Soil Science

Revision

 
 

How much would you expect illite to swell when wetted??

Correct! Illite has shrink/swell properties intermediate between those of kaolinite and montmorillonite. The interlayer potassium ions hold the crystals together tightly, minimising expansion. However, between some layers and particularly near the edges, potassium ions tend to be displaced by hydrated exchangeable cations, leading to a structure more similar to smectites. and allowing some swelling to occur.

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The thickness of silicate clay crystals depends on how many layers are bonded together. In soils, which clay mineral if any would you expect to for the smallest crystals??

Correct! Montmorillorite has the smallest crystals. The van der Waals forces which hold the layers together are too weak to allow large crystals to form. By contrast, in kaolinite the layers are held together by stronger hydrogen bonds. In illite crystals, interlayer potassium ions serve the same purpose. Kaolinite and illite crystals contain more layers and are therefore larger.

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Which, if any, of the silicate clay minerals would you expect to be most cohesive, plastic and sticky when wet??

Correct! Montmorillorite. Cohesion, plasticity and stickiness increase as the specific surface area of the particles increases. This is why clay makes soil cohesive, but sand and silt do not. Montmorillonite has higher specific surface area than kaolinite and illite because of its small crystal size, and because swelling of the crystals opens up internal surfaces between the crystal layers.

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Which of the following primary minerals would be most likely to survive in strongly weathered soils??

Correct! Quartz is the least weatherable of the common primary minerals.

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Which of the following is false??

Correct! The formula of gibbsite is not FeOOH, it iis A1(OH)3.

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The iron oxide minerals hematite and goethite form under different environmental conditions Which one of the following would most favour hematite formation in preference to goethite??

Correct! Hematite is preferentially formed in tropical soils because of the warm climate.

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Which clay mineral is likely to be a slow-release nutrient source of potassium in soil??

Correct! Illite, because it contains potassium ions between the crystal layers. Although they are not rapidly exchanged with other cations, they may be slowly released if the potassium concentration in the surrounding soil solution falls sufficiently. It is probably for this reason that potassium deficiencies are not common in illite rich agricultural soils in southem Australia

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