Why does OCR put minerals before everything else in Geology?

Minerals are the alphabet of geology: rocks are made of them, and you cannot classify an igneous rock, recognise a metamorphic grade or interpret a sediment without naming minerals first. OCR introduces rock-forming minerals and the silica tetrahedron in Module 2 (Foundations in geology) because the classification of all three rock families, Bowen's reaction series, weathering resistance and economic geology all depend on this base. Master mineral identification early and every later module becomes easier.

How do I tell pyroxene from amphibole in the exam?

Both are dark, iron-magnesium chain silicates, so colour will not separate them. The deciding property is the cleavage angle: pyroxene has two cleavages meeting at about 90 degrees, while amphibole has two at about 120 degrees (and 56 degrees). Habit helps too: pyroxene forms short, stubby crystals; amphibole forms long, prismatic ones. OCR mark schemes reward the cleavage angle as the key discriminator.

What does Bowen's reaction series actually predict?

Three things OCR asks about. First, the order minerals crystallise from a cooling magma (olivine first, quartz last). Second, how fractional crystallisation works: removing early, silica-poor crystals enriches the residual melt in silica, so a basic magma can evolve towards acid. Third, weathering resistance: quartz crystallises last at the lowest temperature, so it is most stable at the surface, while olivine crystallises first and weathers fastest.

How are igneous rocks classified, and why two ways?

By silica content (acid, intermediate, basic, ultrabasic), which the magma chemistry sets and which controls colour and density, and by grain size (coarse intrusive versus fine extrusive), which records the cooling rate. The two axes are independent: basalt and gabbro share a basic composition but differ in grain size; granite and rhyolite share an acid composition but differ in grain size. Always state both to name a rock correctly.

What is the single most common mistake in this topic?

Putting chilled and baked margins in the wrong rock. A chilled margin is in the igneous body (its edge cooled fast against cold country rock, so it is finer-grained). A baked margin is in the country rock (it was heated and recrystallised). The presence of a baked margin proves the intrusion is younger than the rock it baked. Swapping the two, or forgetting that a lava flow bakes only the rocks below it, costs easy marks.

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OCR A-Level Geology: rock-forming minerals and igneous processes overview

A deep-dive OCR A-Level Geology guide to rock-forming minerals and igneous processes. Covers the silica tetrahedron and silicate groups, mineral identification, Bowen's reaction series and fractional crystallisation, igneous classification by silica content and texture, and intrusive and extrusive forms with the exam patterns OCR repeats.

Generated by Claude Opus 4.818 min readOCR-H414-Module-2Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic actually demands
Rock-forming minerals and silicate structures
Bowen's reaction series and magma differentiation
Igneous rock classification and textures
Igneous intrusions and volcanic forms
How this topic is examined
Check your knowledge

What this topic actually demands

Rock-forming minerals and igneous processes are the foundation of OCR A-Level Geology. The topic runs from the atomic structure of the silicate minerals, through how to identify them in hand specimen, to how magmas crystallise, how igneous rocks are classified, and how intrusive and extrusive bodies are recognised in the field. The examiners test two linked skills: precise identification and classification, and the interpretation of textures and field relationships as a record of how a rock formed.

This guide walks through the four clusters of the topic in a sensible build order, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Rock-forming minerals and silicate structures

Almost every common mineral in the crust is a silicate, built from the $\mathrm{SiO_4^{4-}}$ silica tetrahedron. The silicate groups are defined by how many corner oxygens each tetrahedron shares: isolated (olivine, no sharing), single-chain (pyroxene, two shared), double-chain (amphibole), sheet (mica, three shared) and framework (quartz and feldspar, all four shared). More sharing (polymerisation) means higher silica and lower iron and magnesium, which is why ultrabasic rocks are full of isolated and chain silicates and acid rocks are full of framework silicates.

You identify minerals using a fixed property list: colour and lustre, hardness on Mohs' scale (tested against a fingernail, a coin and a steel blade), cleavage versus fracture (record directions and angles), streak, density and habit. The high-value discriminators are the hardness of quartz ( $7$ , scratches steel), the perfect single cleavage of mica (a sheet silicate), and the cleavage angles that separate pyroxene ( $90^{\circ}$ ) from amphibole ( $120^{\circ}$ ).

Bowen's reaction series and magma differentiation

A magma crystallises in a set order, captured by Bowen's reaction series. The discontinuous branch runs olivine, pyroxene, amphibole, biotite (each reacting to form the next), while the continuous branch is plagioclase changing from calcium-rich to sodium-rich; below these come potassium feldspar, muscovite and quartz. Olivine and calcium plagioclase form first at high temperature; quartz forms last at low temperature.

The series explains fractional crystallisation: if early, dense, silica-poor crystals are removed (by settling or filtering), the residual melt becomes enriched in silica, so magma differentiation can drive a basic parent magma towards intermediate and acid compositions. The same order predicts weathering resistance, since quartz (last, lowest temperature) is most stable at the cool, wet surface and olivine (first, highest temperature) least.

Igneous rock classification and textures

Igneous rocks are classified on two independent axes. Composition (silica content) gives acid (over $63\%$ , pale, quartz-rich), intermediate ( $52$ to $63\%$ ), basic ( $45$ to $52\%$ , dark) and ultrabasic (below $45\%$ ). Grain size records cooling rate: coarse phaneritic rocks cooled slowly at depth (intrusive: granite, gabbro), fine aphanitic rocks cooled fast at the surface (extrusive: rhyolite, basalt).

Texture is the evidence you read in the exam. A porphyritic texture (large phenocrysts in a fine groundmass) records two cooling stages, slow then fast; a glassy texture (obsidian) records a quench; a vesicular texture (pumice, scoria) records trapped gas in a degassing lava. Name a rock from both composition and texture: coarse and acid is granite, fine and basic is basalt.

Igneous intrusions and volcanic forms

Magma that solidifies underground forms intrusive bodies, classified by shape and by their relationship to bedding: batholiths and dykes are discordant (they cut across bedding), while sills and laccoliths are concordant (parallel to bedding). Intrusion is recognised from a chilled margin (finer crystals at the edge of the igneous body, from rapid cooling against cold rock) and a baked margin or contact aureole (altered country rock). The baked margin proves the intrusion is younger than the rock it baked.

Cross-cutting relationships then give relative age: any intrusion that cuts another rock is younger than it. A lava flow, being extrusive, bakes only the rocks below it and is then buried, which is how you distinguish a buried flow from a sill (a sill bakes both above and below and has chilled margins on both sides).

How this topic is examined

A typical OCR profile for rock-forming minerals and igneous processes:

Specimen and identification questions (Paper 3). Identifying a mineral or igneous rock from described properties, classifying it by silica content, and naming it from composition plus texture.
Process and explanation questions (Paper 1). Explaining fractional crystallisation with reference to Bowen's series, why a porphyritic rock has two crystal sizes, or why quartz resists weathering.
Field-relationship and dating questions (Paper 3). Reading chilled and baked margins, distinguishing a sill from a lava flow, and ordering igneous events by cross-cutting relationships.
Level-of-response extended answers (Papers 1 and 2). The link from Bowen's series through fractional crystallisation to magma differentiation is a predictable six-mark question.

Check your knowledge

A mix of recall and application questions covering the whole topic. Attempt them under timed conditions, then check against the solutions.

Name the five main silicate groups in order of increasing oxygen-sharing, with a mineral example of each. (5 marks)
Explain how the cleavage angle distinguishes pyroxene from amphibole. (2 marks)
Describe the discontinuous branch of Bowen's reaction series. (2 marks)
Explain how fractional crystallisation can produce an acid residual magma from a basic parent. (4 marks)
A dark, fine-grained rock is made of pyroxene and calcium plagioclase. Name it and give its composition and origin. (3 marks)
Explain what a porphyritic texture shows about a rock's cooling history. (3 marks)
Distinguish a concordant intrusion from a discordant one, with an example of each. (2 marks)
A sill and a dyke both occur in a sandstone, and the dyke cuts the sill. Place the three rocks in order, oldest first, and justify the order. (3 marks)

Solutions

Q1: Isolated (nesosilicates), for example olivine; single-chain (inosilicates), for example pyroxene; double-chain (inosilicates), for example amphibole; sheet (phyllosilicates), for example mica; framework (tectosilicates), for example quartz or feldspar.
Q2: Pyroxene has two cleavages meeting at about $90^{\circ}$ ; amphibole has two cleavages meeting at about $120^{\circ}$ (and $56^{\circ}$ ). The angle is the diagnostic difference.
Q3: The discontinuous branch is the sequence olivine, pyroxene, amphibole, biotite, in which each high-temperature mineral reacts with the melt to form the next as the magma cools, and the crystal structure changes at each step.
Q4: As the magma cools, early, dense, silica-poor crystals (olivine, pyroxene, calcium plagioclase) crystallise first and are removed by settling (fractional crystallisation). Because they are silica-poor, the residual melt is left enriched in silica, sodium and potassium, evolving the composition from basic towards acid.
Q5: Basalt. It is dark and rich in iron-magnesium minerals with calcium plagioclase, so it is basic ( $45$ to $52\%$ silica). It is fine-grained, so it cooled quickly at or near the surface and is extrusive (volcanic).
Q6: A porphyritic texture has large phenocrysts in a fine groundmass, recording two cooling stages: slow cooling at depth grew the large early crystals, then faster cooling at or near the surface froze the rest as a fine groundmass.
Q7: A concordant intrusion is parallel to the bedding (for example a sill); a discordant intrusion cuts across the bedding (for example a dyke).
Q8: Sandstone first (it is cut by both intrusions); then the sill (intruded into the sandstone but cut by the dyke); then the dyke (it cuts the sill, so it is youngest). Cross-cutting relationships: the cutting feature is always younger.