Today`s lecture

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Today's lecture
• More Aqueous Geochemistry
• First a look back at calcite solubility as a
function of pH and Temperature
SOLUBILITY OF CALCITE IN AN OPEN
SYSTEM
a
3
H

KK

p
1 CO2 CO2
 K
2
2 Kcal HCO 
2 Ca 2 
3
M
3
Ca 2 

K1K cal KCO2 pCO2
2
4 K 2 Ca 2   HCO

3
2
The pH of pure water in equilibrium with calcite at 25°C as a function of the partial pressure
of CO2. Note that pH decreases linearly with increasing CO2 partial pressure.
8.5
pH
8.0
7.5
7.0
6.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
log pCO
2
3
Changes to Ocean pH as a function of PCO2
Why do we care about mineral precipitation in CO2
sequestration?
Solubility Product
• CaCO3(s) <==> Ca2+(aq) + CO3-2(aq)
• In writing the equilibrium constant expression for a heterogeneous
equilibria, we ignore the concentrations of pure liquids and solids. So
the equilibrium constant expression for the equilibria above is:
• Ksp = [Ca2+][CO3-2]
• This equilibrium constant is called a solubility-product constant. Even
though [CaCO3] is excluded from the equilibrium constant expression,
some undissolved CaCO3(s) must be present in order for the system to
be at equilibrium. In general, the solubility product constant (Ksp) is the
equilibrium constant for the equilibrium that exists between a solid ionic
solute and its ions in a saturated aqueous solution. The rules for writing
the solubility-product expression are the same as those for writing any
other equilibrium constant expression: The solubility product is equal to
the product of the concentrations of the ions involved in the equilibrium,
each raised to the power of its coefficient in the equilibrium equation.
The Common-Ion Effect
•
•
•
•
The solubility of a substance is affected not only by temperature but also by the
presence of other solutes. We now consider the case in which the other solute
has an ion in common with the substance involved in the equilibrium, in other
words we are going to look at the common-ion effect in solubility. Just as in the
case of the common-ion effect in acid-base equilibria, the common-ion effect in
solubility is just a special case of LeChattelier's principle. Consider the
equilibrium of CaCO3(s) with its saturated solution, if we add a solute containing
Ca2+ or a solute containing CO3-2 to the saturated solution, the equilibrium will
shift to the left making CaCO3 less soluble.
CaCO3(s) <==> Ca2+(aq) + CO3-2(aq)
<----Addition of Ca2+ or CO3-2 shifts equilibrium left decreasing solubility--------<<
• Example: What if a solution undersaturated with respect to calcite passes
through an evaporite bed with anhydrite (Ca SO4). Anhydrite will dissolve
putting CA++ and SO4– into solution. The sulfate will not affect the
solubility of calcite but the added Calcium might raise the Ca++
concentration enough to exceed the Solubility Product of Calcite.
Carbonates ( such as calcite) are one of the mineral groups that may form to
sequester CO2. But remember that in the last class we say that adding CO2 to
the aqueous solution caused a pH decrease. How does that affect the
precipitation of calcite?
http://www.ldeo.columbia.edu/~sanpisa/Ocea
nSed%20project/class%20project/caco3phsol.J
PG
Hydrolysis reactions neutralize acid
These are very common reactions in weathering and hydrothermal alteration
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3HCO3- = H+ + CO3-2
Feldspar compositional variation
• Or - Orthoclase KAlSi3O8
• Ab - Albite NaAlSi3O8
• An - Anorthite CaAl2Si2O8
Feldspars exhibit two solid solutions
One from Or to Ab and the other from
Ab to An.
Labradorite: An50 to An70
Feldspars are the most common mineral in the earths crust !!
Solid Solution
Solid solution is not seen just in Feldspar.
One other example is Carbonate which is important for this class
Feldspar compositional variation
• Or - Orthoclase KAlSi3O8
• Ab - Albite NaAlSi3O8
• An - Anorthite CaAl2Si2O8
Feldspars exhibit two solid solutions
One from Or to Ab and the other from
Ab to An.
Labradorite: An50 to An70
Feldspars are the most common mineral in the earths crust !!
Where do we find Feldspar?
• Feldspar rich sandstone (Arkose)-- Reservoir
• Shales—Caprock
• Basalts
I. Gaus et al. / Chemical Geology 217 (2005) 319–337
I. Gaus et al. / Chemical Geology 217 (2005) 319–337
If you have kinetic rate constants you can calculate how long
these reactions will take.
• Feldspar is not the only mineral you need to
be concerned with in a sandstone reservoir.
• The amount of initial carbonate (probably as
cement) is also important
Effects of Carbonate buffers
Siliciclastic vs carbonate buffered
reservoirs.
Ca-rich plagioclase feldspar is an essential mineral in basalt.
In felsic rocks both plagioclase and
Alkali feldspars are important
Reaction of Ca-rich feldspar to calcite
Volcanic glass may be important too. It is easily reacted.
Another important mineral in basalt is OLIVINE
Olivine reactions
Mineral Carbonation for Sequestration
http://www.esc.ethz.ch/research/efficientconv
ersion/cotwo/mineralization.jpg

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