---
title: "Feldspar and mica groups of minerals"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:feldspar-and-mica-groups-of"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/feldspar-and-mica-groups-of
site: BCT Engineering
---

# Feldspar and mica groups of minerals

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2064 Jestha (old course), 3 marks; 2059 Chaitra (old course), 3 marks (asked 2 times)
- Video: [15) Feldspars and Quartz](https://www.youtube.com/watch?v=AiAu7eMGaFI) — CVshorey
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/feldspar-and-mica-groups-of
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Describe the properties of the feldspar and mica groups of minerals.

## Short answer

Feldspars are aluminosilicates of K, Na, Ca with hardness 6, two good cleavages near 90°, and weather to clay, reducing rock strength. Micas are sheet silicates with perfect basal cleavage, hardness 2-3, and occur as muscovite (light) and biotite (dark); their flaky nature makes rocks weak, slippery, and unsuitable for foundations and aggregate.

## Answer

#### Feldspar group
The most abundant minerals of the crust (about 50-60%). They are aluminosilicates of potassium, sodium and calcium.
- **Types**: potassium feldspars (orthoclase, microcline) and plagioclase (albite to anorthite, a series of sodium-calcium feldspars).
- **Crystal system**: monoclinic (orthoclase), triclinic (microcline, plagioclase).
- **Hardness**: 6. **Specific gravity**: 2.55-2.76.
- **Cleavage**: two good cleavages at about 90° (or near 86° in plagioclase); plagioclase shows fine striations.
- **Colour**: white, pink, cream or grey; vitreous to pearly lustre; white streak.
- **Occurrence**: granite, gneiss, syenite, basalt.
- **Engineering**: durable when fresh, but weathers to clay, lowering rock strength.

#### Mica group
Sheet silicates with very perfect cleavage in one direction, splitting into thin, flexible, elastic sheets.
- **Types**: muscovite (white mica, light) and biotite (black mica, iron-magnesium rich).
- **Crystal system**: monoclinic.
- **Hardness**: 2-3. **Specific gravity**: 2.7-3.1.
- **Lustre**: pearly to vitreous; colourless to brown or black.
- **Occurrence**: granite, schist, gneiss, phyllite, sandstone.
- **Engineering**: mica-rich rocks split easily along planes, are weak and slippery, so unsuitable as foundation, aggregate or for stable slopes.

## Check yourself

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

1. Feldspars are divided into:
   A. Muscovite and biotite
   B. Potassium feldspars and plagioclase (sodium-calcium series)
   C. Calcite and dolomite
   D. Olivine and pyroxene
   Answer: B. Orthoclase and microcline are K-feldspars; albite to anorthite are plagioclase. (Key point: Feldspar types)

2. Feldspar's cleavage is:
   A. One perfect direction
   B. Three directions at non-right angles
   C. Two good directions at about 90 degrees
   D. None
   Answer: C. Orthoclase has two cleavages near 90 degrees. (Key point: Feldspar cleavage)

3. Muscovite and biotite are:
   A. Both K-feldspars
   B. White mica (light) and black mica (iron-magnesium rich)
   C. Black and white forms of quartz
   D. Hard minerals of hardness 6
   Answer: B. Muscovite is white, biotite black and Fe-Mg rich. (Key point: Mica types)

4. Why is mica-rich rock unsuitable for foundations or stable slopes?
   A. It is too hard to excavate
   B. It is soluble in water
   C. It splits easily along planes and is weak and slippery
   D. It has no cleavage
   Answer: C. Perfect cleavage makes weak planes. (Key point: Mica engineering)

## 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 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×
- [Engineering significance of rock-forming minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-rockforming-minerals) — Engineering Geology I, chapter 3, asked 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