---
title: "Structural units of clay minerals"
subject: "Soil Mechanics"
chapter: 4
key: "semester_4/soil_mechanics:clay-structural-units"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/clay-structural-units
site: BCT Engineering
---

# Structural units of clay minerals

Soil Mechanics · Chapter 4: Soil Structure and Clay Minerals

- Asked in: 2079 Jestha, 1 marks; 2075 Baisakh, 2 marks; 2076 Baisakh, 2 marks (asked 3 times)
- Video: [Soil Mineralogy - Clay Mineralogy](https://www.youtube.com/watch?v=Qh6wSfVN45s) — Elementary Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/clay-structural-units
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-4

## Question

What are the basic structural units (building blocks) of clay minerals? Describe the silica sheet, gibbsite sheet and brucite sheet.

## Short answer

The basic units are silica tetrahedra and alumina octahedra. Silica sheet: Si-centred tetrahedra sharing basal oxygens. Gibbsite sheet: Al-centred octahedra with two-thirds of cation sites filled. Brucite sheet: Mg-centred octahedra with all three sites filled.

## Answer

#### Basic structural units
Clay minerals are made from two basic building blocks:
1. **Silica tetrahedron:** a silicon atom at the centre surrounded by four oxygen atoms (or hydroxyls).
2. **Alumina octahedron:** an aluminium (or magnesium) atom at the centre surrounded by six hydroxyls (or oxygens).

Their repeated units form sheets, which are stacked to form clay mineral layers.

#### Silica sheet
Many silica tetrahedra are joined by sharing the three oxygens at the base, forming a flat sheet with hexagonal pattern; the apexes point in one direction. Formula $(\text{Si}_4\text{O}_{10})^{4-}$ or $\text{Si}_4\text{O}_6(\text{OH})_4$. Thickness about 4.93 Å.

#### Gibbsite sheet
Octahedral units with **aluminium** in the centre, joined by shared hydroxyls. Only two out of three possible positions of Al are filled (dioctahedral). Formula $\text{Al}_2(\text{OH})_6$ (Al$^{3+}$). Thickness about 5.05 Å.

#### Brucite sheet
Similar octahedral sheet with **magnesium** (Mg$^{2+}$) at the centre, in which all three positions are filled (trioctahedral). Formula $\text{Mg}_3(\text{OH})_6$.

```text
 Tetrahedron      Octahedron
     Si              Al / Mg
    /|\            /| |\
   O O O          OH OH OH
     O            OH OH OH
```

## Check yourself

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

1. The two basic units of clay minerals are
   A. silica tetrahedron and alumina octahedron
   B. silica octahedron and alumina tetrahedron
   C. kaolinite and illite
   D. gibbsite and brucite only
   Answer: A. Si-O tetrahedra and Al/Mg-OH octahedra. (Key point: Basic units)

2. Gibbsite sheet has
   A. Mg with all positions filled
   B. Al at centre with only two of three positions filled
   C. Si at centre
   D. K ions
   Answer: B. Dioctahedral, Al2(OH)6. (Key point: Gibbsite)

3. Brucite sheet contains
   A. Al with two positions filled
   B. Si
   C. Mg with all three positions filled
   D. Fe only
   Answer: C. Trioctahedral, Mg3(OH)6. (Key point: Brucite)

## Related in Soil Mechanics

- [Types and structure of clay minerals](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-mineral-types-structure) — Soil Mechanics, chapter 4, asked 6×
- [Double diffuse layer in clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adouble-diffuse-layer-explain) — Soil Mechanics, chapter 4
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- [Clay 4 m, 90% in 4 hours, time in years](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-bed-90-percent-years) — Soil Mechanics, chapter 9
- [Clay 10 m, 2.54 cm sample, half settlement](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-10m-half-settlement-time-ratio) — Soil Mechanics, chapter 9

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