---
title: "Silicate versus carbonate minerals in the field"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:silicate-versus-carbonate-minerals-in"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/silicate-versus-carbonate-minerals-in
site: BCT Engineering
---

# Silicate versus carbonate minerals in the field

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2081 Kartik (new course), 2 marks; 2076 Chaitra, 0.5 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/silicate-versus-carbonate-minerals-in
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

How do you identify (differentiate) silicate minerals and carbonate minerals in the field?

## Short answer

Apply dilute HCl: carboantes effervesce (calcite vigorously, dolomite when powdered), while silicates do not react. Carboantes are soft (hardness 3-4) with rhombohedral cleavage; silicates are harder (5.5-7, except micas 2-3) and lack such cleavage. A knife scratch and acid test quickly distinguish them.

## Answer

Silicate and carbonate minerals can be distinguished in the field by a few simple tests.

| Test | Silicate minerals | Carbonate minerals |
|---|---|---|
| Dilute HCl | No reaction | **Effervescence** (calcite fizzes strongly; dolomite only when powdered) |
| Hardness | Mostly 5.5-7 (quartz and feldspar scratch glass); micas are soft (2-3) | Soft, 3-4 (scratched by knife) |
| Cleavage | Varies (mica one, feldspar two, quartz none) | Perfect rhombohedral cleavage (3 directions) |
| Specific gravity | 2.5-3.5 | About 2.7-2.9 |
| Lustre | Vitreous, pearly | Vitreous to dull |
| Examples | Quartz, feldspar, mica, olivine | Calcite, dolomite |

The fastest field test is the acid test with a drop of dilute HCl, together with a knife scratch.

## Check yourself

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

1. What is the fastest field test separating carbonates from silicates?
   A. Streak: carbonates are black
   B. Magnet: carbonates are attracted
   C. Taste: carbonates are salty
   D. Dilute HCl: carbonates effervesce, silicates do not
   Answer: D. Calcite fizzes with dilute HCl; silicates do not react. (Key point: Acid test)

2. Compared with carbonates, silicates are usually:
   A. Harder (5-8), while carbonates are soft (3-4)
   B. Softer, below 3
   C. Equal in hardness
   D. Softer than a fingernail
   Answer: A. Carbonates are scratched by a knife; many silicates scratch glass. (Key point: Hardness)

3. Which cleavage is characteristic of carbonate minerals like calcite?
   A. None, with conchoidal fracture
   B. Perfect rhombohedral, three directions
   C. One direction, in thin sheets
   D. Two directions at 90 degrees
   Answer: B. Calcite has three perfect cleavages; quartz has none. (Key point: Cleavage)

## Related in Engineering Geology I

- [Identify the three rock types in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-the-three-rock-types) — Engineering Geology I, chapter 4, asked 5×
- [Identify metamorphic rock in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-metamorphic-rock-in-the) — Engineering Geology I, chapter 4, asked 2×
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- [Classification of minerals](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aclassification-of-minerals) — Engineering Geology I, chapter 3

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