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:
aK  a Al 3 aH3 4 SiO4
K
aH4 
• from Gibbs free energies of formation. The IAP could
then be determined from a water analysis, and the
saturation index calculated.
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.
6
Quartz
7
Amorphous
silica
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.
Muscovite
log (aK+/aH+)
5
K-feldspar
4
3
Gibbsite
Kaolinite
2
1
Pyrophyllite
0
-6
-5
-4
-3
log aH SiO 0
4
4
-2
-1
Aluminum
2
0
Gibbsite
Al
+++
, T = 25 C , P = 1. 013 bars , a [ H 2O] = 1
-4
-
-6
Al(OH)4
-8
++
+++
+
AlOH
Al(OH)2
Diagram A l
log a Al
+++
-2
25oC
-10
2
3
4
5
6
7
pH
8
9
10
11
12
Greg Mon Nov 01 2004
14
12
-
Fe(OH)4
pH
8
, T = 25 °C , P = 1.013 bars, a [H 2O] = 1; Suppressed: FeO(c), Hematite, Goethite
10
Fe(OH)3 (ppd)
Fe(OH)3
6
+
4
Fe(OH)2
++
FeOH
2
++++
Fe2 (OH)2
0
–10
–8
–6
Diagram Fe
+++
+++
Fe
25°C
–4
+++
log a Fe
–2
0
Lab user Fri Oct 02 2009
Linking nutrient cycling and redox
chemistry in fresh water lakes
P cycling and sediment microbes
PO43- PO43-
Iron reducers
FeOOH
PO43-
Fe2+
H2S
PO43-
Sulfate Reducers
FeS2
PO43-
PO43- PO43Blue Green Algae blooms?
N cycling and sediment microbes
NO3-
Ammonifying
bacteria
NH4+
Solid State Analysis
• X-Ray Fluorescence Spectroscopy (XRF)
• X-ray Diffraction (XRD)
Extractions
• Using selected acids, bases, organics, chelators
to dissolve a subset of minerals
• Many possibilities for this
• We will look at 2 used to investigate Fe, Mn, P:
– Aqua Regia – Combination of Nitric Acid and
Hydrochloric acid, heated to just below boiling –
dissolved most stuff (but not all!!)
– Ascorbic Acid extraction – Combination of
ascorbic acid and bicarbonate
FeOOH/MnOOH in sediments
• Ascorbic acid extraction
technique specific to
‘reactive’ Fe and Mn
(Kostka and Luther, 1994)
• XRD data confirms reactive
stuff is ferrihydrite
• Size is nanloparticulate,
sensitive to degree of
recycling!
Where is the Phosphorus?
P mobility measured
through hundreds of
sediment digests for
samples in Missisquoi
Bay collected at different
times, places, and depths
Only strong and
consistent correlation is
with reactive Fe, present
predominantly as
nanoparticulate FeOOH
particles
2008 Sediment Extraction Correlation Statistics
Ascorbic Acid (reactive)
R2
p-value
n
Aqua Regia (total)
R2
p-value
n
P vs. Fe
0.8952
6.4 x 10-100
202
0.1940
3.5 x 10-10
185
P vs. Mn
0.6561
3.1 x 10-48
202
0.3026
5.0 x 10-16
185
P vs. Ca
0.6895
2.7 x 10-27
184
0.1709
5.0 x 10-9
185
P vs. Al
0.4653
1.5 x 10-26
184
0.6883
3.4 x 10-48
185
2008 Reactive Fe vs P
2007 Reactive Fe & Mn vs Reactive P
0.700
1.2
Fe vs P
0.500
P vs. Mn
0.400
0.300
y = 0.0779x + 0.0928
R² = 0.8862
0.200
0.100
y = 0.4726x + 0.2519
R² = 0.5681
0.000
0.000
y = 0.0626x + 0.1195
R² = 0.8952
1
mg P/g dry sed.
0.600
0.8
0.6
0.4
0.2
0
1.000
2.000
3.000
4.000
5.000
6.000
7.000
0
5
10
Fe mg/g dry sed.
15
20
2007 Total Fe vs Total P
2008 Total Fe and Mn vs P
2.50
y = 0.6168x + 0.6817
R² = 0.6537
y = 0.04x - 0.4575
R² = 0.5946
2.00
mg P/g dry sed
mg P/g dry sed.
2.00
2.50
1.50
y = 0.5057x + 0.6956
R² = 0.6325
1.00
0.50
1.50
1.00
T Fe vs.
P
TMn vs.
P
y = 0.026x + 0.081
R² = 0.909
0.50
p < 0.001
0.00
0.00
20.00
40.00
mg Mn or Fe/ g dry sed.
60.00
0.00
0.00
20.00
40.00
60.00
80.00
mg Mn or Fe/g dry sed
100.00