2
Weathering and Water-rock Reactions
• Relative mineral stability under weathering
Not to scale!
• Dissolution and Hydrolysis
• Formation of clays
• Cation exchange and adsorption
Understanding Earth
• Oxidation-Reduction
O id ti R d ti
• Carbonate deposition
• Post-depositional changes (diagenesis)
• Weathering and soils
Relative mineral stability during weathering
3
Brownlow’s Geochemistry
5
Weathering Products
Brownlow’s Geochemistry
Weathering Products: Relative Solubility
6
Brownlow’s Geochemistry
1
7
Mineral/Rock Solubility
Rock/Mineral Solubility: Halides and Sulfates
8
The relative solubilities of halides and sulfates can be inferred from their
sequence of precipitation from evaporating seawater.
Mineral solubilities may be affected by temperature, pH, Eh, and the
concentrations of other species in the solution
The first to precipitate are at the bottom of the table, similar to the
stratigraphic sequence sometimes found in evaporite deposits..
Except for carbonates, most minerals become more soluble at higher
temperatures.
The solubility of most minerals in pure water is very low
(e.g. silicates, oxides, sulfides)
Halides, sulfates, and carbonates are generally much more soluble.
Brownlow’s Geochemistry
Rock/Mineral Solubility: Carbonates
9
10
Rock/Mineral Solubility: Silica
Dissolution of quartz is via a hydration reaction:
SiO2(qtz) + 2 H2O ↔ H4SiO4(aq)
At low to neutral pH, the solubility is low
K = aH4SiO4 = 10-4 mol/kg at 25°C
~8 ppm
For amorphous silica, the solubility is much higher, ~115 ppm
Surface waters and ground waters typically have silica concentrations
between these two values
H4SiO4 is also a weak acid (silicic acid) and dissociates at higher pH
levels increasing the total solubility:
H4SiO4 ↔ H3SiO4- + H+
then
H3SiO4 ↔ H2SiO42- + H+
Brownlow’s Geochemistry
11
Silica Solubility
Brownlow’s Geochemistry
12
Silica solubility
White’s Geochemistry
2
Dissolution of other silicate minerals
13
14
Surface Leaching
To the limited extent that they do dissolve (via hydrolysis), many
silicates react to produce dissolved ions plus a solid residue composed
of new minerals. This is known as incongruent solution.
For example:
Al2Si2O5(OH)4(s) + 5 H2O ↔ 2 Al(OH)3(s) + 2 H4SiO4(aq)
kaolinite
gibbsite
KAlSi3O8(s) + H+(aq) + 7 H2O ↔ Al(OH)3(s) + K+(aq) + 3 H4SiO4(aq)
K-feldspar
gibbsite
The degree of completion of these reactions is also affected by pH.
pH further influences the form that Al will take: Al3+, Al(OH)3, Al(OH)4Water flow through rock or sediment is also a factor as it can remove the
soluble products. Water in general is highly important in silicate weathering.
15
Surface Leaching
Brownlow’s Geochemistry
Kinetics of silicate mineral dissolution
16
Dissolution rates for silicates are limited by the kinetics of the various
processes involved. The two possible limitations are the same as those
for crystallization .
• Transport-limited
- limited by the kinetics of diffusion through the leached layer
(Diffusion in the adjacent aqueous solution is sufficiently rapid to
be ignored here
here. For other materials
materials, this may be different.)
different )
• Reaction-limited
-limited by the kinetics of surface reactions (breaking of bonds and
formation of new minerals - e.g. hydrolysis)
Silicate weathering tends to be reaction-limited.
Brownlow’s Geochemistry
17
Clay formation
Note also that reaction rates (and the equilibrium conditions) are affected by
both temperature and pH. Biological processes are also important for
enhancing silicate weathering through the production of acids, for example.
Stability Fields of Clay Formation
18
Clays are a common product of weathering silicate rocks, particularly
those containing feldspars.
For example:
2 NaAlSi3O8(s) + 2 H2CO3(aq) + 9 H2O
albite
carbonic acid
↔
Al2Si2O5(OH)4(s) + 2 Na+(aq) + 4 H4SiO4(aq) + 2 HCO3-(aq)
kaolinite
silicic acid
Note that we are dealing with the mineralogical definition of clay here, not the
particle size definition. Clay minerals have sheet structures (broadly similar to
micas), and are typically identified using XRD.
Brownlow’s Geochemistry
3
(cat)Ion-Exchange
19
in Clays
(cat)Ion-Exchange
20
in Clays
Different types of clays have significantly different ion-exchange
properties. Smectite-type clays (e.g. montmorillonite) have much
greater capacity for ion-exchange than does the more common
kaolinite. Some smectites can gain or lose water as well, leading to
shrink-swell behavior.
A key property of clay minerals is their ability to exchange ions with
solutions. This exchange can involve ions attached to mineral
surfaces and ions that are part of the mineral structure.
Among other things, ion-exchange involving clays is important for
properties
p
and fertility.
y
soil p
White’s Geochemistry
21
Cation Exchange in Clays
22
Adsorption on Surfaces
Dissolved ions or molecules that become attached to the surface of a
solid are adsorbed. Since fine-grained sediments, including clay
minerals, have large amounts of surface area, their potential to
exchange material with a solution through absorption/desorption is
relatively high.
These reactions can significantly modify the composition of waters in
contact with sediment. Adsorption also plays a key role in the transport
rate of pollutants (both organic and inorganic) in soils, groundwater,
and surface water.
Water molecules may also become adsorbed to surfaces through the
attractive force of hydrogen bonding. Adsorption of water on mineral
surfaces is the first step in weathering by hydrolysis.
Brownlow’s Geochemistry
Rock/Mineral Weathering: Oxidation-Reduction
Oxidation-reduction reactions are common at/near the Earth’s surface
as it is the interface between the atmosphere (which contains free
oxygen) and the Earth’s interior (where free oxygen is absent). Thus,
there is a transition from more oxidizing to more reducing conditions
with depth.
As igneous and metamorphic rocks largely form under more reducing
conditions, rock weathering often includes significant oxidation.
23
Rock/Mineral Weathering: Oxidation-Reduction
24
Biological process are also important as photosynthesis produces both
free oxygen (an oxidizing agent) and organic matter (a strong reducing
agent during respiration and decomposition).
C, O, N, S, Fe, and Mn are key elements involved in oxidationreduction reactions under near
near-surface
surface conditions. All have more than
one oxidation state, and all four are sufficiently abundant to be
important. Cr, V, As, and Ce also undergo redox reactions, but these are
generally present at trace abundances.
Elements that react strongly with the above can also be affected by
redox conditions. For example, Cu and Ni abundances in solution drop
dramatically at low Eh since reduced S combines with Cu and Ni to
form solid sulfides.
4
25
Rock/Mineral Weathering: Oxidation-Reduction
26
Rock/Mineral Weathering: Oxidation-Reduction
Examples:
Examples:
• oxidation of organic carbon to form CO2 and H2CO4
• oxidation of pyrite and the production of acid mine drainage
• reduction of magnetite:
- relatively insoluble Fe3+ converted to more weakly bonded and
therefore more soluble Fe2+
- reaction may be driven by organic matter as the reducing agent
(1) initial oxidation (underground)
4FeS2(s) + 14O2(g) + 4H2O ↔ 4Fe2+(aq) + 8SO42-(aq) + 8H+(aq)
produces water with dissolved Fe and sulfate
(2) second oxidation (often upon exposure of water to air)
4Fe2+(aq) + O2(g) + 4H+(aq) ---> 4Fe3+(aq) + 2H2O
eliminates some of the acid from the first step
(2) third reaction (hydrolysis)
4Fe3+(aq) + 12 H2O ---> 4Fe(OH)3(s) + 12H+(aq)
produces solid Fe-hydroxide and more acidity
27
Oxidation-Reduction
28
Redox Effects
on Metals
White’s Geochemistry
Recap/Summary
Brownlow’s Geochemistry
30
Marine Carbonate Deposition
Most seawater is close to carbonate saturation.
Deposition of carbonates from seawater is strongly dependent upon
temperature, PTotal, PCO2, and biological activity.
Decreasing temperature, increasing PTotal, and greater PCO2, all increase
carbonate solubility. These changes all occur with increasing depth,
thus solubility increases with depth.
Supersaturation and carbonate deposition thus require relatively shallow
water. Much of the carbonate precipitated in this environment results
from biological activity.
5
31
Processes that occur in sediment after deposition. Includes reactions
between pore water and sediment.
32
Diagenesis
Diagenesis
The distributions here
may be explained by
oxidation-reduction
during diagenesis – i.e.
Mn reduction, diffusion,
and reprecipitation as
MnCO3.
Examples:
redox reactions
- may vary with depth
- may be influenced or controlled by microbial activity
(e.g. oxidation of organic matter coupled with
reduction of sulfate)
mineral dissolution
mineral precipitation (e.g. the cement)
dolomitization of carbonates
White’s Geochemistry
33
Diagenesis
34
Dolomitization
The changes in sulfate
and alkalinity may be
explained by bacterial
reduction of sulfate.
Variations in Ca2+ may be
explained by carbonate
precipitation/ dissolution.
Brownlow’s Geochemistry
Brownlow’s Geochemistry
35
Weathering and Soil
Soil formation is a product of multiple
weathering processes.
Soils may be divided into horizons based
on their composition. O represents
organic matter. A-C represent the
mineral soil. R represents minimally
weathered bedrock.
Due to leaching processes, the A horizon
is enriched in relatively insoluble
weathering products. The A horizon also
contains organic matter.
The B horizon has lower organic
contents and higher clay contents than
the A horizon above. It also accumulates
material leached from the A horizon.
White’s Geochemistry
6
37
Weathering and Soil
White’s Geochemistry
7