---
title: "Physical properties of minerals"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:physical-properties-of-minerals"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/physical-properties-of-minerals
site: BCT Engineering
---

# Physical properties of minerals

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2081 Baisakh (new course), 2 marks; 2081 Bhadra, 2 marks; 2068 Baisakh, 2 marks; 2066 Bhadra (old course), 6 marks; 2065 Shrawan (old course), 5 marks; 2063 Baisakh (old course), 4 marks (asked 6 times)
- Video: [10 Physical Characteristics / Properties of Minerals](https://www.youtube.com/watch?v=DyNjp3fQRIo) — Amit Sengupta
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/physical-properties-of-minerals
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the physical properties of minerals.

## Short answer

Key physical properties are colour, streak (powder colour on porcelain), lustre (metallic or vitreous etc.), hardness (Mohs scale), cleavage (smooth planar breaks), fracture, tenacity, specific gravity, and crystal form/habit. Some minerals show magnetism, effervesscence with acid, or taste.

## Answer

The physical properties of minerals depend on chemical composition and crystal structure, and are used to identify minerals in hand specimens.

#### 1. Colour
Appearance in reflected light. **Idiochromatic** minerals have constant colour (malachite green); **allochromatic** minerals vary because of impurities (quartz can be white, pink, purple). Colour is not always reliable.

#### 2. Streak
Colour of the powder of a mineral, seen on an unglazed porcelain plate. It is more constant than colour: hematite gives a red-brown streak even when the specimen looks black.

#### 3. Lustre
Appearance of a fresh surface in reflected light.
- Metallic (galena, pyrite).
- Non-metallic: vitreous (quartz), pearly (talc), silky (asbestos), resinous, greasy, earthy (kaolin), adamantine (diamond).

#### 4. Transparency
Transparent, translucent or opaque.

#### 5. Hardness
Resistance to scratching, measured on the Mohs scale (1 to 10).

#### 6. Cleavage
Tendency to split along smooth planes of weak bonding. Described as perfect (mica, one direction), good (feldspar, two directions), poor or absent. Calcite has three directions, not at right angles.

#### 7. Fracture
Irregular breaking where there is no cleavage: conchoidal (quartz), uneven, splintery, hackly.

#### 8. Tenacity
Behaviour under stress: brittle, malleable, sectile, flexible, elastic.

#### 9. Specific gravity
Ratio of the mineral's density to that of water (quartz 2.65, galena 7.5).

#### 10. Form / habit
Crystal shape: prismatic, tabular, fibrous, granular, massive.

#### 11. Other properties
Magnetism (magnetite), taste (halite), effervescence with dilute HCl (calcite), feel (talc greasy), and fluorescence.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is streak more reliable than colour for identification?
   A. Streak measures hardness
   B. The colour of the powder stays constant even when the mineral's colour varies
   C. Streak shows the cleavage direction
   D. Streak depends on crystal shape
   Answer: B. Powder colour is fairly constant; hematite is red-brown streak although it may look black. (Key point: Streak)

2. How do cleavage and fracture differ?
   A. Cleavage is irregular breaking; fracture is smooth splitting
   B. Cleavage is splitting along smooth planes of weak bonding; fracture is irregular breakage without such planes
   C. Both are resistance to scratching
   D. Cleavage is colour of powder; fracture is shape
   Answer: B. Cleavage follows weak planes; fracture is irregular, e.g. conchoidal in quartz. (Key point: Cleavage vs fracture)

3. Allochromatic minerals are those whose colour:
   A. Is constant, as in malachite
   B. Is only seen in powder
   C. Varies because of impurities, as in quartz
   D. Depends on cleavage
   Answer: C. Idiochromatic minerals have constant colour; allochromatic vary. (Key point: Colour types)

4. Specific gravity of a mineral is:
   A. Its hardness on Mohs scale
   B. Its resistance to breaking
   C. Its shine in reflected light
   D. The ratio of its density to that of water
   Answer: D. SG is a density ratio relative to water, e.g. quartz 2.65, galena 7.5. (Key point: Specific gravity)

## Related in Engineering Geology I

- [Physical and optical properties of minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-and-optical-properties-of) — Engineering Geology I, chapter 3, asked 2×
- [Physical properties; orthorhombic symmetry elements](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-properties-orthorhombic-symmetry-elements) — Engineering Geology I, chapter 3
- [Feldspar and mica groups of minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afeldspar-and-mica-groups-of) — Engineering Geology I, chapter 3, asked 2×

## Related in other subjects

- [Physical and mechanical properties of wood](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aphysical-mechanical-properties-wood) — Civil Engineering Materials, chapter 7
- [Physical properties of OPC and concrete behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-physical-properties) — Concrete Technology and Masonry Structure, chapter 1