Rock Forming Minerals Questions and Answers

Answer: C

Correct answer: sulphuric acid and sulphate

Pyrite is iron sulphide, and on exposure to air and moisture it oxidises to iron oxide and sulphuric acid, the acid then reacting with the cement to form sulphates. The products occupy more volume than the pyrite did, so the aggregate expands and stains, and the sulphate attacks the paste as well.

Answer: D

Circulating water dissolves it along joints and bedding to produce karst caverns and sinkholes, which can leak a reservoir and undermine a structure. Grouting and cut-offs are the usual response once such features are found.

Answer: A

Hydrolysis of feldspar is the main source of the clay in soils, so a granite decays to a sandy clay with quartz grains in a clay matrix. The strength and swelling of that residual soil follow from which clay mineral forms.

Answer: B

Correct answer: soluble, and its dissolution leaves voids

Gypsum dissolves readily in circulating groundwater, so a foundation on gypsiferous rock may lose material over time and develop cavities beneath it. The rate depends on the flow, which is why the hazard is greatest where the water table fluctuates or where construction alters the drainage.

Answer: C

Correct answer: fluorite at 4 but not quartz at 7

The Mohs scale is a scratch order, so a mineral scratches anything below it and is scratched by anything above. At 5 it is harder than fluorite at 4 and softer than quartz at 7, so it scratches the first and not the second. The scale gives the order only and says nothing about how much harder.

Answer: D

Correct answer: 2.6

Specific gravity is the mass divided by the mass of an equal volume of water, and 30 cm³ of water has a mass of 30 g: \[ \frac{78}{30} = 2.6. \] The figure is characteristic of quartz and the feldspars, so a specimen at 2.6 in an ordinary rock is most likely one of them.

Answer: A

Correct answer: pyrite or magnetite

Quartz and feldspar sit at about 2.6 to 2.7, and the micas little higher, so a specific gravity of 5.0 is far outside the range of the rock-forming silicates and points to a metallic mineral. Pyrite at about 5.0 and magnetite at about 5.2 are the two common candidates, and a magnet separates them at once.

Answer: B

It is the feldspar that hydrolyses to clay, while the quartz survives as sand and the biotite contributes both clay and iron oxide. The residual soil is therefore a clay with about a third of its mass as quartz grains.

Answer: C

Fifteen per cent of volume distributed equally in three directions gives about five per cent linear strain, and five per cent of two metres is a hundred millimetres. Such a heave is quite enough to crack a light structure.

Answer: D

Correct answer: oblique, giving a rhombohedron

Calcite’s three cleavages meet at oblique angles, so a broken fragment is a rhombohedron - a cube leaned over. The shape is diagnostic and unmistakable once seen, and together with effervescence in acid it identifies calcite without any further test.