How do geologists identify a mineral from its physical properties?
Minerals are identified using physical properties: colour, crystal size, hardness (tested against fingernail, copper coin, steel and glass), cleavage and fracture, lustre, streak, and the reaction of carbonates with dilute hydrochloric acid; common minerals include quartz, feldspar, mica, calcite, halite, galena and haematite.
A focused answer to the Eduqas GCSE Geology statement on identifying minerals. Covers the physical properties used (colour, crystal size, hardness, cleavage and fracture, lustre, streak and the acid test) and the diagnostic features of quartz, feldspar, mica, calcite, halite, galena and haematite.
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What this dot point is asking
Eduqas wants you to identify a mineral from its physical properties rather than by guessing from its colour. You need to know the properties geologists test (colour, crystal size, hardness, cleavage and fracture, lustre, streak, and the reaction of carbonates with dilute hydrochloric acid), how each test is carried out, and the diagnostic features of the common minerals quartz, feldspar, mica, calcite, halite, galena and haematite. This is the practical skill at the heart of Component 2, where you identify minerals in hand specimen.
The answer
Why no single property is enough
A mineral is a naturally occurring solid with a definite chemical composition and an ordered internal structure. To identify one, you combine several properties, because any one on its own can mislead. Colour is the least reliable; streak, hardness and cleavage are the most reliable.
The properties you test
- Colour. Useful as a first impression but unreliable, because impurities, coatings and crystal size change it. Quartz alone can be clear, white, pink, purple or grey.
- Crystal size. Whether crystals are coarse (large, easily seen with the naked eye) or fine (too small to see individually). This also helps with rocks later.
- Hardness. Resistance to scratching. Test against common objects: a fingernail (), a copper coin (), a steel blade or nail () and glass (). Quartz scratches glass; calcite does not. You record relative hardness, for example "harder than a coin but softer than steel".
- Cleavage and fracture. Cleavage is the tendency to split along flat planes in definite directions (mica has one perfect cleavage and peels into sheets; galena has cubic cleavage). Fracture is breakage along uneven or curved surfaces (quartz has a curved, glassy "conchoidal" fracture and no cleavage).
- Lustre. The way the surface reflects light: metallic (galena, pyrite) or non-metallic (glassy, dull, pearly or earthy).
- Streak. The colour of the mineral's powder, made by rubbing it on an unglazed porcelain plate. Far more reliable than colour: haematite is red-brown, whatever the specimen looks like.
- Reaction with dilute hydrochloric acid. Carbonates fizz (effervesce), releasing carbon dioxide. Calcite fizzes vigorously; this also identifies the limestone and marble that contain it.
The named minerals you must know
| Mineral | Key diagnostic features |
|---|---|
| Quartz | Hard (scratches glass), no cleavage, glassy (conchoidal) fracture, often clear or white |
| Feldspar | Hard, two good cleavages at about a right angle, often pink or white, common in granite |
| Mica | One perfect cleavage, peels into thin flexible sheets, soft, shiny |
| Calcite | Soft (hardness 3), rhombic cleavage, fizzes in dilute acid |
| Halite | Cubic cleavage, soft, salty taste, dissolves in water (an evaporite mineral) |
| Galena | Very dense (heavy heft), metallic lustre, grey, cubic cleavage, an ore of lead |
| Haematite | Red-brown streak whatever the colour, an ore of iron, can be earthy or metallic |
Examples in context
Example 1. The acid bottle in the field. A geologist carries a small bottle of dilute hydrochloric acid precisely because the fizz test instantly separates the carbonate rocks (limestone, marble) from silica-rich rocks that look similar but do not react.
Example 2. Mistaking pyrite for gold. "Fool's gold" (pyrite) is brassy and metallic like gold, but pyrite is hard and brittle with a black streak, while gold is soft, dense and has a golden streak. Streak and hardness settle it.
Try this
Q1. State which two properties are usually the most reliable for identifying a mineral, and which is the least reliable. [3 marks]
- Cue. Most reliable: streak and hardness (also cleavage). Least reliable: colour, because impurities and coatings change it.
Q2. Describe how you would test the hardness of an unknown mineral in the field. [2 marks]
- Cue. Try to scratch it with a fingernail (2.5), a copper coin (3.5), a steel blade (5.5) and glass (5.5), and record which objects do and do not scratch it.
Q3. Name the test that identifies calcite and state what you would observe. [2 marks]
- Cue. Add dilute hydrochloric acid; calcite fizzes (effervesces), releasing carbon dioxide.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksA student is given an unknown mineral. Describe four physical tests they could carry out to help identify it, and state what each test tells them.Show worked answer →
Give four distinct tests, each with what it reveals. One mark each.
- Hardness
- Try to scratch the mineral with a fingernail (2.5), a copper coin (3.5), a steel blade (5.5) and a piece of glass (5.5). This places the mineral on a relative hardness scale (for example "harder than a coin but softer than steel").
- Streak
- Rub the mineral across an unglazed porcelain streak plate. The colour of the powder is more reliable than the specimen colour (haematite always gives a red-brown streak).
- Cleavage or fracture
- Look at how the mineral breaks. Cleavage gives flat shiny planes in set directions (mica splits into sheets), whereas fracture gives uneven or curved surfaces (quartz fractures like glass).
- Acid test
- Add a drop of dilute hydrochloric acid. Calcite (and the limestone and marble it forms) fizzes, releasing carbon dioxide; quartz and most silicates do not.
Other acceptable tests are lustre (metallic versus non-metallic), colour, crystal size and density by heft. Markers reward four different tests each paired with the property it identifies.
Eduqas 20213 marksExplain why streak is often a more reliable identification test than colour, using haematite as an example.Show worked answer →
A short explain question; make the comparison explicit.
Colour is unreliable because small amounts of impurity, surface coatings, weathering and crystal size all change the colour you see in a hand specimen. The same mineral can appear in several colours.
Streak is consistent because grinding the mineral to a powder on the streak plate removes the effect of impurities and coatings, so the powder colour stays the same.
Haematite shows this clearly: a specimen may look silvery-grey, black or red, but its streak is always red-brown, which identifies it.
Top answers state that colour varies with impurities and coatings, that powdering removes that effect, and give the haematite red-brown streak as the worked example.
Related dot points
- Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.
A focused answer to the Eduqas GCSE Geology statement on igneous rocks. Covers how magma and lava crystallise, how cooling rate controls crystal size (intrusive granite versus extrusive basalt), classification by silica content (felsic to mafic), and the crystallisation of minerals from hydrothermal fluids in veins.
- Sedimentary rocks form by weathering, erosion, transport, deposition, and lithification (compaction and cementation); they are classified as clastic (conglomerate, breccia, sandstone, shale), biological (limestone) or chemical (evaporites); grain size, shape, sorting, sedimentary structures and fossil content are used to interpret the depositional environment; fossils form by preservation of hard parts and record past life.
A focused answer to the Eduqas GCSE Geology statement on sedimentary rocks. Covers weathering, transport, deposition and lithification, the clastic, biological and chemical classes (conglomerate, sandstone, shale, limestone, evaporites), reading the depositional environment from grain size, sorting and structures, and how fossils form and what they record.
- Metamorphic rocks form by recrystallisation of existing rocks in the solid state under heat and pressure, without melting; contact metamorphism (heat from an intrusion) produces non-foliated rocks such as metaquartzite and marble; regional metamorphism (heat and directed pressure over a wide area) produces foliated rocks such as slate and schist; protolith and conditions determine the product.
A focused answer to the Eduqas GCSE Geology statement on metamorphic rocks. Covers solid-state recrystallisation under heat and pressure, the difference between contact metamorphism (non-foliated metaquartzite and marble) and regional metamorphism (foliated slate and schist), foliation, and how the protolith and conditions set the product.
- The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.
A focused answer to the Eduqas GCSE Geology statement on the rock cycle. Covers the three rock families and the processes that connect them (weathering, erosion, transport, deposition, lithification, melting, crystallisation and metamorphism), the two energy sources that drive the cycle, and how any rock can become any other.
- Fieldwork involves recording observations systematically: making annotated field sketches, recording rock type, colour, grain size, texture, structures and fossils, measuring features such as dip and bed thickness, and identifying hand specimens of minerals and rocks using their physical properties; observations must be objective, located on a map or grid reference, and recorded safely and accurately so they can be interpreted later.
A focused answer to the Eduqas GCSE Geology statement on field observation. Covers recording observations systematically (annotated field sketches, rock type, grain size, texture, structures, fossils), measuring features in the field, identifying hand specimens by physical properties, and recording objectively, located and safely.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)