Model Answer
0 min readIntroduction
Crystallography, the science dealing with the arrangement of atoms in crystalline solids, relies heavily on understanding symmetry elements. Rotational axes of symmetry are fundamental symmetry elements defining a crystal’s structure. Twinning, a common phenomenon in crystals, involves the intergrowth of two or more crystals of the same substance in a symmetrical manner. Quartz (SiO2), a ubiquitous mineral, exhibits various twinning types due to its trigonal crystal system and structural characteristics. Understanding these symmetry elements and twinning types is crucial for mineral identification and understanding crystal growth processes.
Rotational Axes of Symmetry in Crystals
A rotational axis of symmetry is an imaginary line around which a crystal can be rotated by a specific angle (less than 360°) and appear identical to its original orientation. The order of the axis is determined by the angle of rotation.
- 1-fold (or C1) Axis: Present in all crystals. Rotation of 360° is required to achieve symmetry. It essentially means the crystal has no rotational symmetry.
- 2-fold (or C2) Axis: Rotation of 180° results in an identical view. Common in tetragonal, orthorhombic, and monoclinic systems.
- 3-fold (or C3) Axis: Rotation of 120° results in an identical view. Found in trigonal (rhombohedral) and hexagonal systems.
- 4-fold (or C4) Axis: Rotation of 90° results in an identical view. Characteristic of tetragonal and cubic systems.
- 6-fold (or C6) Axis: Rotation of 60° results in an identical view. Exclusive to the hexagonal system.
Twinning in Quartz
Quartz, belonging to the trigonal crystal system, commonly exhibits three main types of twinning:
1. Contact Twinning
Contact twinning occurs when two quartz crystals grow together, sharing a common crystallographic plane. This plane is known as the twin plane. In quartz, the most common contact twin plane is the {1122} plane. The twin law is such that one crystal is a mirror image of the other across the twin plane. This type of twinning is often observed in quartz crystals from Brazil and Madagascar.
2. Polysynthetic Twinning
Polysynthetic twinning involves multiple, parallel twin planes spaced closely together within a single crystal. This creates a lamellar or striated appearance. In quartz, polysynthetic twinning is often induced by stress during deformation, such as during mountain building. The twin planes are typically {1122} and are very fine, giving the crystal a characteristic ‘herringbone’ pattern when viewed under polarized light.
3. Cyclotwin
Cyclotwin is a more complex type of twinning where multiple twin planes intersect, forming a cyclic pattern. In quartz, cyclotwinning results from a rotation of 180° around a two-fold axis. This creates a pseudo-hexagonal appearance in the crystal, even though quartz is fundamentally trigonal. Cyclotwins are relatively rare but can be found in quartz crystals from certain pegmatite deposits.
The presence and type of twinning in quartz can be used as diagnostic features for identifying the mineral and understanding its geological history.
Conclusion
In conclusion, understanding rotational axes of symmetry is fundamental to characterizing crystal structures, while twinning provides insights into the growth conditions and deformation history of minerals. Quartz, with its diverse twinning types – contact, polysynthetic, and cyclotwin – serves as an excellent example for illustrating these concepts. The identification of these features is crucial for geologists in mineral identification, petrogenesis studies, and understanding tectonic processes.
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.