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

# Physical properties; orthorhombic symmetry elements

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2073 Shrawan, 2 marks
- 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-orthorhombic-symmetry-elements
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the physical properties of minerals. What are the elements of symmetry of the orthorhombic system?

## Short answer

Physical properties include colour, streak, lustre, hardness, cleavage, fracture, tenacity, and specific gravity. The orthorhombic normal class has 3 planes of symmetry, 3 diad axes, and 1 centre of symmetry.

## Answer

#### Physical properties of minerals
- **Colour** and **streak** (colour of powder).
- **Lustre**: metallic or non-metallic (vitreous, pearly, silky, earthy).
- **Hardness**: Mohs scale (1-10).
- **Cleavage** and **fracture**.
- **Tenacity**: brittle, malleable, elastic.
- **Specific gravity**.
- **Transparency** and **crystal habit**.
- Special: magnetism, taste, effervescence in HCl.

#### Symmetry elements of the orthorhombic system
Axes: $a \ne b \ne c$, all at 90°. The normal class has:
- **3 planes of symmetry** (each parallel to a pair of axes).
- **3 diad axes** (along the a, b and c axes).
- **1 centre of symmetry**.

Total: 7 elements.

## Check yourself

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

1. Which is a physical property used to identify minerals?
   A. Streak, lustre, hardness and cleavage
   B. Seismic intensity
   C. Dip and strike
   D. Isoseismal lines
   Answer: A. These are hand-specimen properties. (Key point: Properties)

2. How many planes of symmetry does the orthorhombic normal class have?
   A. 9
   B. 3
   C. 1
   D. 5
   Answer: B. Each parallel to a pair of axes. (Key point: Orthorhombic planes)

3. The orthorhombic axes of symmetry are:
   A. 3 tetrad axes
   B. 4 triad axes
   C. 3 diad axes along a, b and c
   D. 6 diad axes
   Answer: C. Total of 7 elements. (Key point: Orthorhombic axes of symmetry)

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