Notes · GeologyUK · A-Levels

Elements, minerals and rocks

This opening chapter sets out the raw materials of geology: the chemical elements that build the Earth and its crust, the minerals those elements crystallise into, and the three great classes of rock that minerals aggregate to form. It introduces the silicate structures that dominate the crust, the physical properties used to identify minerals, and the features that distinguish igneous, sedimentary and metamorphic rocks in the hand specimen.

4 sections~15 min reading time3 competenciesLevel Foundation 1 · Standard 3

T·0111 / 13

Exam profile

AO1 · Describe the composition of the Earth and crust, the silicate structures and the properties of rock-forming mineralsAO2 · Apply diagnostic physical properties to identify minerals and classify rocksAO3 · Interpret and evaluate evidence to distinguish minerals and the three rock classes

Operators:describeexplainidentifyclassifycalculatededuce

basic level

AS-Level expects you to name the abundant elements, define a mineral, know the common rock-forming minerals and their properties, and recognise the three rock classes.

higher level

The full A-Level requires you to explain the silicate structures, use combinations of properties to identify minerals confidently, and justify a rock classification from evidence.

Depth

Text

Contents · 4 sections

Elements, minerals and rocks

The composition of the Earth and crust#

FoundationWJEC/Eduqas A level Geology (Component 2) — Elements, minerals and rocks

The most abundant elements of the crust

Fig. 1The eight most abundant elements of the continental crust by mass (approximate values). Oxygen and silicon together exceed 70%, which is why silicate minerals dominate.

The Earth is chemically layered. As a whole it is dominated by iron, oxygen, silicon and magnesium, but these are distributed very unevenly between the layers because the young Earth differentiated: dense iron and nickel sank to form the core, while lighter silicates rose to form the mantle and crust. The core is therefore metallic (iron-nickel), the mantle is made of dense iron- and magnesium-rich silicates such as olivine and pyroxene, and the crust is the thin, light, silicate-rich outer skin on which geology at the surface is played out. Understanding this layering is the foundation for the whole subject, because it explains both what rocks are made of and where the Earth's internal heat and movement come from.

The continental crust and the oceanic crust differ in composition. Continental crust is thick (typically 30 to 70 km), less dense and broadly granitic (silica-rich, or 'acidic'), whereas oceanic crust is thin (about 7 km), denser and basaltic (silica-poorer, or 'basic'). This density contrast is why continents stand high and ocean basins lie low, and why oceanic crust, being denser, is the crust that subducts at destructive margins. The distinction between acidic (silica-rich) and basic (silica-poor) compositions runs right through igneous petrology and is worth fixing now.

By mass, just eight elements make up almost 99% of the crust, and only two of them dominate. Oxygen is by far the most abundant (about 46% by mass) followed by silicon (about 28%); together these two exceed 70% of the crust. Aluminium, iron, calcium, sodium, potassium and magnesium make up most of the remainder. Because oxygen and silicon are so dominant, and because they bond together so readily, the crust is overwhelmingly built of silicate minerals — compounds of silicon and oxygen with the other abundant metals — which is why the silicate structures are the key to mineralogy.

It is important to distinguish the abundance of elements from the abundance of minerals and from economic value. The commonest elements make the commonest minerals (the feldspars, quartz and ferromagnesian silicates), but the metals we mine — copper, gold, lead, zinc — are rare crustal elements that must be concentrated by special geological processes into ores before they are worth extracting, a theme returned to in the resources chapter. Abundance in the crust and concentration into a workable deposit are quite different things.

\text{density} = \frac{\text{mass}}{\text{volume}}

Density

Density (in g/cm^3) is a diagnostic property of a mineral and explains the layering of the Earth: denser materials sank to form the core, less dense silicates rose to form the crust.

The layered structure of the Earth by composition

Fig. 2The Earth is layered by composition: a light silicate crust, a dense silicate mantle, and a metallic iron-nickel core.

Worked example

Using density to identify a mineral

A cube of a mineral has sides of 2.0 cm and a mass of 21.2 g. Calculate its density and comment on whether it is more likely to be quartz (density about 2.65 g/cm^3) or a ferromagnesian mineral such as olivine (density about 3.3 g/cm^3).

Find the volume
The cube has volume $2.0 \times 2.0 \times 2.0 = 8.0$ cm^3.
Apply the density formula
Density = mass / volume = $21.2 / 8.0$ .
$\rho = \frac{21.2\ \text{g}}{8.0\ \text{cm}^3} = 2.65\ \text{g/cm}^3$
Interpret
A density of 2.65 g/cm^3 matches quartz almost exactly and is well below the 3.3 g/cm^3 of olivine, so the mineral is far more likely to be quartz.

Result: The density is 2.65 g/cm^3, identifying the mineral as quartz rather than a denser ferromagnesian mineral.

Typical mistakes

Confusing the composition of the whole Earth (iron-dominated because of the core) with the composition of the crust (oxygen- and silicon-dominated).
Assuming that an abundant element is easy to mine; abundance in the crust is not the same as concentration into a workable ore.

Active revision

Explain why oxygen and silicon dominate the crust yet iron dominates the Earth as a whole, referring to the differentiation of the early Earth.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — rocks and minerals (British Geological Survey)

Minerals and the silicate structures#

StandardWJEC/Eduqas A level Geology (Component 2) — Elements, minerals and rocks

Silicate classification by structure

Fig. 3The silicate minerals classified by how far the SiO4 tetrahedra are linked (polymerised): sharing more oxygens raises the silica content and lightens the mineral.

A mineral is a naturally occurring, inorganic solid with a definite (though sometimes ranging) chemical composition and an ordered internal atomic structure. Each of these words matters: 'naturally occurring' excludes synthetic laboratory crystals, 'inorganic' excludes materials made by life such as pearl or coal, 'definite composition' means a mineral can be written as a formula, and 'ordered atomic structure' means the atoms are arranged in a regular, repeating lattice — the property that gives crystals their flat faces, their cleavage and their characteristic hardness. A rock, by contrast, is an aggregate of one or more minerals, so minerals are the building blocks and rocks are what they build.

The basic unit of every silicate is the silica tetrahedron: a small silicon ion sitting at the centre of four oxygen ions arranged at the corners of a tetrahedron, written SiO4 and carrying a net negative charge. This charged unit is the Lego brick of the crust. Tetrahedra can either stay separate, held together by metal cations between them, or share their corner oxygens with neighbouring tetrahedra to build chains, sheets and three-dimensional frameworks. The degree of this sharing, called polymerisation, controls the whole family of silicate minerals and, remarkably, also controls the shape of their crystals and their cleavage.

As the tetrahedra share more oxygens, the mineral family changes in a predictable way. Isolated (unshared) tetrahedra give the olivines; single chains give the pyroxenes; double chains give the amphiboles; continuous sheets give the micas; and a fully linked three-dimensional framework gives the feldspars and quartz. The more oxygens are shared, the higher the silica content and the fewer metal cations are needed to balance the charge, so the framework silicates (quartz, feldspar) are the silica-rich, lower-density, lighter-coloured minerals, while the isolated- and chain-silicates (olivine, pyroxene) are the silica-poor, denser, darker, iron- and magnesium-rich 'ferromagnesian' minerals. This single idea links composition, density, colour and crystal structure.

The structure is also expressed in the way a mineral breaks. Sheet silicates such as mica have strong bonds within each sheet but weak bonds between sheets, so they split into thin flexible flakes — perfect cleavage in one direction. Chain silicates cleave along the gaps between chains, giving pyroxene two cleavages meeting at about 90 degrees and amphibole two cleavages at about 120 degrees, a difference used to tell them apart. Framework silicates such as quartz have equally strong bonds in all directions and so have no cleavage, breaking instead along curved (conchoidal) fractures. Cleavage is therefore a direct read-out of the hidden atomic structure.

The silica tetrahedron

Fig. 4The SiO4 tetrahedron: one silicon surrounded by four oxygens. Sharing corner oxygens with neighbours (right) builds chains, sheets and frameworks.

Worked example

Deducing structure from cleavage

A dark, elongate mineral in an igneous rock shows two cleavage planes meeting at about 120 degrees. Deduce its silicate structure and likely identity, and contrast it with a mineral showing cleavages at about 90 degrees.

Read the cleavage
Two cleavages at about 120 degrees are characteristic of a double-chain silicate, because the mineral splits along the gaps between the double chains.
Identify
A dark, elongate double-chain silicate is an amphibole, most commonly hornblende.
Contrast
Two cleavages meeting at about 90 degrees indicate a single-chain silicate, a pyroxene such as augite. The cleavage angle is thus the key discriminator between the two dark, chain silicates.

Result: The mineral is an amphibole (double chain, cleavages at about 120 degrees), distinguished from pyroxene (single chain, cleavages at about 90 degrees) by its cleavage angle.

Typical mistakes

Using 'mineral' and 'rock' interchangeably; a rock is an aggregate of one or more minerals.
Forgetting that cleavage angles differ between pyroxene (about 90 degrees) and amphibole (about 120 degrees), a key way to tell the two apart.

Active revision

Explain why mica splits into thin flexible sheets whereas quartz breaks with a curved fracture, referring to their atomic structures.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — discovering geology (British Geological Survey)

Physical properties and mineral identification#

StandardWJEC/Eduqas A level Geology (Component 2) — Elements, minerals and rocks

The Mohs scale of hardness

Fig. 5Mohs' scale of hardness (1 to 10) with everyday reference hardnesses: fingernail ~2.5, knife/glass ~5.5, quartz 7.

Minerals are identified in the hand specimen by a small set of physical properties, no one of which is conclusive alone but which together are decisive. The properties routinely used are hardness, cleavage and fracture, lustre, colour and streak, density, and crystal habit. The examiner's expectation is that you can take a described specimen or a table of properties and work systematically to an identification, rather than guessing from colour, which is the least reliable property because many minerals occur in several colours.

Hardness is measured against Mohs' scale, a relative scale from 1 (talc, marked by a fingernail) to 10 (diamond), in which each mineral scratches those below it. Useful reference points are that a fingernail is about 2.5, a copper coin about 3.5, a steel knife or nail about 5.5, and window glass about 5.5, so a mineral that scratches glass is harder than 5.5. Quartz, at 7, scratches glass easily and cannot be scratched by a knife, which is one of the quickest field tests. The scale is ordinal, not linear: the gap in absolute hardness between 9 (corundum) and 10 (diamond) is far larger than between 1 and 2.

Cleavage is the tendency to break along planes of weakness in the atomic structure, and it is described by the number of cleavage directions and the angles between them; fracture is how a mineral breaks where there is no cleavage (for example the conchoidal fracture of quartz). Lustre describes how the surface reflects light — metallic (like pyrite or galena) or non-metallic (glassy/vitreous like quartz, pearly like mica, dull, or earthy). Streak, the colour of the powder made by rubbing the mineral on a tile, is more reliable than the colour of the whole specimen: haematite, for example, may look grey or red but always gives a red-brown streak.

In practice, identification is a branching key. You test the most diagnostic properties first and narrow down: is the lustre metallic or not; is it harder or softer than a knife or glass; does it have cleavage and in how many directions; what is its streak, and its density? Working through such a key, quartz emerges as hard (7), no cleavage, glassy lustre and conchoidal fracture; the feldspars as hard (6), two cleavages at about 90 degrees; calcite as soft (3), three cleavages giving rhombs and a fizz with dilute acid; and mica as very soft flakes with one perfect cleavage. Confident identification comes from combining several properties, never one.

A simple mineral identification key

Fig. 6Identification proceeds as a branching key: test lustre, then hardness against a knife/glass, then cleavage, combining properties to an identity.

Worked example

Identifying a mineral from its properties

A specimen is glassy, has no cleavage, cannot be scratched by a steel knife, scratches glass, and has a density of about 2.65 g/cm^3. Identify it.

Lustre and cleavage
A glassy (vitreous) lustre with no cleavage and a conchoidal fracture points to a framework silicate.
Hardness
Scratching glass but not being scratched by a knife means the hardness is about 7, above the 5.5 of a knife or glass.
Density and conclusion
A density of 2.65 g/cm^3 combined with hardness 7, glassy lustre and no cleavage is diagnostic of quartz.

Result: The mineral is quartz (hardness 7, no cleavage, glassy lustre, density 2.65 g/cm^3).

Typical mistakes

Identifying a mineral from colour alone, when many minerals occur in several colours (for example quartz and fluorite).
Treating Mohs' scale as linear; it is an ordinal ranking, so equal steps do not mean equal differences in absolute hardness.

Active revision

A colourless-to-white mineral has hardness 3, three cleavages that give rhomb-shaped fragments, and fizzes with dilute hydrochloric acid. Identify it and justify each step.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — minerals (British Geological Survey)

The three rock classes in hand specimen#

StandardWJEC/Eduqas A level Geology (Component 2) — Elements, minerals and rocks

Recognising the three rock classes

Fig. 7A key to the three rock classes by diagnostic texture: interlocking crystals (igneous), clasts and bedding with fossils (sedimentary), or foliation and recrystallisation (metamorphic).

All rocks belong to one of three great classes defined by how they form: igneous rocks crystallise from molten magma or lava, sedimentary rocks are built from the products of weathering and erosion or from chemical and biological precipitation, and metamorphic rocks are pre-existing rocks recrystallised in the solid state by heat and pressure. These three classes are linked in an endless loop, the rock cycle, which is the organising idea of the next chapter; for now the task is to recognise each class in the hand specimen from the texture and fabric it carries as a record of its origin.

Igneous rocks are recognised by a crystalline texture in which interlocking crystals grew from a melt and there are no gaps, no cement and no fossils. The size of the crystals records the cooling rate: slow cooling deep underground (intrusive) grows large, visible crystals as in granite, whereas fast cooling at the surface (extrusive) gives fine crystals, sometimes with a glassy groundmass, as in basalt. The absence of layering or of rounded grains, and the tightly interlocking crystal mosaic, mark a rock as igneous.

Sedimentary rocks are recognised by clasts (fragments) held in a matrix or cement, by layering (bedding), and often by fossils, which occur only in sedimentary rocks. Clastic sedimentary rocks such as sandstone and conglomerate are made of transported grains whose size, sorting and rounding record their journey; other sedimentary rocks form chemically or biologically, such as limestone (calcium carbonate, which fizzes with acid) and rock salt. Bedding planes, ripple marks, graded beds and fossils are all telltale sedimentary features absent from igneous rocks.

Metamorphic rocks are recognised by textures produced in the solid state under directed pressure and heat: a foliation, the parallel alignment of platy minerals such as mica that gives slate its cleavage and schist and gneiss their banding, or a mosaic of recrystallised interlocking grains as in marble and quartzite. Regional metamorphism under directed stress produces foliated rocks; contact metamorphism beside an intrusion, without directed stress, produces non-foliated rocks such as hornfels. The alignment of minerals or the segregation into light and dark bands, combined with an interlocking recrystallised fabric, distinguishes metamorphic rocks from both igneous and sedimentary ones.

Worked example

Classifying a hand specimen

A rock is made of rounded quartz grains cemented together, shows clear layering, and contains a fossil shell. Classify it, name it, and give two lines of evidence for your classification.

Read the fabric
Rounded, cemented grains and visible layering (bedding) indicate a clastic sedimentary origin, not an interlocking igneous mosaic.
Use the fossil
A fossil shell confirms a sedimentary rock, because fossils are found essentially only in sediments.
Name the rock
Rounded, cemented, sand-sized quartz grains make the rock a sandstone.

Result: The rock is a sedimentary sandstone; the evidence is the rounded cemented grains with bedding and the presence of a fossil.

Typical mistakes

Thinking coarse crystals always mean igneous; coarse metamorphic rocks (gneiss) and coarse chemical sediments also exist, so the fabric (interlocking mosaic versus foliation versus cemented clasts) must be read.
Placing fossils in igneous or metamorphic rocks; fossils are essentially confined to sedimentary rocks.

Active revision

A grey rock has flat, parallel bands of light and dark minerals and an interlocking crystalline fabric. State its rock class, name a likely rock, and explain the origin of the banding.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — rock classification (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

British Geological Survey — rocks and minerals

Elements, minerals and rocks

4 sections~15 min reading time3 competenciesLevel Foundation 1 · Standard 3

Elements, minerals and rocks

Key points

Using density to identify a mineral

Key points

Deducing structure from cleavage

Key points

Identifying a mineral from its properties

Key points

Classifying a hand specimen

Sources

Elements, minerals and rocks

Key points

Using density to identify a mineral

Key points

Deducing structure from cleavage

Key points

Identifying a mineral from its properties

Key points

Classifying a hand specimen

Sources