**3. Clay minerals**

### **3.1 Chemical composition & crystallographic structure of clay minerals**

Clay minerals are a product of rock weathering, and form from decomposition of feldspar minerals in hard rocks such as granite. They are commonly

*Review of Geochemical and Geo-Mechanical Impact of Clay-Fluid Interactions Relevant… DOI: http://dx.doi.org/10.5772/intechopen.98881*

described as soil particles with sizes below 2 μm, often labelled as nature's nanoparticles. In terms of chemical composition, clays belong to a group of minerals called alumino-silicates. The alumino-silicates are composed of complex arrangement of atoms to form diverse structural configurations with the basic components being silicon, aluminium and oxygen. Silicon and aluminium atoms bond with oxygen to form silicon tetrahedral sheets and aluminium octahedral sheets respectively. These sheets are subsequently bonded by sharing common oxygen atoms, though the oxygen atoms at the edges of both sheets are left unpaired. These unpaired oxygen atoms at the edges of the sheets impose negative charges on clay mineral surfaces rendering them water sensitive and highly reactive to cations [31, 32].

Another factor contributing to high negative charges in clay minerals is the isomorphic cationic substitution within the sandwiched tetrahedral and octahedral sheets, which leads to imposition of excess negative charges on clay mineral surfaces [33]. The mechanism described above contributes to higher levels of clay sensitivity to water-based engineered fluids in subsurface.
