G6: Minerals & Their Properties
What minerals are, how they are identified by their physical properties, and the common rock-forming minerals.
Minerals & Their Properties
What minerals are, how they are identified by their physical properties, and the common rock-forming minerals.
Key Fact: A mineral is a naturally occurring, inorganic solid with a definite chemical composition and crystalline structure
Key Fact: Silicates (containing SiO4 tetrahedra) are the most common mineral group, making up ~90% of Earth's crust
Key Fact: Key properties for mineral identification: hardness (Mohs scale 1-10), cleavage, fracture, lustre, colour, streak, density and crystal habit
Key Fact: Common rock-forming minerals: quartz, feldspar, mica, calcite, olivine, pyroxene, amphibole
Key Fact: Hardness is measured on the Mohs scale: talc (1) to diamond (10); each mineral can scratch those below it
Key Fact: Cleavage is the tendency of a mineral to break along flat planes of weakness in its crystal structure
📋 Key Vocabulary and Concepts
For Minerals & Their Properties, you must know:
- Silicate: A mineral containing silicon and oxygen in SiO4 tetrahedra; the largest and most abundant mineral group
- Mohs scale: A scale of mineral hardness from 1 (talc, softest) to 10 (diamond, hardest), based on scratch resistance
- Cleavage: The tendency of a mineral to break along flat planes determined by weaknesses in its crystal lattice
- Streak: The colour of the fine powder left when a mineral is scraped across an unglazed porcelain plate
- Lustre: The way a mineral surface reflects light (e.g. metallic, vitreous/glassy, pearly, earthy)
❓ Practice Questions
Q: Define the term 'mineral'.
Q: How does the Mohs hardness scale work?
Q: What is the difference between cleavage and fracture?
Q: Name the five most common rock-forming minerals in Earth's crust.
Q: Why is streak more reliable than colour for mineral identification?
✅ Answers
- A naturally occurring, inorganic solid with a definite chemical composition and a regular crystalline structure.
- Minerals are ranked 1 (talc, softest) to 10 (diamond, hardest). A mineral can scratch any mineral with a lower hardness number. For example, quartz (7) scratches feldspar (6) but not topaz (8).
- Cleavage is a clean break along flat planes of weakness in the crystal structure (e.g. mica splitting into thin sheets). Fracture is an irregular break not along a plane of weakness (e.g. quartz showing conchoidal fracture).
- Feldspar (most abundant), quartz, mica, amphibole and pyroxene, all are silicates.
- Colour can vary due to impurities (e.g. quartz can be clear, pink, purple, black). Streak (the powder colour) is more consistent and diagnostic, for example, haematite is always red-brown in streak regardless of its surface colour.
🎯 Exam Tips
- Mohs scale: 1=talc, 10=diamond, know common minerals at each level (calcite=3, quartz=7)
- Five criteria for a mineral: naturally occurring, inorganic, solid, definite composition, crystalline structure
- Silicates dominate the crust, know that SiO4 tetrahedra are the building blocks
- Cleavage vs fracture is a common question, cleavage is along flat planes, fracture is irregular
📝 Exam Technique
GCSE Geology Exam Tips — Minerals & Their Properties:
1. For Minerals & Their Properties questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Minerals & Their Properties to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Minerals & Their Properties questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Minerals & Their Properties to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ All minerals are silicates ✓ While silicates are the most common group (~90% of crust), other mineral groups include carbonates (calcite), oxides (haematite), sulfides (pyrite), halides (halite) and native elements (gold)
✗ A mineral can have more than one streak colour ✓ Each mineral has one consistent streak colour regardless of surface appearance, this is what makes streak a reliable identification property
✗ Hardness on the Mohs scale is linear ✓ The Mohs scale is ordinal, not linear, the absolute hardness difference between corundum (9) and diamond (10) is far greater than between talc (1) and gypsum (2)
✍️ Model Answer
Full-Mark Response
Describe the physical properties used to identify minerals and explain why multiple properties are needed. [6 marks]
Minerals are identified using several physical properties. Hardness (Mohs scale 1-10) measures scratch resistance: talc (1) is softest, diamond (10) is hardest. Cleavage describes breakage along flat planes of crystal weakness (e.g. mica has perfect basal cleavage splitting into thin sheets); fracture describes irregular breakage (e.g. quartz shows conchoidal fracture). Lustre describes light reflection: metallic, vitreous (glassy), pearly, earthy. Streak is the powder colour on an unglazed plate, more reliable than surface colour, which varies with impurities. Colour is the most obvious but least reliable property (e.g. quartz ranges from clear to purple to black). Crystal habit describes the typical external shape (e.g. cubic for halite, hexagonal for quartz). Density (specific gravity) distinguishes minerals of similar appearance. Multiple properties are needed because no single property is diagnostic alone. For example, quartz and feldspar can both be pink and glassy, but quartz has conchoidal fracture while feldspar has two cleavage directions at 90 degrees. Pyrite and gold are both gold-coloured, but pyrite has a greenish-black streak while gold has a golden streak. Using multiple properties eliminates ambiguity and allows confident identification.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of minerals & their properties, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of minerals & their properties to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to minerals & their properties, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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