Rock Forming Minerals Questions and Answers

Answer: A

Correct answer: quartz

Window glass is about 5.5 and corundum is 9, so the mineral lies between them, and quartz at 7 is the common mineral in that gap. It is also the standard field test: a mineral that scratches glass is at least about 5.5, and among ordinary rock-forming minerals that usually means quartz.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Three minerals with three unmistakable properties. Calcite effervesces in dilute acid, being a carbonate. Magnetite is strongly attracted to a magnet, alone among the common minerals. And muscovite splits into thin elastic sheets along its perfect basal cleavage, a consequence of its layered structure.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

The three represent three points on the silicate scale of polymerisation. Olivine has isolated tetrahedra sharing no oxygens, augite single chains sharing two, and quartz a continuous framework sharing all four. Sharing more oxygens means a stronger, more resistant mineral, which is why quartz survives weathering and olivine does not.

Answer: D

Correct answer: 2, 3, 4, 1

The Mohs scale places gypsum at 2, calcite at 3, fluorite at 4 and quartz at 7, so the order is simply that. Four of the ten standard minerals appear here, and the gap between fluorite and quartz is where the useful field tests - the knife and the glass, both about 5.5 - fall.

Answer: A

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: streak is the colour of the powder and is free of the effects that make apparent colour unreliable, and montmorillonite swells far more than kaolinite because water enters between its layers. Statement 3 is wrong - quartz has no cleavage at all, its framework structure offering no plane of weakness.

Answer: B

Correct answer: gypsum

Gypsum is a sulphate, not a carbonate, so there is no carbon dioxide to be released and no effervescence. The other three are all calcium carbonate in one form or another - chalk, calcite and aragonite - and all react briskly with cold dilute acid.

Answer: C

Correct answer: orthoclase feldspar

Ferromagnesian means containing iron and magnesium, and augite, hornblende and olivine all do, which is why all three are dark and all three weather relatively readily. Orthoclase is the potassium feldspar and contains neither, which is why it is pale and why granite looks as it does.

Answer: D

Correct answer: generate acid and sulphate and expand

Pyrite oxidises in air and moisture to iron oxide and sulphuric acid, and the acid then forms sulphates with the cement. The reaction products take up more space than the pyrite, so the aggregate expands and cracks the concrete, and the sulphate attacks the paste independently. Rust staining is the visible early sign.

Answer: A

Correct answer: montmorillonite rather than kaolinite

Which clay a feldspar weathers to depends on the drainage: free leaching removes the bases and gives kaolinite, while poor drainage retains them and gives montmorillonite. Only montmorillonite takes water between its layers and swells, so an expansive residual soil over granite indicates that the weathering took the second course.

Answer: B

Tarnish, weathering films and the coarseness of the crystal all alter apparent colour, and none of them survives being ground to powder. Haematite, grey in the mass and red-brown in streak, is the standard illustration.