Natural sciences Year 5: Minerals and rocks
Natural sciences, Year 5 (Portugal): A mineral is a natural substance; a rock is made of one or more minerals, and comes as magmatic, metamorphic or sedimentary; each has uses in human life.
Mineral or rock?
A mineral is a natural solid substance with a definite chemical composition, such as quartz or calcite. A rock is a natural material made of one or more minerals joined together. Granite is a rock: it contains quartz, feldspar and mica. Quartz is a mineral: it is a single substance. So a mineral is a «building block» and a rock is what is built from those blocks.
Three groups, three ways of forming
Rocks are grouped by how they form. Magmatic rocks form when magma or lava cools and solidifies: granite and basalt. Sedimentary rocks form when sediments pile up and are compacted and cemented: limestone and sandstone. Metamorphic rocks form when an older rock is changed by heat and pressure without melting: marble comes from limestone, and schist from clay-rich rocks.
Analysing a piece of granite
Look at a granite sample with a magnifying glass. Step 1: you see grains of different kinds: glassy grey grains (quartz), pink or white ones (feldspar) and shiny black flakes (mica). Step 2: more than one mineral, joined together: so it is a rock, not a mineral. Step 3: the grains are large and interlocked, like crystals that had time to grow while cooling slowly. Step 4: conclusion: granite is a magmatic rock.
Quartz is not a rock
Many people use «rock» and «mineral» as if they meant the same. They do not. Quartz, calcite and mica are minerals (each one a single substance). Granite, limestone and marble are rocks (made of minerals). A good test: can you see different grains or ingredients in it? Then it is probably a rock. Another mistake: thinking that a rock keeps the same form forever. Rocks can change group over very long times.
Rocks and minerals in everyday life
Portuguese pavements use white limestone and black basalt. Marble from the Estremoz area is used in buildings and sculpture, granite in walls and kerbs, and schist gives its name to the Schist Villages of the centre. Quartz sand makes glass and limestone is a raw material for cement. Think of the stone in your own region: what is it used for, and who works with it?
Test what you learned
Which statement is correct?
- a) Quartz is a rock and granite is a mineral
- b) Quartz and granite are both minerals
- c) Quartz is a mineral and granite is a rock made of several minerals
Correct answer: c) Quartz is a mineral and granite is a rock made of several minerals — Right: a mineral is one substance, a rock is made of one or more minerals.
A rock is made of large, interlocked crystals of quartz, feldspar and mica, formed as molten rock cooled slowly. What is it?
- a) A magmatic rock, such as granite
- b) A single mineral, of the metamorphic group
- c) A sedimentary rock, because it has several minerals
Correct answer: a) A magmatic rock, such as granite — Right: several minerals joined make a rock, and cooling from molten rock makes it magmatic.
Which of these is a rock, not a mineral?
- a) Calcite
- b) Limestone
- c) Mica
Correct answer: b) Limestone — Right: limestone is a sedimentary rock, made of minerals such as calcite.
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