Soil Science

Clay Crystal Structures

Tetrahedrons and octahedrons

To understand the properties of the silicate clay minerals, you need to comprehend their crystal structures. The fundamental structural units are tetrahedrons and octahedrons.

Tetrahedron - Space filling view

The tetrahedrons usually have an O²⁻ anion at each vertex and a small Si⁴⁺ cation in the center.

Tetrahedron - Polyhedral view

This polyhedral view is a more stylized representation of an Si tetrahedron.

Octahedron - Space filling view

The octahedrons in soil clay minerals most commonly have an Al³⁺ cation at the center and have O²⁻ or OH⁻ anions at the vertices.

Octahedron - Polyhedral view

This is a polyhedral view of an Al octahedron.

Sheet structures

Within the crystals of silicate clay minerals, tetrahedrons are linked into tetrahedral sheets, and octahedrons are linked into octahedral sheets.

Tetrahedral sheet - Tetrahedral sheets are formed by tetrahedrons joining at their vertices (i.e., sharing oxygen anions).

Octahedral sheet - Octahedral sheets are formed by octahedrons joining at edges.

Layer structures

In silicate clay mineral crystals, tetrahedral and octahedral sheets are strongly bonded together to form layers.

1:1 layer structure - In a 1:1 layer structure, one tetrahedral sheet is joined to one octahedral sheet by shared O²⁻ anions.

2.1 layer structure - In a 2:1 layer structure, an octahedral sheet is sandwiched between two tetrahedral sheets.

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