---
title: "Calcite versus quartz"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:calcite-versus-quartz"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/calcite-versus-quartz
site: BCT Engineering
---

# Calcite versus quartz

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2063 Baisakh (old course), 2 marks
- Video: [Quartz vs. Calcite](https://www.youtube.com/watch?v=Af_-7rmgHP4) — Craig the Geologist
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/calcite-versus-quartz
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Differentiate between calcite and quartz.

## Short answer

Calcite is CaCO₃, hardness 3, perfect rhombohedral cleavage, effervesces in HCl, and dissolves in acidic water. Quartz is SiO₂, hardness 7, no cleavage, conchoidal fracture, no acid reaction, and is highly resistant to weathering.

## Answer

| Basis | Calcite | Quartz |
|---|---|---|
| Composition | Calcium carbonate, CaCO₃ | Silica, SiO₂ |
| Hardness | 3 | 7 |
| Cleavage | Perfect, three directions (rhombohedral) | None; conchoidal fracture |
| HCl test | Effervescence | No reaction |
| Crystal system | Trigonal | Trigonal (hexagonal group) |
| Specific gravity | 2.71 | 2.65 |
| Weathering | Dissolves in acidic water | Highly resistant |
| Occurs in | Limestone, marble | Granite, sandstone, quartzite |

## Check yourself

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

1. Calcite and quartz compositions are:
   A. SiO2 and CaCO3
   B. CaCO3 and CaSO4
   C. NaCl and SiO2
   D. CaCO3 and SiO2
   Answer: D. Calcite is carbonate; quartz is silica. (Key point: Composition)

2. Hardness of calcite and quartz:
   A. 3 and 7
   B. 7 and 3
   C. 3 and 6
   D. 5 and 7
   Answer: A. Quartz scratches calcite. (Key point: Hardness)

3. Which reacts with dilute HCl?
   A. Quartz
   B. Calcite, with effervescence
   C. Both
   D. Neither
   Answer: B. Quartz has no reaction. (Key point: HCl)

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