Crystal Structure:
* Calcite: Rhombohedral, with three axes of equal length and angles of 101.9°. It often forms hexagonal crystals.
* Aragonite: Orthorhombic, with three axes of unequal length and angles of 90°. It typically forms needle-like or prismatic crystals.
Physical Properties:
* Hardness: Calcite is slightly softer than aragonite (3 vs. 3.5-4 on Mohs scale).
* Cleavage: Calcite has perfect rhombohedral cleavage, meaning it breaks along smooth, flat surfaces. Aragonite has imperfect cleavage, often breaking unevenly.
* Specific Gravity: Calcite is slightly denser than aragonite (2.71 vs. 2.93-2.95).
* Optical Properties: Calcite exhibits double refraction (birefringence), where a single beam of light is split into two. Aragonite shows less pronounced birefringence.
* Solubility: Calcite is slightly more soluble than aragonite in acidic solutions.
Occurrence:
* Calcite: More common and found in a wider range of geological environments, including limestone, marble, and stalactites/stalagmites.
* Aragonite: Less common and often found in high-pressure environments, such as hydrothermal vents and hot springs. It also forms in biological organisms like shells, corals, and pearls.
Other Differences:
* Stability: Calcite is the more stable form of CaCO3 at standard conditions. Over long periods, aragonite will typically transform into calcite.
* Twinning: Aragonite often exhibits polysynthetic twinning, where crystals are intergrown in parallel layers. Calcite twinning is less common.
In summary:
While both calcite and aragonite are composed of the same chemical compound, their different crystal structures lead to distinct physical, optical, and geological characteristics.
Here's a helpful analogy: Imagine building a house. Calcite is like a brick house, strong and stable. Aragonite is like a house made of popsicle sticks - delicate and more prone to changing over time.
