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

# Physical and optical properties of minerals

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2068 Chaitra, 4 marks; 2074 Chaitra, 3 marks (asked 2 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-and-optical-properties-of
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the physical and optical properties of minerals (in hand specimen).

## Short answer

Physical properties include colour, streak, hardness on Mohs scale, cleavage (e.g., mica one, calcite three), fracture (conchoidal in quartz), specific gravity, and crystal habit. Optical properties are lustre (metallic, vitreous, pearly), transparency, double refraction (calcite), and effects like fluorescence and pleochroism.

## Answer

Minerals in hand specimen are identified by their physical properties, many of which relate to the way they interact with light (optical properties).

#### Physical properties
- **Colour**: general appearance; may vary with impurities (quartz).
- **Streak**: colour of powder on unglazed porcelain; more constant than colour.
- **Hardness**: resistance to scratching on the Mohs scale (1 to 10).
- **Cleavage**: splitting along smooth planes (mica one, feldspar two, calcite three directions).
- **Fracture**: irregular breakage without cleavage (conchoidal in quartz).
- **Tenacity**: brittle, malleable, flexible, elastic.
- **Specific gravity**: relative density (quartz 2.65, galena 7.5), tested by heft.
- **Habit/form**: shape of crystal aggregates (prismatic, fibrous, granular).
- **Others**: magnetism, taste, feel, effervescence with HCl.

#### Optical properties
- **Lustre**: metallic (galena), vitreous (quartz), pearly (talc), silky (asbestos), resinous, greasy, earthy, adamantine.
- **Transparency**: transparent (clear quartz), translucent (milky quartz), opaque (pyrite).
- **Colour and play of colour**: opalescence, iridescence, asterism.
- **Refraction and double refraction**: calcite shows two images of an object seen through it.
- **Fluorescence and phosphorescence**: glow under ultraviolet light (fluorite).
- **Pleochroism**: change of colour with viewing direction (seen under the microscope).

## Check yourself

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

1. Which pairing of mineral and cleavage is correct?
   A. Mica one direction, feldspar two, calcite three
   B. Mica three, feldspar two, calcite one
   C. Mica two, feldspar one, calcite three
   D. Mica one, feldspar three, calcite two
   Answer: A. Mica: one perfect; feldspar: two; calcite: three. (Key point: Cleavage directions)

2. Pleochroism is:
   A. Glow under ultraviolet light
   B. Change of colour with viewing direction, seen under the microscope
   C. Two images through the mineral
   D. Colour of the powder
   Answer: B. Fluorescence is the glow and double refraction gives two images. (Key point: Pleochroism)

3. Which mineral shows two images of an object seen through it?
   A. Quartz
   B. Pyrite
   C. Calcite
   D. Talc
   Answer: C. Calcite exhibits strong double refraction. (Key point: Double refraction)

4. Which pairing is correct?
   A. Clear quartz is opaque, pyrite transparent
   B. Milky quartz is transparent, pyrite translucent
   C. Clear quartz is transparent, milky quartz translucent, pyrite opaque
   D. Pyrite is translucent, clear quartz opaque
   Answer: C. Transparency grades from transparent to translucent to opaque. (Key point: Transparency)

## Related in Engineering Geology I

- [Physical properties of minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-properties-of-minerals) — Engineering Geology I, chapter 3, asked 6×
- [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

- [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6
- [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