Which physical properties let you identify a mineral in hand specimen?
Identifying minerals by physical properties: hardness (the Mohs scale), cleavage and fracture, lustre, colour and streak, habit, density (specific gravity) and special properties (magnetism, reaction with acid); and the use of these diagnostic properties to identify the common rock-forming and ore minerals in hand specimen.
A focused answer to the Eduqas Geology statement on mineral identification. Covers hardness and the Mohs scale, cleavage and fracture, lustre, colour and streak, habit, density and special properties, and how to combine diagnostic properties to identify common minerals in hand specimen for Component 1.
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What this dot point is asking
Eduqas wants you to know the diagnostic physical properties of minerals (hardness on the Mohs scale, cleavage and fracture, lustre, colour and streak, habit, density and special properties such as magnetism and reaction with acid) and to combine them to identify the common rock-forming and ore minerals in hand specimen. This is the practical skill examined directly in Component 1 (Geological Investigations), where you interpret specimens and photographs.
The answer
Hardness and the Mohs scale
Hardness is resistance to scratching, ordered on the Mohs scale from 1 (talc) to 10 (diamond): talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10. The scale is relative, not linear (diamond is far harder than the steps suggest). In the field you use simple references: 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 but not steel is harder than about 5.5; one scratched by a fingernail is softer than 2.5 (for example gypsum or talc).
Cleavage and fracture
Cleavage is the tendency to break along planes of weak bonding, described by the number of directions and the angles between them: mica has one perfect cleavage (flat flakes); feldspar and pyroxene have two near 90 degrees; amphibole two near 60 and 120 degrees; calcite three not at right angles (rhombs); halite and galena three at right angles (cubes). Fracture is irregular breakage where there is no cleavage: quartz shows a smooth, curved conchoidal fracture. Cleavage is one of the most reliable diagnostic properties because it reflects the internal structure directly.
Lustre, colour and streak
- Lustre is the way the surface reflects light: metallic (galena, pyrite, magnetite) or non-metallic, which is subdivided into glassy or vitreous (quartz, feldspar), pearly (mica), resinous, earthy (some hematite) and silky.
- Colour is the least reliable property because impurities and coatings change it (quartz can be colourless, pink, purple or smoky).
- Streak is the colour of the powder, made on an unglazed porcelain streak plate. It is far more reliable than colour: hematite, black or red in the specimen, always gives a diagnostic red-brown streak.
Habit, density and special properties
- Habit is the characteristic shape: cubic (pyrite, galena), prismatic (hornblende), platy (mica), fibrous, massive, botryoidal (rounded, for example some hematite) or bladed.
- Density (specific gravity) is mass relative to water; you can estimate it by "heft". Galena (about 7.5) and magnetite (about 5) feel noticeably heavy, while quartz (2.65) and the feldspars (about 2.6) feel ordinary.
- Special properties clinch many identifications: magnetism (magnetite is strongly magnetic), reaction with dilute acid (calcite fizzes vigorously, releasing carbon dioxide; dolomite only fizzes when powdered), taste (halite is salty) and double refraction (clear calcite, Iceland spar).
Combining the properties
No single property is decisive on its own; you build an identification by combining several. A glassy, colourless mineral with no cleavage, conchoidal fracture and hardness 7 is quartz. A pale mineral with two cleavages near 90 degrees and hardness 6 is a feldspar. A dark mineral with one perfect cleavage into flakes is a mica. A soft (hardness 3) mineral with rhombic cleavage that fizzes in acid is calcite.
Examples in context
Example 1. Calcite versus quartz. Both can be colourless, but calcite is soft (hardness 3, scratched by a knife), shows rhombic cleavage and fizzes in dilute acid, while quartz is hard (7), has no cleavage and conchoidal fracture and does not react. Three quick tests separate them.
Example 2. Identifying ore minerals. Heft, metallic lustre and streak distinguish the ore sulphides (galena very heavy with a grey streak, chalcopyrite brassy with a greenish-black streak), the skill needed in the economic geology specimens.
Try this
Q1. State the references used in the field for hardness 2.5, 3.5 and 5.5. [2 marks]
- Cue. Fingernail about 2.5, copper coin about 3.5, steel knife or window glass about 5.5.
Q2. Explain why streak is more reliable than colour for identifying hematite. [2 marks]
- Cue. Hematite varies from black specular to red earthy in colour, but its powdered streak is always the same diagnostic red-brown, because the powder removes the effect of crystal size and coatings.
Q3. Describe two quick tests that distinguish calcite from quartz. [2 marks]
- Cue. Calcite is soft (scratched by a steel knife) and fizzes in dilute acid; quartz is hard (scratches glass) and does not react with acid.
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 20196 marksA hand specimen contains a black mineral with a metallic lustre, a hardness of about 6, a grey-black streak and which is strongly attracted to a magnet. Identify the mineral and explain how three of these properties helped you, distinguishing it from a similar-looking mineral.Show worked answer →
A levels-of-response answer; name the mineral, then justify with the diagnostic properties.
- Identity
- The mineral is magnetite (an iron oxide).
- The diagnostic properties
- The strong magnetism is the decisive test: very few common minerals are strongly magnetic, so this points almost uniquely to magnetite. The grey-black streak (the colour of the powder) confirms an iron oxide and separates it from hematite, which has a distinctive red-brown streak even when the specimen looks black. The metallic lustre and hardness of about 6 are consistent with magnetite and rule out softer sulphides.
- Distinguishing from a similar mineral
- Specular hematite also looks black and metallic, but it is not strongly magnetic and gives a red-brown streak, so the magnetism and streak together separate the two.
Top-band answers name magnetite, use the magnetism and streak as the decisive tests, and explicitly contrast it with hematite using the streak.
Eduqas 20214 marksExplain why streak is often a more reliable diagnostic property than the colour of a mineral. Illustrate your answer with one example.Show worked answer →
Explain the limitation of colour, then why streak is more reliable.
- Why colour is unreliable
- The colour of a mineral specimen can vary widely because of small amounts of impurities, weathering coatings or trace elements. Quartz, for example, can be colourless, pink, purple or smoky, so colour alone cannot identify it.
- Why streak is reliable
- Streak is the colour of the mineral's powder, made by rubbing it on an unglazed porcelain tile. The powder removes the effect of surface coatings and crystal size, so it gives a more consistent, characteristic colour for a given mineral.
- Example
- Hematite occurs as black specular crystals and as red earthy masses, but both give the same diagnostic red-brown streak, so the streak identifies it whatever the specimen looks like.
Markers reward the variability of colour (impurities, coatings), the definition of streak as the powder colour, and a valid example such as hematite.
Related dot points
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Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)