15-01-2013, 01:53 PM
ADVANCED SOIL MECHANICS
Origin of Clay Minerals
“The contact of rocks and water produces clays, either at or near the surface of the earth” (from Velde, 1995).
Rock +Water Clay
For example,
The CO2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H+ and bicarbonate ions, and make water slightly acidic.
CO2+H2O H2CO3 H+ +HCO3-
The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite.
Feldspar + hydrogen ions+water clay (kaolinite) + cations, dissolved silica
2KAlSi3O8+2H+ +H2O Al2Si2O5(OH)4 + 2K+ +4SiO2
Note that the hydrogen ion displaces the cations.
STRUCTURAL UNITS OF SILICATES LAYER
IN MOST CLAY MINERAL STRUCTURES, THE SILICA TETRAHEDRA ASSOCIATE IN A SHEET STRUCTURE.
THREE OF THE FOUR OXYGENS OF EACH TETRAHEDRON ARE SHARED TO FORM A HEXAGONAL NET.
THE BASES OF THE TETRAHEDRA ARE ALL IN THE SAME PLANE& THE TIPS ALL POINT IN THE SAME DIRECTION .
OCTAHEDRAL SHEET
The sheet structure is composed of Mg and Al coordinated octahederally with oxygen.
If the octahederally coordinated cation is divalent then all possible cations sites are normally filled and the structure is tri-octahedral.
If the cation is trivalent then only 2-3 rd the possible cationic spaces are normally filled and the structure is termed as di octahedral.
In clay mineral structures a sheet of magnesium octahedral is termed as bruited sheets.
Differential Thermal Analysis (DTA)
Differential thermal analysis (DTA) consists of simultaneously heating a test sample and a thermally inert substance at constant rate (usually about 10 ºC/min) to over 1000 ºC and continuously measuring differences in temperature and the inert material T.
Endothermic (take up heat) or exothermic (liberate heat) reactions can take place at different heating temperatures. The mineral types can be characterized based on those signatures shown in the left figure.
(from Mitchell, 1993)