Identifying minerals

What is the streak of a mineral and how does it differ from crystal colour? Why is streak a more reliable property for identifying a mineral?

Distinguish between metallic and non-metallic lustre, and name three types of non-metallic lustre with an example mineral of each (vitreous, greasy, silky, adamantine).

Below are descriptions of five numbered mineral samples:

1. Crystals with a cubic habit, brass-yellow color, and metallic luster.

2. Hexagonal prismatic crystals, colorless, and with a vitreous luster.

3. Transparent and flexible sheet-like aggregate, cleavable into very thin sheets.

4. Crystals with perfect rhombohedral cleavage and vitreous luster.

5. Lead-gray mass, metallic luster, and cubic cleavage.

Indicate which number corresponds to pyrite and which to muscovite mica, and state the visible physical property that allowed you to identify each one.

Quartz can be colourless, white, pink, violet or smoky depending on its impurities. Explain why colour is an unreliable criterion for identifying minerals and which three properties you would combine to identify any sample with confidence.

A laboratory receives three small golden metallic particles from an alluvial placer. The prospector claims that they are all native gold nuggets (theoretical density $\approx 19,3\text{ g/cm}^3$). Hydrostatic density measurements and mechanical tests under a binocular loupe are performed:

• Grain 1: density $5,0\text{ g/cm}^3$. When pressed lightly with the tip of a steel awl, it shatters into splinters and generates a greenish-blackish brown powder.

• Grain 2: density $4,2\text{ g/cm}^3$. It is easily scratched with the tip of a copper pin and leaves a greenish-black streak.

• Grain 3: density $18,9\text{ g/cm}^3$. When pressed with the steel awl, the tip penetrates leaving a permanent indentation without fracturing or shedding powder.

1. a) Identify with reasons which mineral corresponds to each grain (among pyrite, chalcopyrite, and native gold), linking the result to the mechanical properties of tenacity (brittleness vs. malleability) and hardness.

1. b) Explain the chemical basis for why native gold preserves its unaltered lustre in river sediments for millennia, while golden sulphides end up transforming into limonite or alteration patinas.

Order these minerals from the Mohs scale in increasing order of hardness: quartz, talc, corundum, gypsum, and fluorite. In addition, state which two everyday objects are used as hardness references in the field.

An unknown sample scratches gypsum and calcite, but is scratched by fluorite. Narrow down its hardness on the Mohs scale and propose a mineral compatible with this result.

A sample scratches glass but does not scratch quartz, and quartz scratches it. Constrain its hardness on the Mohs scale and propose two minerals compatible with that interval.

A student measures the hardness of a prismatic crystal of kyanite using microindentation tests and controlled scratching with a Mohs kit. They obtain the following verified data:

• In the direction parallel to the prism's longest dimension (crystallographic c-axis), a common steel pocketknife (hardness 5,5) produces a clear continuous groove (estimated hardness: 4,5 to 5).

• In the transverse direction (perpendicular to the elongation of the prism), the pocketknife produces no damage and the surface can only be scratched using a quartz point (estimated hardness: 6,5 to 7).

1. a) Interpret this behavior at the crystal lattice scale: justify why the mechanical anisotropy of kyanite is so pronounced compared to isotropic minerals such as garnet, relating the different hardnesses to the strength and orientation of the chemical bonds in its structure.

1. b) Predict how this mineral will respond regarding its cleavage when a rock containing it is subjected to differential tectonic stresses, and describe a simple experimental design to check whether another physical property (electrical conductivity or light propagation) also exhibits anisotropy in the same crystal.

What is cleavage and how does it differ from fracture? Give an example of a mineral with perfect cleavage.

What is the relative density of a mineral? Describe a simple method to estimate it in the laboratory with a balance and a measuring cylinder of water.

What is the tenacity of a mineral? Define brittle, sectile, malleable and elastic behaviour, with an example mineral for each.

A galena sample has a mass of $66\ \text{g}$ and displaces $8,7\ \text{cm}^3$ of water. Calculate its density and explain why high density is a good criterion to distinguish it from the similar-looking pyrite.

Mica cleaves in one direction, feldspar in two and calcite in three. Explain what relationship the number of cleavage directions has with the crystal lattice and with the angles between the breaking surfaces.

In an orogenic sector, three metapelite samples have been collected with different mineral assemblages of the $\text{Al}_2\text{SiO}_5$ system:

• Sample A: andalusite prisms whose edges are replaced by fibrous aggregates of sillimanite.

• Sample B: deformed kyanite porphyroblasts in a muscovite and quartz matrix, with no other polymorph.

• Sample C: coexistence in apparent textural equilibrium of andalusite, kyanite, and sillimanite in a contact aureole at about 500 °C and 0,38 GPa.

1. a) Interpret the P-T (pressure-temperature) path of sample A: justify whether it reflects prograde metamorphism or a thermal decompression process upon crossing the univariant curve of the phase diagram.

1. b) Apply Gibbs' phase rule ($F = C - P + 2$, setting pressure and temperature as free variables) to demonstrate how many degrees of freedom ($F$) the assemblage of sample C has at the triple point and justify why finding this paragenesis in nature is an extraordinarily precise petrogenetic indicator.

Effervescence with dilute hydrochloric acid and magnetism are quick field tests. Which mineral does each one identify almost unequivocally? Describe how each test is carried out.

Some properties can be perceived with the senses, such as the greasy feel of talc or the salty taste of halite. Name two more examples and explain what hygiene precautions the taste test requires.

In the image, quartz and orthoclase feldspar can have similar colours. Propose two simple tests (hardness, cleavage, or streak) to differentiate them in hand specimen and explain the expected result in each case.

Besides effervescence, calcite shows double refraction: letters seen through a transparent crystal appear doubled. Explain what this property consists of and how to use it as a diagnostic test.

During a mining survey in an iron deposit, a technician collects a specimen with a dull metallic grey lustre and high density. When bringing a neodymium magnet close to it, he observes that the specimen does not experience magnetic attraction, which leads him to conclude erroneously in his preliminary report that it is galena or sphalerite with impurities.

1. a) Refute the technician's conclusion: deduce with reasons which iron ore mineral makes up the specimen and what immediate hand specimen test allows you to rule out lead or zinc sulphides with complete certainty.

1. b) In the laboratory, it is verified that upon heating the specimen or when analysing adjacent deeper samples, a noticeable net magnetism appears and the rock exhibits pseudomorphism (martitisation). Explain what geochemical/redox process accounts for the transformation between the original magnetite and the mineral in the specimen.

In a laboratory practical, a group of students proposes the following protocol to identify four anhedral macrocrystalline specimens (quartz, calcite, halite, and pyrite):

"Step 1: Taste all four specimens in your mouth. Step 2: For the three that do not taste like salt, scratch the streak plate hard to see the colour of the powder. Step 3: Add concentrated $\text{HCl}$ to the one that did not leave a streak."

1. a) Critically evaluate this proposal: point out two serious objections (one methodological regarding chemical safety and another technical regarding the hardness limits of the porcelain streak plate compared to quartz).

1. b) Design an alternative, rigorous, and safe dichotomous key (without tasting specimens and minimising damage to the material) that unambiguously discriminates the four minerals based on relative hardness (abbreviated Mohs scale with fingernail, copper coin, and glass), habit/cleavage, streak, and dilute acid test.

During the expert assessment of a quarry for construction aggregates, a light-coloured mineral fragment with a vitreous lustre is extracted. A disagreement arises within the team: one expert believes it is quartz, another claims it is calcite, and a third argues that it is potassium feldspar (orthoclase). You have only a hardened steel point (hardness 5,5), a commercial glass plate (hardness 5,5), a reference quartz fragment (hardness 7), a $10\times$ hand lens, and a dropper with 10 % $\text{HCl}$.

1. a) Design a sequential field-testing strategy that allows you to conclusively refute two of the three hypotheses and confirm the third, justifying the expected result for each mineral.

1. b) If the specimen scratches glass but is scratched by quartz, does not effervesce, and under the hand lens shows flat surfaces reflecting light forming steps at right angles ($90^\circ$), deduce the identity of the mineral. What textural or morphological feature would you look for with the hand lens to differentiate whether it is orthoclase or a plagioclase?

In a recycling plant for aggregates from demolition, three mineral concentrates that may alter the durability of concrete are analyzed:

• Concentrate A: dark mineral with a submetallic luster, high density, reddish-brown streak, and no acid effervescence.

• Concentrate B: colorless/whitish mineral, with perfect rhombohedral cleavage, hardness 3, and immediate, vigorous effervescence with cold dilute $\text{HCl}$.

• Concentrate C: golden mineral with intense metallic luster, hardness greater than 6, forming cubic aggregates and having a greenish-gray streak.

1. a) Deduce the mineral species of each concentrate, arguing which property is unequivocally diagnostic to rule out its most common counterparts (magnetite for A; dolomite or gypsum for B; chalcopyrite or native gold for C).

1. b) From environmental chemistry and materials pathology, predict what weathering reaction aggregate C will undergo if trapped in concrete in the presence of water and atmospheric oxygen, and what two serious consequences (mechanical and geochemical) it will generate in the structure.