Minerals and rocks

State three physical properties used to identify minerals in a laboratory practical and describe how each one is tested.

Explain what the Mohs scale measures, how it is used in practice with a fingernail, a copper coin and a piece of glass, and which minerals occupy its two extremes.

What is the streak of a mineral and how is it obtained? Discuss why it is a more reliable property than colour for identifying many minerals (give the example of haematite or of pyrite).

Explain the difference between a mineral and a rock, and give an example of a rock made of several minerals and another of a rock made almost entirely of a single mineral.

Obsidian and pumice come from similar felsic magmas, yet one is glassy and the other spongy. Explain what cooling and degassing history the texture of each one tells and why pumice can even float.

During a survey in a volcanic-plutonic complex, three samples (A, B, and C) with identical rhyolitic/granitic chemical composition are analyzed:

• Sample A: millimeter-sized interconnected quartz, potassium feldspar, and biotite with no glassy or microcrystalline matrix.

• Sample B: homogeneous glassy mass and conchoidal fracture, showing no X-ray diffraction signals of crystal lattices.

• Sample C: angular feldspar phenocrysts embedded in a microscopic groundmass with small vesicles.

1. a) A preliminary report states that the textural differences are due to variations in the concentration of silica and water in the original magma. Refute this statement, justifying which is the actual physical variable responsible.
2. b) Reconstruct the cooling history (variation of the rate of heat loss over time) of each sample and deduce in which geological setting or emplacement (deep magma chamber, shallow dike, or surface lava flow) each one solidified.
3. c) Predict what texture a sample D will exhibit if it is collected right at the contact plane between the deep intrusion and the cold host rock.

Explain what a sediment is and through which diagenesis processes (compaction and cementation) it turns into a coherent sedimentary rock, using the example of sand and sandstone.

Distinguish between detrital, chemical and organogenic (biogenic) sedimentary rocks, give two examples of each kind (sandstone and lutite; gypsum and rock salt; limestone and coal) and indicate the environment in which each one forms.

The grain size of a detrital rock informs about the energy of the sedimentary environment. Discuss what a conglomerate of rounded pebbles, a fine-grained sandstone and a lutite indicate, and associate each rock with an environment (a mountain river bed, a beach or delta, the bottom of a calm lake).

Name the three main types of rock based on their origin and summarise in one sentence the process that forms each type.

Classify these rocks as igneous, sedimentary, or metamorphic, and justify each classification based on its origin or texture: granite, sandstone, marble, basalt, slate, and limestone.

Explain the difference between regional metamorphism and contact metamorphism, and order the series of increasing metamorphic grade slate → schist → gneiss, indicating what happens to the minerals and to the texture as intensity increases.