Model Answer
0 min readIntroduction
The hexagonal crystal system is one of the seven crystal systems, characterized by a hexagonal unit cell with two equal axes at 120 degrees to each other and a third axis perpendicular to the plane formed by the first two. This system is common in minerals like quartz, apatite, and beryl. Understanding the crystal forms within this system is crucial for mineral identification and understanding their physical properties. The projection diagrams help visualize the three-dimensional structure of these crystals in two dimensions, aiding in their classification and analysis. This answer will briefly describe the common crystal forms of the hexagonal system, illustrating them with simplified projection diagrams.
Hexagonal Crystal System: Key Characteristics
The hexagonal system is defined by the following characteristics:
- Unit Cell Parameters: a = b ≠ c, α = β = 90°, γ = 120°
- Symmetry: Possesses a six-fold rotation axis (6/mmm) and a mirror plane perpendicular to the c-axis.
- Bravais Lattices: Includes one primitive and one centered hexagonal lattice.
Common Hexagonal Crystal Forms with Projection Diagrams
1. Hexagonal Prism
The hexagonal prism is formed by the repetition of hexagonal faces perpendicular to the c-axis. It is the most basic form in the hexagonal system.
Projection: The projection shows a hexagon as the base, with vertical lines representing the edges extending upwards, forming a prism.
2. Hexagonal Pyramid
A hexagonal pyramid is formed by faces that converge towards a point (apex) on the c-axis. It is denoted by {pyramid}.
Projection: The projection shows a hexagon as the base, with lines converging towards a central point above it, representing the pyramid faces.
3. Hexagonal Dipyramid
A hexagonal dipyramid consists of two hexagonal pyramids joined base-to-base. It is denoted by {dipyramid}.
Projection: The projection shows two hexagons, one above the other, with lines converging towards two apex points, one above and one below the hexagons.
4. Hexagonal Scalenohedron
Scalenohedra are formed by faces that are scalene triangles. In the hexagonal system, they are denoted by {s}.
Projection: The projection shows a more complex arrangement of lines converging towards apex points, forming triangular faces.
Combination of Forms
Often, crystals exhibit a combination of these basic forms. For example, a hexagonal prism may be capped with a hexagonal pyramid, resulting in a more complex crystal shape. The notation used to describe these combinations follows specific crystallographic conventions.
Conclusion
In conclusion, the hexagonal crystal system exhibits a variety of distinct crystal forms, including prisms, pyramids, and dipyramids, each characterized by unique arrangements of faces and symmetry elements. Understanding these forms and their projection diagrams is fundamental to mineralogical analysis and crystal identification. The ability to visualize these three-dimensional structures in two dimensions is a crucial skill for geologists and materials scientists. Further study of crystal morphology and crystallography provides a deeper insight into the internal structure and properties of 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.