Soil Colloids

Kaolinite

1:1 clay, low charge, typically highly weathered, non expanding clay because of hydrogen bonds, no isomorphous substitution, low surface area

Smectites

2:1 clays, large negative charges; isomorphous substitution in octahedral layer, expanding clay due to weak bonds between sheets, high surface area, aqueous environment

Fine Micas

2:1 clay, low negative charge; moderate isomorphous substitution in tetrahedral layer, Non expanding clay from K⁺, moderate surface area, in wet and acidic soils

Vermiculite

2:1 clay, large negative charge; isomorphous substitution on octahedral or tetrahedral layers, expanding clay, high surface area, wet alkaline to neutral environments

Chlorite

2:1:1 clay, moderate negative charge, moderate isomorphous substitution, nonexpanding clay due to extra Mg in octahedral layer, moderate surface area, low to medium grade metamorphic environment

Geothite

Fe-oxide clay, little to no negative charge but can get positive at low pHs, non expanding, moderate surface area, yellow/red colors in soils

Gibbsite

Al-oxide clay, little to no negative charge but can develop positive charge at low pHs, non expanding, moderate surface area, in highly weathered soil

Humus

Organic, very negative charge which is all pH dependent, isomorphous substitution doesn’t apply, non expanding, surface area difficult to measure, dark color