Minerals and rocks: the rock cycle explained

It is mid-October 2026. In vocational schools that teach integrated sciences, and also in technical schools and the first two years of the licei (Italian academic high schools), Year 1 of high school has reached basic geology. After the atmosphere and the hydrosphere, or before them, depending on the textbook, you study minerals, rocks and the rock cycle. The test usually comes four to seven weeks from now, between mid-November and early December, and it is often a mix of closed-answer questions and one or two open questions.

Many students treat this topic as a list to learn by heart, and that is exactly where marks are lost: they learn the names of the rocks without understanding what the rocks are sorted by. Here is the scheme I use in my lessons: three questions, always the same, and the names come by themselves. If you want the picture of the rest of the programme, the neighbouring pages are atmosphere, hydrosphere, pressure and winds, plate tectonics, volcanoes and earthquakes and the Earth, the solar system and the seasons.

What a mineral is

A mineral is a solid, natural, inorganic substance with a defined chemical composition and an ordered crystal structure. Each of these words rules something out:

• solid: liquid water is not a mineral, natural ice is;
• natural: a crystal made in a laboratory is not a mineral in the strict sense;
• inorganic: coal, which comes from plant remains, is not a mineral;
• defined composition: it can be written with a chemical formula, for example ${\text{SiO}}_2$ for quartz or ${\text{CaCO}}_3$ for calcite;
• crystal structure: the atoms are arranged in a regular, repeating pattern in space, the so-called crystal lattice. Glass, whose atoms are arranged at random, is not a crystal.

The internal structure explains the outward properties. If the lattice has planes along which the bonds are weaker, the mineral breaks along those planes. If the bonds are all very strong, the mineral is hard.

Physical properties

To recognise a mineral you do not look at a single feature, but at a set of properties.

Hardness. This is resistance to scratching. It is measured with the Mohs scale, which runs from 1 to 10 and is a relative scale: a mineral scratches all those with a lower number.

Two things that are often asked. First: the numbers are not equally spaced, because the scale only gives the order, not how much harder one mineral is than another; diamond, for example, is much harder than corundum, not “one more”. Second: in practice you use everyday objects to help. A fingernail scratches talc and gypsum (hardness about 2.5), glass and a steel blade scratch fluorite and apatite, and quartz scratches glass.

Lustre. This is the look of the surface when it reflects light: metallic (pyrite, galena), vitreous (quartz), pearly, greasy, dull. It should not be confused with colour.

Colour. This is the least reliable property: the same mineral can change colour because of tiny impurities. Quartz can be colourless, pink, purple (amethyst), smoky. For this reason we also use the colour of the powder (the streak left on a rough piece of porcelain), which is far more constant.

Cleavage and fracture. Cleavage is the tendency to break along well-defined parallel planes, as mica (in very thin sheets) and calcite (in rhombohedra) do. If the mineral breaks irregularly, with no preferred planes, we speak of fracture: quartz, for example, has a conchoidal fracture, with curved surfaces like those of broken glass.

The families of minerals

Minerals are grouped according to the main anion or chemical group. In Year 1 textbooks you find five:

Silicates are the most important family: on their own they make up most of the Earth’s crust. Their building block is the silicon-oxygen tetrahedron, one silicon atom surrounded by four oxygen atoms; the tetrahedra link to each other in different ways (isolated, in chains, in sheets, in networks) and that way of linking explains why micas cleave into sheets and quartz does not. Carbonates have a useful property for recognising them: calcite fizzes in contact with dilute hydrochloric acid, because carbon dioxide is released. Graphite and diamond are both pure carbon, but with different crystal structures: one is soft (Mohs 1-2) and the other is the hardest (Mohs 10). It is the best example there is of structure explaining properties.

The three families of rocks

A rock is an aggregate of one or more minerals (or, in some cases, of fragments of other rocks). Rocks are divided according to how they formed. The three questions are: where does the material come from, where did it solidify or settle, and in what conditions.

Igneous rocks

They form from the cooling and solidification of a magma, molten rock.

• Intrusive (or plutonic): the magma cools slowly at depth. The minerals have time to grow and the rock has large crystals, visible to the naked eye. Typical example: granite (quartz, feldspars, mica).
• Extrusive (or volcanic): the magma reaches the surface, as lava, and cools quickly. The crystals are small or absent, and sometimes there is volcanic glass. Typical example: basalt, dark and very fine-grained.

The rule to remember: cooling rate and crystal size are linked. Slow, large crystals; fast, small crystals.

Sedimentary rocks

They form from the accumulation and lithification (compaction and cementation) of sediments. They fall into three groups:

• Clastic (or detrital): made of fragments of pre-existing rocks, carried by water, wind or ice and then deposited. Examples: sandstone (cemented sand grains), conglomerate (rounded pebbles), clay.
• Chemical: they form by precipitation of salts dissolved in water, often through evaporation. Examples: rock salt, gypsum, some limestones.
• Organic (biogenic): they come from the accumulation of the remains of organisms, shells and skeletons. Example: limestone made of shells and corals.

Sedimentary rocks can be recognised because they are layered and because they can contain fossils: the other two families do not contain them, for obvious reasons (the heat of the magma would destroy them, and metamorphism wipes them out).

Metamorphic rocks

They form from a pre-existing rock (of any type) that, while staying solid, changes structure and minerals because of high temperature and pressure. Underline “while staying solid”: if it melted completely, it would become magma and we would be back to igneous rocks.

Examples to know:

Many metamorphic rocks have a banded or sheet-like structure, called schistosity (foliation), because the minerals line up perpendicular to the pressure. Marble, on the other hand, is made of calcite crystals joined together and has no schistosity.

The rock cycle

The rock cycle is the set of processes that turn one family of rocks into the others. It is not a one-way circle: there are many shortcuts. The essential scheme:

1. a magma cools and solidifies, and an igneous rock is born;
2. any rock exposed at the surface undergoes weathering and erosion (wind, water, ice, temperature changes) and breaks up into sediments;
3. the sediments are transported and deposited, then through diagenesis (compaction and cementation) become a sedimentary rock;
4. a sedimentary (or igneous) rock buried deep down, under high pressure and temperature, changes and becomes metamorphic;
5. if the temperature rises further and the rock melts, magma forms, and the cycle starts again.

The shortcuts: an igneous rock can become metamorphic directly (granite turning into gneiss), a metamorphic rock can be eroded and become sediment, a sedimentary rock can melt without going through full metamorphism. The engine of the cycle has two parts: the internal energy of the Earth, which pushes magmas upward and drags rocks down, and the energy of the Sun, which drives the water and wind that erode. The link with plate tectonics is that plate movements carry rocks down into the depths and lift them up again, forming mountain ranges.

Exercise 1: classify rocks

For each rock, give the family and subgroup and justify it with one sentence: granite, basalt, sandstone, limestone with shells, marble, gneiss.

Solution.

• Granite: intrusive igneous. Large crystals, because the magma cools slowly at depth.
• Basalt: extrusive igneous. Very small crystals, because the lava cools quickly at the surface.
• Sandstone: clastic sedimentary. It is made of sand grains, fragments of pre-existing rocks, cemented together.
• Limestone with shells: organic (biogenic) sedimentary. The material comes from the remains of organisms and the fossils are the proof.
• Marble: metamorphic. It comes from limestone, which recrystallises under temperature and pressure without melting.
• Gneiss: metamorphic. It generally comes from granite, with minerals arranged in bands.

Exercise 2: order by hardness

You have four samples: quartz, gypsum, calcite, orthoclase. Put the samples in order from softest to hardest, and then say what you can deduce from these facts: a fingernail scratches gypsum, a fingernail does not scratch calcite, quartz scratches orthoclase.

Solution. With the Mohs scale: gypsum 2, calcite 3, orthoclase 6, quartz 7. The order from softest to hardest is therefore

The deductions:

• a fingernail scratches gypsum: its hardness is greater than 2, and it is about 2.5;
• a fingernail does not scratch calcite: calcite has a hardness greater than 2.5 (on the scale, 3);
• quartz scratches orthoclase: quartz is harder, 7 against 6.

This is the logic of the scale: a mineral scratches another only if its number is higher. If two minerals scratch each other, they have a similar hardness.

Exercise 3: describe the cycle, open question

A fragment of granite is exposed on a mountain. Describe the transformations it can go through until it becomes a metamorphic rock.

Solution, in five lines.

1. Granite is an intrusive igneous rock; once exposed, it is open to the weather.
2. Through weathering and erosion it breaks up into sediments (sand, clay).
3. Transport, for example by a river, carries them downstream where they are deposited in layers.
4. Through diagenesis, compaction and cementation, they become a sedimentary rock, for example a sandstone.
5. If the sandstone is buried deep down, by plate movements, and subjected to high temperature and pressure without melting, it recrystallises and becomes a metamorphic rock, for example a quartzite.

An answer like this gets full marks because it names all the processes (weathering, erosion, transport, sedimentation, diagenesis, metamorphism) and in order. For a bonus, add that granite can become gneiss directly, without going through the sediments.

The mistakes that cost marks

1. Confusing mineral and rock. A mineral has a formula; a rock is an aggregate of minerals. Granite has no formula, quartz does.
2. Coal among the minerals. It comes from organic matter, so it is not a mineral; diamond, on the other hand, is.
3. Mohs as a linear scale. The numbers are an order, not a measurement. A mineral of hardness 6 is not “twice” one of hardness 3.
4. Metamorphism and melting. A metamorphic rock has never been in the liquid state; if it melts, it is magma.
5. Forgetting the subgroup. “Granite: igneous” is half an answer; you need “intrusive” and the reason why.

How to prepare

Draw the scheme of the cycle once and for all: three rectangles (igneous, sedimentary, metamorphic), a circle for the magma, and arrows with the name of the process on each one (cooling, erosion and sedimentation, diagenesis, metamorphism, melting). Then go over the Mohs scale out loud, with the ten names, and for each family of minerals learn one example and one property. Redo the three exercises without looking at the solutions, changing the rocks. The overview of the subject is on the Science page.

If you want a check before the test

If geology feels like a list of names that won’t stick, what is usually missing is a thread to hold them together, and almost always it is enough to redo the scheme together. Write to me from the contact page and we will do a free introductory call: you send me your notes or a question that doesn’t add up, and I tell you where it gets stuck.