|theImage.com     Notes on Basic Geology|
|Notes created & information organization based on the book:
"The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at www.wiley.com for additional resource information
|Weathering - Page 4|
Many water solutions can act as weak acids. Carbon dioxide in water produces carbonic acid, which is one of the weakest. The chemical decomposition of rock is carried out by the slow replacement of one atom for another, or by the very slow dissolution of portions of a mineral and conversion to another.
( H2O + CO2 --> H+1 + HCO3 -1 )
KAlSi3O8 + H+1 + H2O ---> K+1 + Al3Si4O10(OH)8 + SiO2
This converts the potassium feldspar (orthoclase) to the clay kaolinite. The resultant clay is very insoluble and can resist further hydrolysis. It also has far less strength that the original feldspar and is more susceptible to physical weathering.
The water can leach a mineral like marchasite (FeS2) and remove part of the sulphur, producing one or more of the iron oxides.
For instance: Fe+2 ----> Fe+3 + e-1 (electron)
In this case the iron is oxidized. The iron can come from pyroxenes, amphiboles, or micas. When it is hydrolyzed out of the structure it can then combine with oxygen and through the oxidation process it is converted to an iron hydroxide. Dehydration (loss of water) produces the mineral goethite (FeO (OH)). Goethite can be further dehydrated to hematite (Fe2O3).