Rock Forming Minerals Questions and Answers

Answer: A

Correct answer: strongly magnetic

Magnetite is the only common mineral strongly attracted to a magnet, and a specimen will deflect a compass needle. Haematite and limonite are the other common iron oxides and are at most weakly magnetic, so the test separates the three quickly - with the streak, red-brown for haematite and black for magnetite, as confirmation.

Answer: B

Correct answer: red-brown, whatever the specimen's apparent colour

Haematite occurs as a bright red earthy mass and as a grey metallic crystal, and the two look nothing alike; both give the same red-brown streak. This is why streak is more reliable than colour: it is the colour of the powdered mineral, free of the effects of crystal size, surface films and impurities.

Answer: C

Correct answer: softer, and not effervescent in acid

Gypsum at 2 can be scratched by a fingernail, while calcite at 3 cannot; and gypsum is a sulphate, so it does not effervesce in acid as calcite does. Either test alone distinguishes them, and both are quick to make in the field.

Answer: D

Kaolinite's two-layer sheets are held together by hydrogen bonds that water cannot easily enter, while montmorillonite's three-layer sheets admit water between them. That difference decides how a clay soil behaves under a foundation.

Answer: A

Correct answer: between its structural layers

Montmorillonite has weakly bonded layers, and water molecules and cations enter the space between them, forcing them apart. The volume change can be several times, and it is reversible on drying - which is why an expansive soil heaves in the wet season and shrinks and cracks in the dry.

Answer: B

Correct answer: potassium ions between the sheets

Illite has the same three-layer structure as montmorillonite but with potassium ions occupying the interlayer space and binding the sheets together. Water cannot readily displace them, so illite swells much less than montmorillonite while remaining more active than kaolinite, whose layers are bonded by hydrogen bonds.

Answer: C

Correct answer: montmorillonite

Black cotton soil is a montmorillonite clay derived from the weathering of basalt, and its heave in the wet season and shrinkage in the dry are the direct consequence of water entering and leaving the interlayer spaces. Foundations on it must either reach below the zone of seasonal moisture change or be designed to tolerate the movement.

Answer: D

Correct answer: hard and chemically resistant

Quartz is hard at 7 on the Mohs scale, has no cleavage to provide a plane of weakness, and is chemically inert to almost everything encountered in service. Those three properties together are why quartz sand and quartzite make durable aggregate and why quartz is the mineral that survives in a sediment after everything else has weathered away.

Answer: A

Correct answer: provides planes of weakness and a poor bond

Mica cleaves into thin flakes along its basal plane, so each flake is a plane of weakness in the aggregate and splits under load. The flat faces also bond poorly to the cement paste and increase the water demand of the mix. A few per cent is tolerable; more is a reason to reject the source.

Answer: B

Isolated tetrahedra and single chains held by cations offer far easier attack than a continuous framework, and the ferrous iron oxidises readily. A basalt therefore decays much faster than a granite in the same climate.