Minerals: definition and properties
A seller offers you a very cheap "diamond". How could you tell whether you are being cheated without taking it to a laboratory? Minerals have properties that can be checked with simple tests, and those tests don't lie.
What a mineral is
If you look at grains of table salt and fragments of broken glass through a magnifying glass, you will see that the salt forms perfect geometric cubes, whereas the glass breaks into irregular curves: why do some solids arrange themselves naturally into such precise geometric shapes while others do not?
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…
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…
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.
Hardness and the Mohs scale
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.
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 Mohs scale? Name the softest and the hardest mineral on the scale and explain how it is used to compare hardnesses.
What is the Mohs scale? Name the softest and the hardest mineral on the scale and explain how it is used to compare hardnesses.
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…
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?
Lustre, colour and streak
Quartz can occur in nature as a transparent, milky white or dark violet (amethyst) crystal, yet all of them leave exactly the same white powder when rubbed: which visual properties are really reliable for telling minerals apart?
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.
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.
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.
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.
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…
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.
Identifying a mineral by combining properties
Two stones picked up in the countryside can have exactly the same golden colour and metallic lustre, yet one of them may be valuable gold and the other common pyrite: how can we combine several simple tests to identify a mineral without fear of making a mistake?
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…
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).
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…
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…
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.
Back to the start: To check whether the "diamond" is fake, try scratching it with a piece of quartz: a genuine diamond has a hardness of $10$ on the Mohs scale and no other mineral can scratch it, whereas a glass imitation ($5{,}5$) will be marked instantly.