Chemical Weathering Lecture

Cochise College
Geology Home Page

Roger Weller, geology instructor 
last edited: 12/16/15

soil created by breakdown of basalt- Maui, Hawaii

Chemical Weathering of Igneous Minerals

     On Earth, the principle chemical agents responsible for the chemical decomposition
of silicate minerals in igneous rocks are water (H2O), carbon dioxide (CO2), and
oxygen (O2).  Dominant silicate minerals are olivine, pyroxene, amphibole, biotite
mica, muscovite mica, plagioclase feldspar, and potassium feldspar.  These are the
same minerals described in Bowen’s Magmatic Reaction Series.  The exception is the
mineral quartz which is chemically resistant to these chemicals at surface temperatures
and pressures.  Basic or mafic igneous rocks, such as basalt and gabbro, consist
primarily of calcium-rich plagioclase and pyroxene with minor amounts of olivine and
magnetite.  At the other end of the spectrum the felsic or silica-rich igneous rocks, such
as granite and rhyolite, contain sodium-rich plagioclase feldspar, potassium feldspar,
muscovite mica and quartz.

     The most important ingredient in all of the following reactions is liquid water.
Carbon dioxide and oxygen on their own will not do anything without the presence of

     Temperature is also very important.  The higher the temperature, the faster the
chemical reaction.  Remove heat and the chemical reaction slows down or even stops.

     Consequently, the fastest chemical weathering takes place in warm, humid areas
like the tropics.  Negligible chemical weathering is found in cold, dry areas like deserts
in the Antarctic.

Weathering Products

     The most common minerals produced by chemical weathering are usually designated
as Secondary Sedimentary Minerals.  Of these minerals, many are water soluble and are
deposited by the evaporation of minerals in solution; these minerals are referred to as
Evaporites.  Common evaporates are halite (salt) gypsum, calcite, dolomite, sylvite,
borates, and a large collection of other soluble salts.  Others, such as clay, hematite,
and goethite are not soluble.

Calcite -  CaCO3
Dolomite-  (Ca, Mg)CO3
Clay Minerals -kaolinite, smectite, halloysite, montmorillonite, illite,etc.
Hematite - Fe2O3
Goethite – 2FeO.OH (often referred to as limonite)
Gypsum - CaSO4.2H2O
Halite – NaCl
Sylvite- KCl
Quartz- SiO2


Weathering of Specific Minerals


olivine in basalt, San Carlos Indian Reservation, Arizona

Olivine reacts with carbonic acid (carbon dioxide and water)
     Mg2SiO4  + 4 H2CO3 -------------2Mg+2 + 4HCO3- +H4SiO4
    The silicic acid breaks down into water and silica.

Olivine can also undergo oxidation to magnetite and silica.
     6FeSiO3 + O2  ------ 2Fe3O4 + 6SiO2

magnetite nodule, Huachuca Mountains, Arizona

magnetite crystals in basalt, Maui, Hawaii

Magnetite can be oxidized to hematite (red iron oxide)
     4Fe3O4 + O2 ----6Fe2O3

Hematite process a brownish-red streak

magnetite oxidized to hematite, Hawaii

Hematite can then hydrated to produce goethite, a yellow iron oxide.
     Fe2O3 + H2O ---- 2FeO.OH

goethite producing yellow streak

Plagioclase Feldspar

The Ca-Na plagioclase plagioclase feldspar reacts with CO2 and H2O.
The following reaction is the one that used the smallest amount of water.
     4CaNaAl3SiO5O16 + 8CO2 + 14H2O------
              4CaCO3 +4Na+1 + 4HCO3-1 + 8SiO2 + 6Al2Si2O5(OH)
CaCO3 is calcite and Al2Si2O5(OH) is kaolinite (some attribute more
2H2O in kaolinite instead of (OH).

feldspar crystal in igneous rock, altering to clay,
10x magnification



Potassium Feldspar

orthoclase, potassium feldspar

Orthoclase or microcline can be decomposed by hydrolysis, yielding kaolin
and silicic acid..
     4KAlSi3O8 + 4H+1+ + 2H2O ------ 4K+1 + Al4Si4O10(OH) + 8SiO2
The H+1 ions are provided by carbonic acid.


pyrite in quartz

Pyrite is a trace mineral in many igneous rocks.  Chemical destruction of pyrite yields
sulfuric acid and iron oxides, such as hematite and goethite. 
     4FeS2 + 14O2 + 4H2O ------4Fe+2 + 8SO4-2 + 8H+1
Sulfuric acid (H2SO4) can also be produced by volcanic eruptions of sulfur dioxide
that chemically combines with water.  Sulfuric acid can then attack calcite to produce
  CaCO3 + H2SO4 + 2H2O  -----CaSO4.2H2O + H2CO3
or, sulfate ions in solution can combine with the calcium released by the breakdown of
plagioclase  and pyroxene to produce gypsum
     SO4-2 + Ca+2 + 2H20 ----- CaSO4.2H2O

layered gypsum

Pyroxene, Amphiboles, Biotite, and Muscovite

augite, a pyroxene
CaMgSi2O6 to (Mg,Fe)(Al,Fe)2SiO6

hornblende, an amphibole
(Ca,Na) 2(Mg,Fe,Al) 5(Al,Si) 8O 22 (OH)2

biotite mica

muscovite mica

Weathering of pyroxenes, amphiboles, biotite, and muscovite produces clays, silicic
acid (which converts to silica), and many cat-ions which are released into solution
(Ca+1, Na+1, K+1. Mg+2, Fe+2).  Calcium and magnesium cat-ions are free to combine
with carbonic acid to produce dolomite.
     Ca+2 + Mg+2 + H2CO3 ----- (Ca, Mg) CO3 + H2O


Sodium ions in solution can react with chlorine ions to produce halite (salt).
     Na+1 + Cl-1 ----- NaCl


Potassium ions in solution can combine to produce sylvite.
     K+1 + Cl-1 --------KCl


Smectite can also form from the presence of iron, magnesium, calcium, sodium,
alumina, and silica all in solution.
     (Ca, 2Na) (Al, Mg, Fe)8(Si, Al)16040(OH)8.nH20
Many other salts can also form during the evaporation of these ion-rich solutions.
A few are listed below.

     Hanksite-   9Na2SO4.2Na2CO3.KCl
     Sulphohalite-     2NaSO4.NaCl.NaF
     Carnallite-     KMgCl3.6H2O
     Epsom’s salt-   MgSO4 7H2O
     Kiesert-   MgSO4 *  H2O),
     Sanderite- MgSO4 . 2H2O
     Starkeyite- MgSO4 . 4H2O
     Pentahydrite- MgSO4 . 5H2O
     Hexahydrite- MgSO4 . 6H2O
     Heptahydrate-  MgSO4. 7H2O
     Meridianiite- MgSO4. 11H2O
     Mmirabilite- Na2SO4.10H2O
     Szomolnokite- FeSO4.H2O

Other Possible Secondary Minerals

If the decomposing igneous contains significant amounts of boron or fluorine, many
other secondary minerals can be created.