---
title: "Orthorhombic system; isometric mineral examples"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:orthorhombic-system-isometric-mineral-examples"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/orthorhombic-system-isometric-mineral-examples
site: BCT Engineering
---

# Orthorhombic system; isometric mineral examples

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2062 Baisakh (old course), 3+2 marks
- Video: [The 7 Crystal Systems!](https://www.youtube.com/watch?v=11Ng8CJNE7Y) — Rock Talk
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/orthorhombic-system-isometric-mineral-examples
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the orthorhombic crystal system with a suitable diagram and give examples of three minerals belonging to the isometric system.

## Short answer

The orthorhombic system has three unequal axes at right angles, with 3 planes, 3 diad axes, and a centre of symmetry. Three isometric minerals are galena, pyrite, and halite.

## Answer

#### Orthorhombic system
The **orthorhombic system** has three crystallographic axes at right angles to each other, all of **different lengths**.

$$
a \ne b \ne c, \qquad \alpha = \beta = \gamma = 90^\circ
$$

```text
        c
        |
        |_____ b
       /
      a        a, b, c unequal, all 90 deg
```

Symmetry of the normal class: 3 planes of symmetry, 3 diad axes and 1 centre of symmetry (7 elements). Typical forms are rhombic prisms and pyramids. Minerals: topaz, barite, olivine, sulphur, aragonite.

#### Three isometric minerals
The isometric (cubic) system has $a = b = c$, all axes at 90°. Examples:
1. **Galena** (PbS)
2. **Pyrite** (FeS₂)
3. **Halite** (NaCl); fluorite, garnet, magnetite and diamond are also isometric.

## Check yourself

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

1. Which describes the orthorhombic axes?
   A. Three perpendicular axes, all of different lengths
   B. Three equal axes at 90 degrees
   C. Three unequal axes, none at 90 degrees
   D. Four axes, three at 120 degrees
   Answer: A. a not equal b not equal c with all angles 90 degrees. (Key point: Orthorhombic axes)

2. Total symmetry elements of orthorhombic normal class:
   A. 23
   B. 7 (3 planes, 3 diad axes, 1 centre)
   C. 13
   D. 9
   Answer: B. 3 + 3 + 1 = 7. (Key point: Symmetry count)

3. Which three are isometric minerals?
   A. Topaz, barite, olivine
   B. Gypsum, orthoclase, mica
   C. Galena, pyrite, halite
   D. Quartz, calcite, dolomite
   Answer: C. Galena (PbS), pyrite (FeS2) and halite (NaCl) are cubic. (Key point: Isometric minerals)

## Close variants set in other exams

- [Orthorhombic system; monoclinic mineral examples](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aorthorhombic-system-monoclinic-mineral-examples) — Engineering Geology I, chapter 3, asked 2×

## Related in Engineering Geology I

- [Isometric crystal system with symmetry elements](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aisometric-crystal-system-with-symmetry) — Engineering Geology I, chapter 3, asked 5×
- [Symmetry elements, crystal systems, rock-forming minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Asymmetry-elements-crystal-systems-rockforming) — Engineering Geology I, chapter 3
- [Axis of symmetry; isometric system](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aaxis-of-symmetry-isometric-system) — Engineering Geology I, chapter 3
- [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