Rock Forming Minerals Questions and Answers

Answer: C

Correct answer: isolated and linked only through cations

In a nesosilicate the tetrahedra share no oxygens with one another and are held together by iron and magnesium ions between them. Olivine therefore has no plane of markedly weaker bonding and shows poor cleavage, and its structure is the most open to attack, which is why it is the first mineral to weather in a basic rock.

Answer: D

Correct answer: single chains of tetrahedra

Pyroxenes are single-chain silicates, each tetrahedron sharing two oxygens with its neighbours along the chain. The chains are bonded to one another more weakly than within themselves, which gives the two cleavages at close to ninety degrees that distinguish augite from hornblende in hand specimen.

Answer: A

Correct answer: cleavage angles of about 56 and 124 degrees

Both are dark ferromagnesian minerals of similar colour and hardness, and the cleavage angle is what separates them. Hornblende is a double-chain amphibole, and the wider chain gives cleavages meeting at about 56 and 124 degrees; augite, a single-chain pyroxene, cleaves at close to ninety.

Answer: B

Correct answer: all four of their oxygens

In a framework silicate every oxygen is shared between two tetrahedra, so each tetrahedron shares all four. Quartz is the pure case; in the feldspars some silicon is replaced by aluminium, and the resulting charge deficiency is balanced by potassium, sodium or calcium sitting in the cavities of the framework.

Answer: C

Correct answer: potassium rather than sodium and calcium

Orthoclase is the potassium feldspar; the plagioclases form a continuous series from sodium-rich albite to calcium-rich anorthite. All are framework silicates of the same essential structure, and the difference lies entirely in which cation balances the aluminium substitution.

Answer: D

Correct answer: half of it

Feldspar accounts for something close to sixty per cent of the crust, more than any other group, and it is the dominant mineral of the granites, the gabbros and most of the sediments derived from them. It is also the mineral whose weathering supplies most of the world’s clay.

Answer: A

Correct answer: colourless or pale, and free of iron and magnesium

Both micas share the sheet structure and the perfect basal cleavage; the difference is chemical. Muscovite is the potassium-aluminium mica, pale or colourless, while biotite carries iron and magnesium and is dark brown or black. Being a ferromagnesian mineral, biotite also weathers more readily.

Answer: B

Its isolated tetrahedra are bonded through cations in a compact structure with no plane of weakness, giving a hardness of 7 or more. Its toughness and hardness are why it is used as an abrasive and in waterjet cutting.

Answer: C

Correct answer: rhombohedral cleavage and effervescence in acid

Two tests settle it at once. Calcite cleaves into rhombohedra, a shape quartz never shows, and it effervesces in dilute acid, which quartz does not. Hardness confirms it: calcite at 3 is scratched by a knife, and quartz at 7 scratches glass.

Answer: D

Correct answer: only slowly, or when powdered or warmed

Dolomite is a carbonate and does react, but the magnesium in its structure makes it far less soluble in dilute acid than calcite, so a solid piece shows little or nothing while a powdered one effervesces plainly. The contrast is the standard field test for distinguishing dolomite from limestone.