2.3 - Types of Weathering - Chemical

Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions. These reactions include oxidation,  hydrolysis, and carbonation. These processes either form or destroy minerals, thus altering the nature of the rock’s mineral composition. Temperature and, especially, moisture are critical for chemical weathering; chemical weathering of rock minerals generally occurs more quickly in hot, humid climatic regions.

Oxidation is the reaction of rock minerals with oxygen, thus changing the mineral composition of the rock. When minerals in rock oxidize, they become less resistant to weathering. Iron, a commonly known mineral, becomes red or rust colored when oxidized.

Figure 5. The iron in olivine (Fe₂SiO₄) is reduced and the iron in limonite (Fe₂O₃.H₂O)  is oxidized. In addition, the release of silicon and hydration makes the mineral more susceptible to physical weathering.  (Image on the left courtesy of USGS. Image on the right courtesy of http://www.csmate.colostate.edu).

Carbonation is the process of rock minerals reacting with carbonic acid. Carbonic acid is formed when water combines with carbon dioxide. Carbonic acid dissolves or breaks down minerals in the rock.

CO₂ + H₂O  →  H₂CO₃

(carbon dioxide + water  →  carbonic acid)

CaCO₃ + H₂CO₃  →  Ca²⁺ + 2HCO_3-

(calcite + carbonic acid  →  calcium + bicarbonate)  

Hydrolysis is a chemical reaction caused by water. Water changes the chemical composition and size of minerals in rock, making them less resistant to weathering. Click on the video clip below to see  hydrolysis of a relatively weathering resistant mineral, feldspar. When this mineral is completely hydrolyzed, clay minerals and quartz are produced and such elements as K, Ca, or Na are released.

A hydrolysis reaction of orthoclase (alkali feldspar), a common mineral found in igneous rock, yields kaolinite, silicic acid, and potassium.

2KAISi₃O₈ + 2H⁺ + 9H₂0  →  H₄Al₂Si₂O₉ + 4H₄SiO₄ + 2K⁺

(orthoclase + water  → kaolinite + silicic acid + potassium)

• A small amount of ground calcium feldspar is placed in the mortar.
• Five drops of distilled H₂O are added (chemical weathering)
• A few drops of cresol red indicator areadded (yellow at pH < 7.2 and pink at pH = 7.8) 
• Color is yellow orange
• The feldspar mixture is ground into a finer powder
• Color is now purplish red

*This animation has no audio.*

Hydration is the absorption of water into the mineral structure.  A good example of hydration is the absorption of water by anhydrite, resulting in the formation of gypsum. Hydration expands volume and also results in rock deformation.

Figure 6. The images above show anhydrite (CaSO₄), which can convert to gypsum (CaSO₄.2H₂O) through hydration. (Image on the left is anhydrite, courtesy of http://csm.jmu.edu/minerals/Anhydrite.html  Image on the right is gypsum, courtesy of http://www.nps.gov/brca/Geodetect/Photo%20book/RM%20pix/pages/Gypsum_jpg.htm)

• A small amount of limonite, Fe₂O₃.H₂O, is placed into a test tube.
• The tube is held with a test-tube holder just above the blue flame of a lighted Bunsen burner.  
• The sample is cooled.  
• Note:  The color change of limonite from yellow to brick red; (Reaction Fe₂O₃).

Dehydration is the removal of water from rock or mineral structures.  A good example of dehydration is the removal of water from limonite, resulting in the formation of hematite. 

Dehydration of limonite to hematite may be observed in the following experiential activity.

Figure 7. The images above show the dehydration reaction of limonite (Fe₂O₃.H₂O) on the left to hematite (Fe₂O₃) on the right. The water, which was a structural component of limonite, has been removed in the process of dehydration. (Image of limonite and hematite, courtesy of http://www.csmate.colostate.edu and http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Slideshow/Slideindex.html, respectively.)