Which of the following does not represent a type of crystal system ?

Options

A. Triclinic
B. Monoclinic
C. Rhombohedral
D. Isotropical

Explanation

To determine which of the following does not represent a type of crystal system, we will analyze each option provided:

1. **Triclinic**: This is indeed one of the seven crystal systems. It is characterized by having no axes of symmetry and all angles being different.

2. **Monoclinic**: This is also one of the seven crystal systems. It has one angle that is not 90 degrees and two axes of equal length.

3. **Rhombohedral**: This is often considered synonymous with the trigonal crystal system. It is also one of the seven crystal systems, characterized by three equal axes that are inclined to each other.

4. **Isotropical**: This term does not refer to a crystal system. Instead, it is often used in the context of isotropic materials, which have the same properties in all directions but does not classify as a crystal system.

Based on this analysis, the answer to the question is:

**Answer**: Isotropical (Option D) does not represent a type of crystal system.