---
title: "Engineering significance of rock-forming minerals"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:engineering-significance-of-rockforming-minerals"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/engineering-significance-of-rockforming-minerals
site: BCT Engineering
---

# Engineering significance of rock-forming minerals

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2065 Shrawan (old course), 2 marks; 2063 Baisakh (old course), 2 marks; 2059 Chaitra (old course), 2 marks (asked 3 times)
- Video: [Common Rock-forming Minerals | Physical and Chemical Properties of Minerals | Earth and Life Science](https://www.youtube.com/watch?v=DWglWyoD_t4) — Science Simplified
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/engineering-significance-of-rockforming-minerals
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the engineering significance of rock-forming minerals (quartz, calcite, feldspar and mica).

## Short answer

Quartz imparts hardness and durability but is abrasive and reactive forms can cause concrete expansion. Calcite dissolves in acidic water, creating solution cavities that threaten foundations and dams. Feldspar weathers to weak, swelling clay, reducing rock strength and causing slope failures. Mica's perfect cleavage makes rocks weak along foliation, leading to instability and poor aggregate.

## Answer

The properties of rock-forming minerals decide the strength, durability and behaviour of rocks used in foundations, tunnels and construction materials.

- **Quartz (SiO₂)**: very hard (7), chemically stable, no cleavage. Rocks rich in quartz (granite, quartzite, sandstone) are strong, durable and resistant to weathering, so good for aggregate, foundations and building stone. Its abrasive nature wears drilling tools and crushers. Reactive (amorphous) silica can cause alkali-aggregate reaction in concrete.
- **Calcite (CaCO₃)**: soft (3), perfect cleavage, dissolves in acidic water. Limestone and marble make cement, lime and building stone. Solution cavities in limestone cause reservoir leakage and foundation settlement, a risk for dams.
- **Feldspar**: hardness 6, two cleavages; weathers to clay minerals, so feldspar-rich rocks lose strength with weathering. The clay formed is weak, swells and is slippery, and can cause slope failure.
- **Mica**: very perfect cleavage, soft, flaky. Rocks rich in mica (schist, phyllite) split easily, are weak along foliation, and cause slope and tunnel instability. Mica in aggregate reduces concrete strength.

## Check yourself

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

1. Why are quartz-rich rocks good for aggregate and foundations?
   A. Quartz is soft and easily shaped
   B. Quartz dissolves to bind concrete
   C. Quartz is hard, chemically stable and has no cleavage
   D. Quartz has perfect cleavage
   Answer: C. Hardness 7, stability and no cleavage make rocks strong and durable. (Key point: Quartz)

2. What problem does calcite (limestone) pose for dams?
   A. It swells like clay
   B. Solution cavities cause reservoir leakage and foundation settlement
   C. It is too hard to excavate
   D. It splits along foliation
   Answer: B. Calcite dissolves in acidic water, forming cavities. (Key point: Calcite)

3. Why does feldspar reduce rock strength with time?
   A. It weathers to weak, swelling, slippery clay minerals
   B. It dissolves completely in rain
   C. It shrinks when dry
   D. It reacts with cement
   Answer: A. Clay formed from feldspar weakens rock and causes slope failure. (Key point: Feldspar)

4. Why do mica-rich rocks cause slope and tunnel instability?
   A. Mica is extremely hard
   B. Mica has no cleavage
   C. Perfect cleavage makes them split easily and they are weak along foliation
   D. Mica expands strongly on wetting
   Answer: C. Soft, flaky mica creates planes of weakness. (Key point: Mica)

## Related in Engineering Geology I

- [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×
- [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
- [Calcite versus quartz](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Acalcite-versus-quartz) — Engineering Geology I, chapter 3
- [Engineering significance of three rock types](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-three-rock) — Engineering Geology I, chapter 4, asked 5×
- [Petrology; engineering significance of rock classes](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrology-engineering-significance-of-rock) — Engineering Geology I, chapter 4
- [Metamorphic rock, cleavage; igneous rock significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphic-rock-cleavage-igneous-rock) — Engineering Geology I, chapter 4

## Related in other subjects

- [Rock forming minerals and significance](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-forming-minerals) — Engineering Geology II, chapter 1, asked 6×
- [Calcite significance for reservoir site](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acalcite-reservoir-significance) — Engineering Geology II, chapter 1
- [Quartz significance for hydropower project](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aquartz-hydropower-significance) — Engineering Geology II, chapter 1