Eduqas GCSE Geology Minerals and rocks: identifying minerals and the three rock families

What this module actually demands

Minerals and rocks is the foundation of Eduqas GCSE Geology: everything later (geological structures, the rock cycle, geological time, economic geology) assumes the named minerals and rocks and how they form. The examiners test two linked skills here. The first is identifying minerals and rocks from their physical properties in hand specimen, which is the practical skill at the heart of Component 2. The second is explaining how each rock family forms and interpreting what a rock records about the past. This overview ties the four dot-point pages together and sets out the exam patterns Eduqas repeats; each cluster has a matching dot point with practice questions.

Identifying minerals by physical properties

A mineral is a naturally occurring solid with a definite composition and an ordered structure. No single property identifies one, so you combine several and treat colour as the least reliable. The reliable tests are hardness (scratch against a fingernail at 2.5, a copper coin at 3.5, and steel and glass at 5.5), cleavage (flat planes in set directions, like mica's one perfect cleavage or galena's cubes) versus fracture (uneven or glassy, like quartz), lustre (metallic or non-metallic), streak (the reliable powder colour, so haematite is always red-brown), and the dilute hydrochloric acid test (carbonates such as calcite fizz). Learn the diagnostic features of quartz, feldspar, mica, calcite, halite, galena and haematite exactly, because Component 2 hands you specimens to identify.

Igneous rocks and cooling rate

Igneous rocks form by the crystallisation of magma or lava, giving interlocking crystals with no bedding. The master rule is that cooling rate controls crystal size: slow cooling at depth grows large crystals (coarse-grained intrusive rocks such as granite), and fast cooling at the surface grows small crystals (fine-grained extrusive rocks such as basalt). Igneous rocks are classified by crystal size (coarse intrusive or fine extrusive) and by silica content (pale felsic like granite and rhyolite, intermediate, or dark mafic like gabbro and basalt). Minerals also crystallise from hot hydrothermal fluids moving along fractures, depositing ore minerals (galena, haematite) and gangue (quartz, calcite) in veins, which links forward to economic geology.

Sedimentary rocks and their fossils

Sedimentary rocks build up at the surface through weathering, erosion, transport, deposition and lithification (compaction plus cementation), giving bedded rocks of grains, fragments or fossils. They are clastic (fragments classified by grain size: conglomerate, breccia, sandstone, shale), biological (limestone, from shells and skeletons) or chemical (evaporites such as rock salt). The big skill is reading the depositional environment from grain size and energy, grain shape (rounding), sorting, sedimentary structures (cross-bedding, ripples, mud cracks) and fossils. Fossils form when hard parts are quickly buried and preserved as the sediment lithifies, recording past life, the age of the rock and the environment it formed in.

Metamorphic rocks and processes

Metamorphic rocks form by recrystallisation in the solid state under heat and pressure, without melting (the moment a rock melts the product is igneous). Contact metamorphism is heat alone from an intrusion, baking a narrow aureole and giving non-foliated rocks: limestone becomes marble, sandstone becomes metaquartzite. Regional metamorphism is heat plus directed pressure over a wide area, at colliding plate margins, and gives foliated rocks as platy minerals align: mudstone becomes slate (low grade) then schist (higher grade). Foliation is that parallel alignment of platy minerals, creating planes of weakness, which is why slate splits into roofing sheets. The product depends on the protolith and the conditions.

The exam patterns Eduqas repeats

• Specimen identification (Component 2). You are given hand specimens or photographs and asked to identify minerals and rocks from their properties, then to justify the identification. Practise describing the tests and what each reveals.
• Cooling-rate explanations. "Explain why granite is coarse and basalt is fine" is a recurring item; always link cooling rate to depth and crystal size.
• Depositional environment interpretation. Given grain size, sorting, structures and fossils, infer the environment (high-energy river, quiet sea, desert, reef). This is AO2 and AO3 and carries the higher marks.
• Contact versus regional metamorphism. Distinguish heat-only and local from heat-plus-pressure and widespread, and pair each with the right rock.
• Acid test and streak. Short factual marks come from knowing that calcite fizzes and that streak beats colour.

How to revise this module

1. Make a mineral identification table. One row per named mineral (quartz, feldspar, mica, calcite, halite, galena, haematite) with hardness, cleavage, lustre, streak and any special test. Drill it until you can name a mineral from three clues.
2. Learn the rock chart. Place each named rock by family and, for igneous rocks, by crystal size and colour (granite, basalt, gabbro, rhyolite).
3. Practise environment interpretation. Take described sediments and infer the environment, justifying each feature (grain size, rounding, sorting, structures, fossils).
4. Drill the two metamorphic settings. Be able to state the setting, the pressure regime and a named product for contact and for regional metamorphism.
5. Work the dot-point questions and the paired quiz under timed conditions, then check your wording against the answer_explainers.

Use the four dot-point pages for the detail and worked exam questions; this guide is the map that connects them.