Bentonite Mineralogy and Chemistry
Montmorillonite has a crystal structure. Essentially the crystal consists of a number of sheet-like plated or crystallites stacked tightly one on top of the other.
Due to replacement of Alumina by other ions in the central part of the sheet, the plate becomes electrically unbalanced and other cations are absorbed onto their surfaces to neutralize these negative charges.
The cations are relatively loosely held and may be readily exchanged for other cations.
Cations of calcium Ca2+, magnesium Mg2+, sodium Na+ , K+, ammonia NH4+, and organic compounds are commonly found in these exchangeable sites. Sodium Na+ ions have the greatest ability to solvate and, in fresh water; distances between particles are on average 200-300 Angstrom.
At this point, edge charges on the clay platelets become important. These charges are the result of broken bonds at the edges and are positive in nature, but are of much lower magnitude than the surface charges.
We thus have the situation where the surfaces of the crystallites are negatively charged while the edges are positively charged. Anions, which are negatively charged particles, are attracted to edge sites.
A chemical presentation of the structure of Montmorillonite can be seen with the theoretical formula: (OH)4 Al4 Si8 O20 .xH2O Depending on the substitution of Al with other minerals such as Fe, Mg, Li, different members of the Montmorillonite group occur.
They can be classified as follows (Ross and Hendricks, 1945): Dioctahedral Montmorillonite Montmorillonite – (OH)4 Si8 (Al3,34 Mg 0,66) O20 Na0,66 Nontronite – (OH)4 (Si7,34 Al0,66) Fe4+ O20 Na0,66 Trictahedral Montmorillonite Hectorite – (OH)4 (Si8 Mg 5,33 Li0,66) O20 Na0,66 Nontronite – (OH)4 (Si7,34 Al0,66) Mg6 O20 Na0,66 In the hydroxyl groups the hydrogen (H) has an acidic character and can be replaced by cations. This gives Bentonite a high ion-exchange capacity and buffer properties.