Exercises: Minerals: concept and properties

The image shows six common minerals numbered. Choose three of them, describe their lustre, colour, and crystal shape, and try to identify them using a mineral field guide.

Define what the hardness of a mineral is, and explain what it means that talc has a hardness of 1 and diamond a hardness of 10 on the Mohs scale.

What is the streak of a mineral and how is it obtained? Explain why streak is a more reliable property than colour for identifying a mineral.

Name the four properties normally used to identify a mineral (hardness, lustre, colour, and streak) and describe in one sentence what each one consists of.

Explain what the lustre of a mineral is and the difference between metallic and non-metallic lustre, giving an example of a mineral of each type.

What is the Mohs scale? Name the softest and the hardest mineral on the scale and explain how it is used to compare hardnesses.

Explain what it means that a mineral has an "ordered crystalline structure" and give an example of a crystal you may have seen, in nature, in a museum, or in the laboratory.

Discuss whether these materials are minerals, applying the definition: a piece of amber with an insect inside, a quartz crystal picked up in the mountains, a pearl, and rock salt from a mine.

Order these minerals on the Mohs scale from softest to hardest: quartz (7), gypsum (2), diamond (10), and calcite (3). Then answer: an unknown mineral scratches calcite but is scratched by quartz; between which values does its hardness lie?

You have a fingernail (approximate hardness 2.5), a copper coin (3), and a steel pocket knife (5.5). Describe how you would use these three objects to estimate the hardness of an unknown mineral, giving a specific example.

One sample of quartz is milky white and another is purple (amethyst), but both are the same mineral. Explain how this is possible and what other properties, besides colour, would confirm that both are quartz.

An unknown mineral scratches a copper coin (hardness 3) but fails to scratch a steel pocket knife (hardness 5.5). Between which values is its hardness? Reason whether it could be gypsum (hardness 2) or fluorite (hardness 4).

Two white minerals look very alike: gypsum (hardness 2) and calcite (hardness 3). You only have your fingernail (hardness 2.5) and a copper coin (hardness 3). Describe a test to tell them apart and what result you would expect from each one.

Describe how the streak of a mineral is obtained with an unglazed porcelain plate and reason what happens if the mineral is harder than the plate: will it leave a streak? What conclusion do you draw in that case?

Mica flakes apart into thin sheets and calcite breaks into regularly shaped blocks. Explain what such orderly breakage tells us about the internal structure of these minerals.

From this list —talc (1), gypsum (2), calcite (3), quartz (7), and diamond (10)— state which minerals you can scratch with your fingernail (hardness 2.5) and which ones scratch window glass (hardness 5.5), explaining how you worked it out.

A sample has a metallic lustre, golden colour, greenish-black streak, and scratches glass. A classmate claims that it is gold. Use the properties mentioned to argue that it is pyrite, "fool's gold", and state two other properties you would test to confirm it.

You have on the table two pairs of minerals that are easily confused: a pair with a golden appearance and metallic luster (pyrite and gold) and another with a whitish appearance and vitreous luster (quartz and calcite).

1. a) Explain why color and luster are not sufficient diagnostic properties to distinguish the minerals in either pair.
2. b) Design an identification strategy indicating what specific test (streak or hardness) you would apply to each pair to differentiate them without error, and predict the exact result you would obtain in each case.

Graphite and diamond are composed exclusively of pure carbon atoms, but they exhibit radically opposite properties: graphite wears away when rubbed against paper and has a hardness of 1, whereas diamond has a hardness of 10 and scratches any other mineral.

1. a) Explain, relating the arrangement of their atoms and the type of bond between them, why graphite is so soft and exfoliative while diamond is extremely hard.
2. b) Based on this example, refute with scientific arguments the following statement: "Two mineral samples having exactly the same chemical composition will always have the same hardness and the same physical properties".

In the laboratory, you are given three unknown minerals (A, B, and C) and you have the following materials available: a hand lens, a white porcelain streak plate, your fingernail (hardness 2,5), a copper coin (hardness 3,0), and a steel pocketknife (hardness 5,5).

1. a) Design the experimental protocol, ordering the tests from least to greatest physical impact on the samples, and justify why it is essential to follow that order.
2. b) After applying your protocol, you obtain these results:

• Sample A: vitreous luster; white streak; cannot be scratched by the fingernail, but is scratched by the copper coin.

• Sample B: metallic luster; greenish-black streak; scratches the steel pocketknife.

• Sample C: silky luster; easily scratched by the fingernail.

Determine the hardness range of each sample on Mohs scale and deduce, justifying your answer, which corresponds to calcite, which to gypsum, and which to pyrite.

In the school laboratory, the labels on four mineral samples have been erased: magnetite, calcite, sulfur, and halite. To identify them, you must rely on their special properties.

1. a) Design a simple, non-destructive test to safely identify magnetite and another one for sulfur, describing what phenomenon you will observe in each.
2. b) A student suggests identifying halite by tasting it with their tongue and calcite by pouring a bottle of concentrated hydrochloric acid over it. Critically justify why both proposals are incorrect in a laboratory and propose a safer alternative to verify each one.

Obsidian (natural volcanic glass) and quartz crystal are naturally occurring inorganic solids composed mainly of silicon dioxide. However, in geology, quartz is classified as a mineral, while obsidian is not.

1. a) Justify why obsidian does not meet the scientific definition of a mineral despite being a natural, inorganic, and solid product.
2. b) If you strike both samples with a geological hammer, predict how each will break (type of breakage or fracture) and explain the relationship between their mechanical behavior and their internal atomic structure (ordered or disordered).

A gemstone dealer suspects that a batch of "rubies" contains fake pieces made of red-dyed quartz or red glass. Knowing that ruby is a variety of the mineral corundum (hardness 9 on the Mohs scale) and quartz has a hardness of 7:

1. a) Propose a physical method based on hardness that allows distinguishing an authentic ruby from a quartz or glass fake without destroying the gem. Justify your proposal.
2. b) Common corundum is not used in jewelry, but in industry as an abrasive to polish and cut other materials. Justify what physical property gem ruby and industrial corundum share that explains their enormous usefulness in both such different fields.

A street vendor offers a "diamond" at a very good price, but the stone is scratched by a steel pocket knife (hardness 5.5). Prove, reasoning with the Mohs scale, that the stone cannot be a diamond, and explain what other test you could do to confirm it.