Clay Minerals and Properties Questions and Answers

Answer: A

Tetrahedral silica sheets and octahedral alumina sheets stack in characteristic ways, and it is the layered habit that gives clay its plate-like particles. Feldspar and quartz are framework structures and behave quite differently. Slip rheology depends on both charges acting together.

Answer: B

The alternating single pair of sheets is held to its neighbours by hydrogen bonding, which is strong enough to prevent water entering between the layers. This is why kaolinite does not swell in the way the two-to-one minerals do.

Answer: C

Weak bonding between the stacked units allows water and cations to enter the interlayer space. This is the source of the very large swelling that makes bentonite so useful and so awkward in turn. A few per cent of bentonite transforms a body's workability.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the structural cause. Montmorillonite is a 2:1 mineral - an octahedral sheet between two tetrahedral ones - and the interlayer is weakly held, so water and hydrated cations enter it and force the layers apart. Kaolinite is 1:1 with hydrogen bonding directly between successive layers, which holds them together and leaves no interlayer to swell.

Answer: A

Correct answer: False

Only the smectites swell substantially, montmorillonite being the important case. Kaolinite does not, because its layers are hydrogen bonded together, and illite does not, because its interlayer potassium locks the layers. That difference in swelling is what separates a clay used as a body from one used in a few per cent as a plasticiser or a drilling additive.

Answer: B

Substitution within the lattice leaves a net negative charge that is balanced by cations held at the surface and in the interlayer. Those cations can be exchanged, which is what deflocculation of a casting slip actually exploits.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. Deflocculation works by making the particles repel one another: sodium ions replace the divalent calcium on the exchange sites, and because a single charge holds the double layer less tightly than a double one, the layer expands and the particles are kept apart. The slip then flows at a much lower water content, which is what casting requires.

Answer: D

Dividing 100 by 5 gives twenty. The difference reflects the much larger accessible surface of an expanding lattice compared with a mineral whose interlayer is closed. Exchange capacity is measured routinely on clays used industrially.

Answer: A

Aluminium replacing silicon, or magnesium replacing aluminium, leaves a charge deficit that is permanent and independent of pH. Edge charges by contrast do vary with pH and behave quite differently. Slip behaviour depends on both kinds of charge acting together.

Answer: B

Faces carry a permanent negative charge from lattice substitution while edges carry a pH-dependent charge that can be positive in acid conditions. Edge-to-face attraction between the two is what produces a flocculated card-house structure.