Difference Between Calcite and Aragonite

The key difference between calcite and aragonite is that the crystal system of calcite is trigonal, whereas the crystal system of aragonite is orthorhombic.

Both calcite and aragonite are two different forms of the same compound, i.e., calcium carbonate (CaCO₃). Since they are different structures of the same chemical compound, we call them polymorphs. However, they have distinct physical properties.

CONTENTS

1. Overview and Key Difference
2. What is Calcite
3. What is Aragonite
4. Relationship Between Calcite and Aragonite
5. Side by Side Comparison – Calcite vs Aragonite in Tabular Form
6. Summary

What is Calcite?

Calcite is the most stable polymorph of calcium carbonate. It is a carbonate mineral. Its crystal system is trigonal. Moreover, it is mainly colourless or white, but sometimes it can be grey, yellow or green as well. The lustre of this mineral is vitreous to pearly on cleavage surfaces while the mineral streak is white.

Calcite mineral has a considerable hardness; its Mohs hardness value is 3. The specific gravity of calcite is 2.71. Moreover, this mineral can be transparent or opaque. Occasionally it may show phosphorescence or fluorescence. Single crystals of calcite show birefringence; if we observe an object through this crystal, it appears doubled.

Furthermore, calcite can dissolve in many acid forms. Similarly, it can dissolve in groundwater. Sometimes, it gets precipitated by groundwater; however, factors like temperature and pH of the groundwater have an effect on this precipitation. Moreover, the construction industry is the main consumer of calcite; they use this mineral in the form of limestone and marble to produce cement and concrete. Besides, microbiologically precipitated calcite has many applications including soil remediation, soil stabilization and concrete repair.

What is Aragonite?

Aragonite is a stable polymorph of calcium carbonate. This mineral is formed as a result of precipitation from marine and freshwater environments. The crystal structure of this mineral is orthorhombic. Aragonite mainly occurs in either columnar or fibrous forms. There can be different coloured aragonite minerals: white, red, yellow, orange, green, purple, etc.

The fracture of this mineral is subconchoidal. Its hardness in the Mohs scale is between 3.5 to 4.0. Its specific gravity is 2.96. When considering the lustre, it has vitreous, resinous on fracture surfaces. Moreover, its mineral streak is white.

More importantly, this mineral is thermodynamically unstable at standard temperature and pressure. Therefore, it tends to convert into calcite on scales of 10⁷ to 10⁸ years. That means; calcite is more stable than aragonite. When considering the uses of this mineral, it is essential for the replication of reef conditions in aquaria. Furthermore, it keeps the pH of the seawater close to its natural level.

What is the Relationship Between Calcite and Aragonite?

• Calcite and aragonite are polymorphs of calcium carbonate.
• At surface conditions, aragonite spontaneously turns into calcite over geologic time.

What is the Difference Between Calcite and Aragonite?

Calcium carbonate has three polymorphs: calcite, aragonite and vaterite. The key difference between calcite and aragonite is that the crystal system of calcite is trigonal, whereas the crystal system of aragonite is orthorhombic. There is also a difference between calcite and aragonite in terms of stability. Calcite is the most stable polymorph of calcium carbonate. Although aragonite is also a stable polymorph, it is not stable as calcite.

Below infographic provides more details regarding the difference between calcite and aragonite.

Summary – Calcite vs Aragonite

Calcite and aragonite are polymorphs of calcium carbonate. The key difference between calcite and aragonite is that the crystal system of calcite is trigonal, whereas the crystal system of aragonite is orthorhombic. Moreover, calcite is more stable than aragonite.

Reference:

1. Hobart M, King. “Calcite.” Geology, Available here.
2. Alden, Andrew. “Calcite or Aragonite: Which Forms When?” ThoughtCo, 13 Aug. 2018, Available here.

Image Courtesy:

1. “Calcite-20188” By Rob Lavinsky, iRocks.com – (CC-BY-SA-3.0) via Commons Wikimedia
2. “Aragonite Salsigne France” By Didier Descouens – Own work (CC BY-SA 3.0) via Commons Wikimedia