Minerals and how to identify them
A mineral is natural, non-living and solid, has a fixed chemical make-up and an orderly crystal arrangement of atoms. Quartz (SiO2), halite (NaCl) and calcite (CaCO3) are minerals. A rock is a mixture of minerals: granite = quartz + feldspar + mica.
Identification tests
- Colour: easy but can mislead (impurities change it).
- Streak: colour of the powder when rubbed on a tile.
- Lustre: how it shines: metallic, glassy, dull.
- Hardness: Mohs scale 1 (talc) to 10 (diamond); a harder mineral scratches a softer one.
- Cleavage: splits along flat planes (mica peels in sheets).
- Density and crystal shape.
Geologists also cut rocks into very thin slices and look at them with a polarising microscope; each mineral shows its own colours in polarised light.
Rock-forming minerals and uses
Quartz (glass, electronics), feldspar (ceramics), mica (electrical insulation), calcite (cement), iron ores like haematite (steel), bauxite (aluminium).
Igneous rocks
Deep inside the Earth, rock can melt into magma. When magma cools and hardens it makes igneous rock (from a word meaning fire).
- Intrusive (plutonic): cools slowly inside the crust → large crystals. Example: granite.
- Extrusive (volcanic): lava cools fast at the surface → tiny crystals or glass. Examples: basalt, pumice (full of gas holes, it floats), obsidian.
Volcanoes and earthquakes are caused by forces inside the Earth (endogenous forces) that also bring magma up.
Sedimentary and metamorphic rocks
Sedimentary rocks: weathering breaks rocks; water, wind and ice carry the bits (erosion) and drop them in layers (deposition). Over time the layers are pressed (compaction) and glued (cementation). Examples: sandstone, shale, limestone, coal (from plant remains). Older layers lie below younger ones, and many contain fossils.
Metamorphic rocks: heat and pressure deep underground change the minerals and texture without melting. Shale → slate, limestone → marble, sandstone → quartzite, granite → gneiss. Minerals often line up in bands.
The rock cycle and Earth's structure
The Earth has a thin rocky crust, a hot mantle and an iron–nickel core. The crust is broken into plates that move a few centimetres a year. Where they push, pull or slide, rocks are melted, lifted into mountains or buried.
The rock cycle: igneous → weathering and deposition → sedimentary → heat and pressure → metamorphic → melting → magma → cooling → igneous. Shortcuts happen too: igneous rock can be metamorphosed directly, and any rock can be weathered. Mineral resources (ores, coal, building stone) are found where these processes concentrated them.
Key formulas and definitions
- Mineral: natural, inorganic solid with fixed composition and crystal structure
- Rock: a natural mixture of one or more minerals
- Igneous: cooled magma/lava (granite, basalt)
- Sedimentary: compacted layers (sandstone, limestone, shale, coal)
- Metamorphic: changed by heat + pressure (marble, slate, gneiss, quartzite)
- Mohs scale: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10
Worked examples
1. A mineral scratches glass (hardness 5.5) but is scratched by topaz (8). What could it be?
Step 1: it is harder than 5.5. Step 2: it is softer than 8. Step 3: on the Mohs scale, feldspar (6) or quartz (7) fit. A further test (cleavage, crystal shape) decides: quartz has no cleavage and six-sided crystals.
2. Two basalt samples: one has crystals too small to see, the other has holes like a sponge. Explain.
Both formed from lava that cooled fast at the surface, so crystals had no time to grow. The one with holes (vesicles) cooled while gas bubbles were escaping from the lava.
3. A cliff shows layers with sea shells 100 m above sea level. What does this tell us?
Step 1: shells and layers mean sedimentary rock formed under the sea. Step 2: the bottom layers are the oldest. Step 3: the rock is now high up, so plate movements lifted it after it formed.
Common mistakes
- Using 'rock' and 'mineral' as the same word. A rock is a mixture; a mineral has one fixed composition.
- Thinking metamorphic rocks form by melting. They change while still solid; melting makes magma.
- Trusting colour alone to identify minerals. Streak and hardness are more reliable.
- Saying fossils are found in igneous rock. They are almost always in sedimentary rock.