Molecular Sandwich of Black Cotton Soil

The expansion potential of Black cotton soil is the combined influence of clay particle type and its quantity in the soil. Clay particles which cause a soil to be expansive are extremely small. Their shape is determined by the arrangement of their constituent atoms which form thin clay crystals.

The principal elements in clay are silicone, aluminum and oxygen. Silicone atoms are positioned in the center of a pyramid structure called a tetrahedron with one oxygen atom occupying each of the four corners. Aluminum atoms are situated in the center of an octahedron with an oxygen atom occupying each of the eight corners.

Octahedral sheet is sandwiched between two tetrahedral sheets to create the mineral structure of Black Cotton soil

Because of electron sharing, the silicon tetrahedrons link together with one another to form thin tetrahedral sheets. The aluminum octahedrons also link together to form octahedral sheets. The actual clay crystals are a composite of aluminum and silicon sheets which are held together by intra-molecular forces.

Silicon Tetrahedral sheets are formed sharing electron in Black Cotton Soil

There are many other elements which can become incorporated into the clay mineral structure Black cotton soil such as hydrogen, sodium, calcium, magnesium, sulfur, etc. The presence and abundance of various dissolved elements or “ions” can impact the composition and behavior of the clay minerals.

Aluminum octahedral sheets are formed sharing electron in Black Cotton Soil

One octahedral sheet is sandwiched between two tetrahedral sheets to create the mineral structure. In Black cotton soil, groupings of the constituent clay crystals will attract and hold water molecules between their crystalline sheets in a sort of “molecular sandwich”.