Mass Action & Mass Balance

• mCa2+=mCa2++MCaCl+ + mCaCl20 + CaCL3- +

CaHCO3+ + CaCO3

0 + CaF+ + CaSO40 +

CaHSO4+ + CaOH+ +…

• Final equation to solve the problem sees the mass action for each complex substituted into the mass balance equation

lc

nc

i HLC

HCL

][][

][][

nxLmCamCa 22

Mineral dissolution/precipitation

• To determine whether or not a water is saturated with an aluminosilicate such as K-feldspar, we could write a dissolution reaction such as:

• KAlSi3O8 + 4H+ + 4H2O K+ + Al3+ + 3H4SiO40

• We could then determine the equilibrium constant:

• from Gibbs free energies of formation. The IAP could then be determined from a water analysis, and the saturation index calculated.

4

3

443

H

SiOHAlK

a

aaaK

INCONGRUENT DISSOLUTION

• Aluminosilicate minerals usually dissolve incongruently, e.g.,

2KAlSi3O8 + 2H+ + 9H2O

Al2Si2O5(OH)4 + 2K+ + 4H4SiO40

• As a result of these factors, relations among solutions and aluminosilicate minerals are often depicted graphically on a type of mineral stability diagram called an activity diagram.

ACTIVITY DIAGRAMS: THE K2O-Al2O3-SiO2-H2O SYSTEM

We will now calculate an activity diagram for the following phases: gibbsite {Al(OH)3}, kaolinite {Al2Si2O5(OH)4}, pyrophyllite {Al2Si4O10(OH)2}, muscovite {KAl3Si3O10(OH)2}, and K-feldspar {KAlSi3O8}.

The axes will be a K+/a H+ vs. a H4SiO40.

The diagram is divided up into fields where only one of the above phases is stable, separated by straight line boundaries.

log aH4SiO4

0

-6 -5 -4 -3 -2 -1

log

(aK

+/a

H+)

0

1

2

3

4

5

6

7

KaoliniteGibbsite

Muscovite

K-feldspar

Pyrophyllite

Qua

rtz

Am

orph

ous

silic

a

Activity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica.

2 3 4 5 6 7 8 9 10 11 12-10

-8

-6

-4

-2

0

2

pH

log

a A

l++

+

Al+++

Al(OH)2

+

Al(OH)4-

AlOH++

Gibbsite

25oC

Greg Mon Nov 01 2004

Dia

gram

Al+

++,

T =

25

C,

P

= 1

.013

bar

s, a

[H

2O

] =

1

Seeing this, what are the reactions these lines represent?

Redox Geochemistry

WHY?• Redox gradients drive life processes!

– The transfer of electrons between oxidants and reactants is harnessed as the battery, the source of metabolic energy for organisms

• Metal mobility redox state of metals and ligands that may complex them is the critical factor in the solubility of many metals– Contaminant transport– Ore deposit formation