Exercises: Minerals and rocks

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.

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

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

Name three very common silicate minerals in the rocks of the crust (quartz, feldspar, micas, olivine...) and one diagnostic property of each (hardness, cleavage, colour).

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.

Distinguish between the cleavage and the fracture of a mineral, and give an example of a mineral that cleaves perfectly (a mica) and another that fractures in a conchoidal way (quartz).

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 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.

Arrange these processes and materials from the rock cycle into a cycle, explaining each transition: sedimentary rock, magma, metamorphic rock, igneous rock, sediments.

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.

Observe the three numbered rock samples in the image. a) Describe the texture visible in each one (grain size, presence of strata or mineral bands). b) Relate each texture to the process that originated that type of rock. c) In which of the three would fossil remains be most likely to be found? Reason your answer.

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.

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.

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.

Limestone and marble have the same chemical composition (calcium carbonate) but a different origin. Explain the transformation of one into the other, how they are distinguished with the naked eye, and what happens when a few drops of hydrochloric acid are dripped onto both (and what that test demonstrates).

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).

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.

The layout of a highway is planned to dismantle a slope where an inverted stratigraphic sequence with dinosaur tracks and continuous fossil ripples outcrops. The construction company requests to cover the slope in concrete, claiming that "the photographs already taken are sufficient as a record and the rock cut lacks practical utility."

1. a) Assess the uniqueness and fragility of this outcrop by applying the assessment criteria for Geosites (scientific, educational, and sociocultural value).
2. b) Refute with geological and methodological arguments the construction company's claim that photographic documentation replaces the in situ conservation of the outcrop.
3. c) Design an integrative technical proposal that ensures the geotechnical stability of the highway cutting while simultaneously allowing the preservation of the stratum and its use for safe educational itineraries.

You are given two anonymous consolidated rock samples from two Iberian mountain ranges:

• Sample 1: reacts with effervescence to cold $10\,\%$ $\text{HCl}$ and exhibits dissolution microconduits.

• Sample 2: silicic plutonic rock with quartz, feldspar, and mica, crossed by a network of orthogonal microjoints.

1. a) Justify how mineralogy and structural discontinuity condition the genesis of a karst landscape (as in El Torcal) versus a boulder field with corestones and tors (as in La Pedriza).
2. b) If both areas were to experience a high-mountain periglacial climate with continuous freeze-thaw cycles, predict and compare which mechanical weathering process will prevail in each and what morphological modifications the initial landforms will undergo.
3. c) Explain why rainwater loaded with $\text{CO}_2$ congruently dissolves rock 1, whereas in rock 2 it triggers incongruent chemical alteration that mechanically breaks down the matrix.

A granitic pluton solidified at a depth of $6\ \text{km}$ $60\ \text{Ma}$ ago. Today it outcrops at the surface and its sandy sediments are deposited in an alluvial plain $200\ \text{km}$ from the massif.

1. a) If the average surface erosion rate of the sedimentary cover wedge has been $0{,}04\ \text{mm/year}$, calculate whether denudation alone explains why the rock outcrops today. Justify what vertical geodynamic process (isostatic adjustment) must have been coupled with erosion to allow its exhumation.
2. b) A student claims that the quartz and orthoclase grains will reach the alluvial plain in the same quantitative proportion and with the same degree of roundness that they had in the parent granite. Refute their hypothesis by analyzing the physicochemical stability and mechanical behavior of both minerals during fluvial transport.
3. c) Predict what textural and mineralogical transformations these sands will undergo if the basin experiences subsidence and they become buried under $5\ \text{km}$ of sediments with a geothermal gradient of $30\ \text{°C/km}$.

In an orogenic sector, two rock samples derived from the same pelitic protolith (marine mudstone) are collected:

• Sample A: very fine-grained, perfect slaty cleavage, rich in chlorite, muscovite, and quartz.

• Sample B: clear gneissic banding with quartz, potassium feldspar, biotite, garnet, and sillimanite, with small leucocratic aggregates segregated by incipient partial melting.

1. a) Interpret the relative thermobarometric conditions (metamorphic grade and facies) under which each sample formed and deduce its approximate crustal depth in the root of the orogen.
2. b) In a third sample from the same unit, relict kyanite crystals are observed to be surrounded and replaced by aggregates of sillimanite. Interpret the pressure-temperature (P-T) path experienced by this massif during the collision and deduce whether it reflects crustal burial or tectonic/thermal exhumation.
3. c) Reason in detail which physicochemical mechanisms at the atomic scale and rock fabric scale are responsible for the complete disappearance in sample B of any fossil present in the original mudstone.

In a sedimentary basin, open-pit mining of two contiguous formations is planned: a thick layer of massive gypsum underlying a bioclastic limestone formation. Beneath the gypsum lies a regional confined freshwater aquifer intended for urban water supply.

1. a) Based on the solubility product and the salinity required for chemical precipitation, justify why gypsum requires more extreme evaporation conditions and more restricted environments than marine limestones.
2. b) Analyze the hydrogeological and geotechnical risk if the gypsum quarry deepens to the point of breaching or depressurizing the lower confined aquifer: predict what changes will occur in the chemical quality of the water and what karst collapse risks may be triggered.
3. c) Design a plan of preventive measures and environmental monitoring during quarrying that controls saline seepage and prevents the reactivation of sinkholes due to accelerated dissolution.

Compare the lithological dynamics of the Earth with those of a rocky planetary body such as the Moon, whose mass is much smaller, lacks an atmosphere and hydrosphere, and has dissipated most of its primary internal heat.

1. a) Reasonably deduce which stages of the terrestrial rock cycle are unfeasible on the Moon and which ones can take place, indicating in each case the driving energy source.
2. b) A classmate makes the following statement: "The terrestrial rock cycle is a thermodynamically closed system where all mass is conserved within the crust and energy is recycled indefinitely without losses." Refute this statement by arguing matter exchanges with the mantle/space and by applying the second law of thermodynamics to energy flow.
3. c) Justify the differential role exerted by Earth's gravity in the two subsystems of the cycle: as a driving force for surface sedimentary transport and as the engine of buoyancy and gravitational instability in plate tectonics.

The Ronda peridotite massif (Málaga) constitutes one of the world's largest surface outcrops of subcontinental mantle, formed by dense ultramafic rocks (approximate density of $3{,}3\ \text{g/cm}^3$) in mechanical contact over less dense continental crust ($2{,}7\ \text{g/cm}^3$).

1. a) Justify the apparent gravitational paradox of finding high-density mantle rocks resting on continental crust and deduce which tectonic regime and regional kinematic mechanism (obduction/crustal thrusting) made this emplacement possible.
2. b) Upon exposure to surface fluids, the olivine in the peridotites undergoes serpentinization. Predict the mineralogical, mechanical, and geochemical consequences of this hydrothermal process on the strength of the rock massif and on plant colonization (serpentine soils poor in nutrients and rich in heavy metals).
3. c) A company applies to open an aggregate gravel pit in the massif. Write three expert geological arguments justifying its priority designation as an international non-renewable Geosite (LIG) and the absolute incompatibility of such mining activity.