bonding of some of these element oxides in class. Before coming to class:
  1. Carefully read the information on this page. If it is not clear or if you want additional information, you may also want to review material from your textbook in sections 5.10 and 5.11.

  2. Watch the 15 minute video lecture Carbonates on the MediaSite page.

  3. Take a short quiz on WebBoard.




From Carbon Dioxide to the
Cathedral of Notre Dame


Carbonate minerals are formed by reactions of atmospheric carbon dioxide. Hydration to carbonic acid, H2CO3, and reaction with basic calcium oxide produces the calcium carbonate in limestone. Calcium carbonate is also produced by living things. Coral exoskeletons are biologically synthesized from carbon dioxide.

The first step, the hydration of CO2, is the nucleophilic attack of the oxygen center of water on the electrophilic carbon of carbon dioxide. This forms a new C-O bond. A proton then migrates to one of the CO2 oxygen atoms. Note that this is an equilibrium process. Does the equilibrium favor CO2 or H2CO3?



Carbonic acid is a relatively weak, diprotic acid. Reaction with one equivalent of a strong base converts carbonic acid into the bicarbonate anion, [HCO3]-. Reaction with an additional equivalent of strong base produces the carbonate dianion, [CO3]2-. The bicarbonate anion is in equilibrium with its conjugate acid and its conjugate base in aqueous solution.




Acid Base Chemistry

Acids are formed when atmospheric molecules react with water or hydroxyl radical. Let's begin by reviewing acid-base reactions in general.

A strong acid has a weak (stable) conjugate base. A strong base has a weak (stable) conjugate acid.



The lower the pKa, the stronger is the acid (and the weaker is the conjugate base) in the table below.

acid base pKa
HClO4 ClO4- -10
HI I- -10
H2SO4 HSO4- -9
HBr Br- -9
HCl Cl- -7
PhSO3H PhSO3- -6.5
CH3H C(OH)2 CH3COOH -6.5
RCH2OH2+ RCH2OH -2
H3O+ H2O -1.7
HNO3 NO3- -1.4
[CH3C(OH)=NH2]+, CH3C(O)NH2
acetamide 		0
HSO4- SO4-2 1.99
HF F- 3.17
HN3 N3- 3.17
RCOOH RCOO- 4 - 5
[C6H5NH3]+, C6H5NH2
aniline 		4.6
HCOCH2CHO [HCOCHCHO]- 5
[C5H5NH]+, C5H5N
pyridine 		5.29
H2CO3 HCO3- 6.35
H2S HS- 7
[C3H5N2]+, C3H4N2
imidazole 		7.0
CH3COCH2COCH3 [CH3COCHCOCH3]- 9
HCN CN- 9.22
NH4+ NH3 9.24
ArOH ArO- 8 - 11
RCH2NO2 [RCHNO2]- 10
R3NH+ R3N 10 - 11
HCO3- HCO3-2 10.33
[CH3CH2NH3]+ CH3CH2NH2 10.8
[(CH3CH2)3NH]+ (CH3CH2)3N 10.8
[(CH3CH2)2NH2]+ (CH3CH2)2NH 10.8
EtOOCCH2COOEt [EtOOCCHCOOEt]- 13
CH3OH CH3O- 15.2
H2O HO- 15.74
cyclo-C5H6 cyclo-C5H5- 16
RCONH2 RCONH- 17
PhCOCH3R [PhCOCH2]- 18
RCOCH2R [RCOCHR]- 19 - 20
CH3CH2C(O)OCH2CH3 [CH3CHC(O)OCH2CH3]- 24.5
CH3CN [CH2CN]- 25
H2C2 HCC- 25
H2 H- 35
NH3 NH2- 38
cyclo-C6H6 cyclo-C6H5- 43
CH2CH2 CH2CH- 44
CH4 CH3- 48


Weak bases stabilize the excess negative charge from the electron pair by having:
  • electron-withdrawing atoms or groups
  • delocalization of the electron density.

What is pKa? Essentially, pKa is a convenient method to describe acid strength. The smaller the value of pKa, the stronger the acid, and conversely, the higher the number, the more basic.

Calculate pKa:
  1. The first step is to know the reaction you are going to look at. For example:
    HA + H2O ---> H3O+ + A-

  2. Now determine the equilibrium constant, K, which is the concentration of the products divided by the concentration of the reactants. For our example:
    K = ([H3O+][A-]) / ([HA][H2O])

  3. Next determine the acid dissociation constant, Ka. This is calculated by treating the water as a constant and so you get:
    Ka = K[H2O]
    or
    Ka = ([H3O+][A-]) / ([HA])

  4. Now that you have Ka, you can find the pKa:
    pKa = -log10 Ka



Acidic and Basic Element Oxides

Non-metal oxides (such as carbon dioxide) are acidic. Specifically, they are acid anhydrides that react with water to form protic acids. These elements form covalent bonds with oxygen so the "oxides" are not ionic oxides and do not abstract protons from water. Instead, these elements react with hydroxyl groups in water and liberate protons.

Most metal oxides are basic and are protonated by water. The basicity tends to increase down a column in the periodic table as the electropositive character of the metal increases along with the ionic character of the metal-oxide interaction. For example, BeO is amphoteric but CaO is a base.

Some transition metal oxides are acidic when the metals are in higher oxidation states. Why? Metal oxides such as OsO4 are completely covalent. The Os=O unit is similar to the C=O unit in carbon dioxide. It reacts with hydroxide in water at the osmium center rather than reacting with protons at oxygen.



Calcium oxide (a basic oxide) and carbon dioxide (an acidic oxide) combine in the presence of water to give calcium carbonate.

Carbonate Salts



    Nitrate (NO3-), carbonate (CO32-), phosphate (PO43-), silicate (SiO44-), borate (BO45-), and related oxyanions form salts with main group cations and transition metals. While there is some degree of covalent character to the bonds, they are mainly ionic in character. Imagine close-packed oxyanions with cations in the holes.

    This picture shows crystalline calcite, a natural form of calcium carbonate. Below is aragonite, another form of calcium carbonate.

    Click here for a full listing of carbonate minerals.




The carbonate minerals have a structure that is similar to the cubic structure found in halite (NaCl) where the Na cations are replaced by divalent cations (Ca, Mg, Fe(2+), Mn, Sr, Ba, Pb, etc.) and the Cl anions are replaced by CO32- polyatomic trigonal planar ions.

    Think of the ions as being located on two face-centered cubic latices that interpenetrate one another. Minerals in the carbonate group differ do not have cubic symmetry because the ions on the two lattices differ significantly in size.

    When the size diference is very large (i.e., for the +2 cations Ca, Mg, Fe, and Mn) the symmetry of the lattice is decreased to trigonal. When the size difference is smaller (i.e., for the larger cations Sr, Ba, and Pb) the symmetry is higher (orthorhombic).




Consequently, carbonates can be split into trigonal carbonates that have uniaxial optical character and orthorhombic carbonates that have biaxial optical character.

The unit cells of the trigonal carbonates are not chosen based on the distorted face centered cubic lattice.

The diagram at right shows the trigonal unit cell for calcite. Basically, this structure consists of layers of carbonate anions alternating with layers of cations in octahedral coordination.

Trigonal - Uniaxial - Carbonates
Name Formula Epsilon Omega Birefringence
Calcite CaCO3 1.486 1.658 0.172
Magnesite MgCO3 1.509 1.700 0.191
Siderite FeCO3 1.635 1.87 0.240
Rhodochrosite MnCO3 1.597 51.816 0.219
Dolomite CaMg(CO3)2 1.500 1.679 0.179
Ankerite Ca(Mg,Fe,Mn)(CO3)2 1.51-1.55 1.69-1.75 0.182-0.202


Thermal Decomposition: Size Effects

How stable are these solids? Salts containing the carbonate anion decompose with loss of carbon dioxide. This is an endothermic reaction and produces metal oxide materials. The carbonates are more stable, in general, with larger cations.



Sinks for atmospheric CO2

Carbon dioxide is released into the atmosphere from volcanos, by the combustion of hydrocarbons (fossil fuels), and by the complete oxidation of organic compounds. The concentration of CO2 is slowly increasing (probably because of human activity). It would rise at a much higher rate if not for three carbon dioxide sinks.

  • ocean water
      Carbon dioxide dissolves in water and is in equilibrium with carbonic acid. Carbon dioxide concentrations in the ocean affect the stability of the biominerals in the reefs.
  • products of photosynthesis
      Plants convert water and carbon dioxide into oxygen and sugars. Most of the CO2 is released to the atmosphere when the plant decomposes but a small percentage is retained in the earth.
  • minerals
      Carbon dioxide reacts with basic oxides to form stable solids.


Reaction with Acids, Base

The carbonate minerals are all relatively insoluble in water. Those with small cations are more soluble than those with large cations.

All carbonate will react with acids to generate soluble bicarbonate salts. This is responsible for "acid rain" damage to limestone and marble.

Another way of looking at that same process is that a way to remove excess carbon dioxide from the atmosphere is by the dissolution of carbonates such as limestone and marble. The removal of carbon from the atmosphere and recycling in the ocean is part of the carbon cycle.


Minerals

For more on these and related minerals, go to Galleries.com

Magnesite, MgCO3

Commonly occurs as fine-grained to extremely fine-grained alteration product of Mg-rich rocks; in metamorphic rocks, may occur as disseminated grains or stratified layers in schists; in sedimentary rocks, may occur as evaporite deposits.

Siderite, FeCO3

Siderite is common as disseminated grains in sedimentary iron formations, metamorphic iron formations.

Rhodochrosite, MnCO3

Rhodochrosite commonly occurs with sulfide minerals, other carbonates in hydrothermal vein and replacement deposits.

Dolomite, CaMgCO3

Dolomite is a common sedimentary rock-forming mineral that can be found in massive beds several hundred feet thick. They are found all over the world and are quite common in sedimentary rock sequences. These rocks are called appropriately enough dolomite or dolomitic limestone. Dolomite at present time, does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. It appears that dolomite rock is one of the few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. They are originally deposited as calcite/aragonite rich limestones, but during a process call diagenesis the calcite and/or aragonite is altered to dolomite.