In layer silicates of clay minerals, there are a lot of isomorphic substitutions in 2:1 minerals, both in the tetrahedral and octahedral layers (e.g., montmorillonite, hectorite, nontronite, etc.); yet there are very few such substitutions in 1:1 minerals (there is no substitution in kaolinite or halloysite). Any reasonable explanations are appreciated.
Have a look at the link http://www.swac.umn.edu/classes/soil2125/doc/s11ch1.htm
Take not of the terms non-expansible (1;1 minerals, e.g. kaolinite) and non-expansible (2:1, e.g. smectite group minerals) and possible substitution sites. This will explain
The isomorphous substitution originates during the formation of minerals, for example during the magma solidification. According to the thermodynamic conditions of the magma, minerals are formed so as to reach the lowest thermodynamic potential. Thus, it is possible that there isomorphic substitutions, where aluminum can coordinate with four oxygens rather than six, comprising a mixed tetrahedron blade silicon aluminum. The same occurs for the Mg in the octahedral blade of Al and Mg. The residual charge is compensated by a counter ion in the structure (cations K, Na, Ca, Mg ...).
However, when this mineral that was formed at high temperatures and pressures comes under natural conditions (low pressure and temperature) the mineral will be out of thermodynamic equilibrium, and Al is not so stable with cordenação 4 but in coordination with 6 oxygens. Thus, the points where there is isomorphic substitution, are the most unstable points in structure and are the points of weakness of the mineral. Those points where atc the weathering with greater intensity, releasing these chemicals to the system. Thus it is expected that 2: 1 minerals also have large amounts of isomorphic susbtituições, but in mineral 1: 1, the local.
In layer silicates 1:1 ,Mg is substituted by Ni and Fe in lizardite in laterite profiles
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