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Isomorphous substitution; 1:1 vs 2:1

Short answer

Isomorphous substitution is replacement of a central atom in silica tetrahedron or alumina octahedron by a similar-size lower-valence atom, creating a net negative charge. 1:1 minerals have one silica and one gibbsite sheet, strong hydrogen bonding, little substitution, low swelling; 2:1 minerals have gibbsite between two silica sheets, weak or K+ bonding, more substitution, higher swelling and specific surface.

What is isomorphous substitution? Compare between 1:1 and 2:1 minerals.

Isomorphous substitution

Isomorphous substitution is the replacement of a central atom in the silica tetrahedron or alumina octahedron by another atom of nearly the same size but lower valence, without any change in the crystal structure. Examples: Al3+^{3+} replacing Si4+^{4+} in a silica sheet, and Mg2+^{2+} replacing Al3+^{3+} in a gibbsite sheet. This leaves the layer with a net negative charge, which attracts cations and water (the origin of the double diffuse layer and cation exchange).

1:1 versus 2:1 minerals

Basis1:1 minerals (kaolinite)2:1 minerals (illite, montmorillonite)
LayerOne silica + one gibbsite sheetOne gibbsite between two silica sheets
ThicknessAbout 7.2 ÅAbout 10 Å
Bond between layersStrong hydrogen bondWeak (van der Waals) or K+^+ ions
SubstitutionAlmost noneConsiderable
Water entryNot possiblePossible in montmorillonite
SwellingLowMedium (illite) to high (montmorillonite)
Specific surfaceAbout 15 m²/g80 - 800 m²/g
Plasticity and activityLowMedium to high

✓ Answer checked by Claude Opus 5.5 (high effort)

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