Lecture 24 sedimentary weathering

Sedimentary Materials
• Sedimentary rocks cover 80% of the earth’s
surface but only comprise ~1% of the volume
of the crust (they are generally NOT dense
either!)


• Once we weather the
source material, the
material is transported,
deposited, compacted, and
lithified, and maybe
changed by reaction with
groundwater (called
diagenesis)


Transport
• All weathered products can be transported
• Dissolved ions are transported until they
get to a final destination (such as the
ocean) and/ or are precipitated
• Physically weathered minerals/ rock
fragments  How are they transported?
– Water, wind, glaciers, gravity

• What processes are more selective to the
size of the particle


Types of sedimentary rocks

• Detrital (a.k.a. clastic)  form by compaction and
lithification of clastic sediments or lithic fragments
– Clasts are little grains or fragments of rocks (i.e. can be
made of 1 or more minerals)
– Classification based on size

• Chemical  form by precipitation of minerals from
water, or by alteration of pre-existing material
– Classification based on chemical composition

• Biogenic  formed of previously living organic
debris
• HOWEVER  Many sedimentary rocks are
combinations of 2-3 of these types… WHY?


Weathering
• Looking at the rock cycle,
key to forming sedimentary
rocks is weathering (or
erosion) of pre-existing
rocks (or organisms…)
• Types of weathering:
– Physical (a.k.a. mechanical)
– Chemical


Physical Weathering
•
•

•
•

Joints and sheeting development in rocks
Frost wedging, salt wedging, biologic wedging
Thermal stress
Abrasion – through water, wind, glaciers,
gravity, waves


Exfoliation or unloading
• Some rocks expand to to pressure release,
uplift, heating/ cooling, etc. and break off in
sheets


Chemical Weathering
• How do we dissolve stuff?
– Ions dissolve into water based on properties
of that ion and how easily the mineral
‘releases’ it into the water
– What properties do you think make the ions in
a mineral dissolve more easily?

SiO2
olivine

Fe2+
Mg2+

SiO2


Chemical Weathering Vocabulary
• Hydrolysate – dissolved material
• Resistate – solid material left behind (did’t
dissolve)
– More easily dissolved elements include alkali
and alkaline earths (Na+, Ca2+, K+)

• Residual – product of hydrolysis reactions
left behind (it can be physically weathered
too…)


Mineral Dissolution
• Write a reaction:
– Mg0.5Fe0.5SiO4 + H2O  0.5 Mg2+ + 0.5 Fe2+ + SiO44-

• Describe that reaction as an equilibrium
expression which defines how much of the
mineral can dissolve in a particular fluid
– What aspects of fluid composition do you think
might affect how much of a mineral can
dissolve?
– Keq=[products] / [reactants]
– Keq=[Mg2+][Fe2+][SiO44-] / [olivine][H2O]


Aqueous Species

• Dissolved ions can then be transported
and eventually precipitate
• Minerals which precipitate from solution
are rarely the same minerals the ions
dissolved out of
• Why would they be transported before
precipitating?
SiO2
K+

feldspar
Na+

SiO2

smectite


Chemical Weathering II - hydrolysis
• Some minerals ‘weather’ directly to other
minerals
• Mineral dissolves and immediately
reprecipitates a new mineral at the surface
of the original
– Feldspars  Clays
– Fe-bearing silicates to iron oxyhydroxides
olivine
olivine

FeOOHs

Acid/base reactions
• Many minerals are affected by the pH of
the solution they are in
– some form H+ or OH- when they dissolve
– Some dissolve much faster/ better in low or
high pH solutions

• Calcite weathering
– CaCO3 + H+ + H2O  H2CO3(g) + CaOH+

• Acid/ base chemistry important in mineral
dissolution and precipitation!!


Oxidation
• Recall that elements exist as different ions in
a particular oxidation state
• Changing that oxidation state can have a big
effect on how well that element will dissolve
and what minerals will form after it dissolves
• Oxidation (where a reduced ion loses an
electron to an oxidant) is important in the
weathering of many minerals at the surface
of the earth where O2 is the oxidant
• Fe(II)2SiO4 + ½ O2 + H2O  2 Fe(III)OOH + SiO2


Chemical Weathering

• Recap: How do minerals dissolve?
– Dissolution reactions
• Ions dissolve in water, do not change

– Acid-base reactions
• Ions dissolve in water through interaction with H+
or OH-

– Redox reactions
• Ions dissolve/ precipitate affected by interaction of
ions in mineral or in water with O 2


Chemical Weathering and Stability
• All minerals are described by a ‘stability’
• Thermodynamics defines this through an
energy  all energies are relative
• Energy changes depending on the conditions
 i.e. some minerals are more stable than
others at high P and T; change the P and T
conditions and different minerals are more
stable
• In weathering environments,
minerals that are weathering
are not stable, minerals
precipitating ARE stable


• Examples of graphical representations of
mineral stability derived from thermodynamic

calculations


Resistance to weathering
• Goldrich series  empirical observation
concerning what minerals weather before
others…
olivine

Ca-plagioclase

pyroxene
amphibole

Na-plagioclase

biotite
K-feldspar
quartz

Remind you of anything??


What happens when granite is
weathered??
• First, unweathered granite contains these minerals:
–
–
–
–

Na Plagioclase feldspar
K feldspar
Quartz
Lesser amounts of biotite, amphibole, or muscovite

• What happens when granite is weathered?
• The feldspars will undergo hydrolysis to form kaolinite
(clay) and Na and K ions
• The Na+ and K+ ions will be removed through leaching
• The biotite and/or amphibole will undergo hydrolysis
to form clay, and oxidation to form iron oxides.


Granite weathering, continued
• The quartz (and muscovite, if present) will remain as
residual minerals because they are very resistant to
weathering.
• Weathered rock is called saprolite.
• What happens after this?
– Quartz grains may be eroded, becoming sediment. The quartz in
granite is sand- sized; it becomes quartz sand. The quartz sand will
ultimately be transported to the sea (bed load), where it accumulates
to form beaches.
– Clays will ultimately be eroded and washed out to sea. Clay is finegrained and remains suspended in the water column (suspended
load); it may be deposited in quiet water.
– Dissolved ions will be transported by rivers to the sea (dissolved
load), and will become part of the salts in the sea.