Model Answer
0 min readIntroduction
Calcite and aragonite are both polymorphs of calcium carbonate (CaCO3), meaning they share the same chemical formula but differ in their crystal structure. Calcite is the more stable form at ambient temperatures and pressures, commonly found in sedimentary rocks like limestone and chalk. Aragonite, however, forms under higher pressure and lower temperature conditions, often found in marine organisms’ shells and in some metamorphic rocks. Understanding their internal structures is crucial for interpreting geological processes and material properties. This answer will illustrate the internal structure of calcite and explain the differences between it and aragonite.
Internal Structure of Calcite
Calcite crystallizes in the rhombohedral system, a variant of the hexagonal crystal system. Its structure is based on a framework of interconnected CaO6 octahedra and CO32- triangles. The key features of calcite’s internal structure are:
- Unit Cell: The basic repeating unit of calcite is a rhombohedral unit cell.
- Ion Arrangement: Calcium ions (Ca2+) occupy specific sites within the octahedra, while carbonate ions (CO32-) are arranged in a triangular fashion.
- Layered Structure: Calcite exhibits a layered structure along the c-axis, with alternating layers of calcium and carbonate ions.
- Spiral Growth: Calcite crystals often exhibit spiral growth patterns due to the arrangement of ions during crystallization.
Figure: Calcite Unit Cell showing arrangement of ions. (Source: Wikimedia Commons)
Internal Structure of Aragonite
Aragonite, also CaCO3, crystallizes in the orthorhombic system. While it shares the same chemical composition as calcite, its internal structure is significantly different:
- Unit Cell: Aragonite’s unit cell is orthorhombic, meaning it has three unequal axes at right angles to each other.
- Ion Arrangement: The arrangement of calcium and carbonate ions differs from calcite. Aragonite features a more distorted arrangement of the CaO6 octahedra and CO32- triangles.
- Stacking Sequence: The stacking sequence of layers in aragonite is different from calcite, leading to its distinct crystal morphology.
- Needle-like Crystals: Aragonite often forms needle-like or prismatic crystals, unlike the more blocky or rhombohedral shapes of calcite.
Figure: Aragonite Unit Cell showing arrangement of ions. (Source: Wikimedia Commons)
Differences between Calcite and Aragonite
The differences in internal structure between calcite and aragonite are primarily due to the different ways the calcium and carbonate ions arrange themselves. This is influenced by factors like temperature, pressure, and the presence of impurities.
| Feature | Calcite | Aragonite |
|---|---|---|
| Crystal System | Rhombohedral (Hexagonal) | Orthorhombic |
| Stability | Stable at ambient conditions | Metastable at ambient conditions; stable at higher pressures and lower temperatures |
| Ion Arrangement | Less distorted CaO6 octahedra and CO32- triangles | More distorted CaO6 octahedra and CO32- triangles |
| Typical Morphology | Rhombohedral, blocky | Needle-like, prismatic |
| Density | 2.71 g/cm3 | 2.93 g/cm3 |
The higher density of aragonite is a direct consequence of its more compact crystal structure. The different stacking arrangements also affect their optical properties, such as birefringence. The transition from aragonite to calcite is a reconstructive phase transition, requiring significant energy input to rearrange the ions.
Conclusion
In conclusion, both calcite and aragonite are polymorphs of calcium carbonate, but their distinct internal structures – rhombohedral for calcite and orthorhombic for aragonite – lead to differences in their physical properties and stability. These structural variations are governed by the arrangement of ions and influenced by environmental conditions like temperature and pressure. Understanding these differences is crucial for interpreting the geological history of rocks and the formation of biogenic minerals.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.