Crystal Systems

Crystal systems are a way of classifying crystals based on their symmetry and the arrangement of their atoms or molecules. There are seven crystal systems: cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic, and triclinic. Each crystal system has certain characteristics that differentiate it from the others.

Explaining the Options

a) Triclinic: The triclinic crystal system is characterized by three unequal axes that intersect at oblique angles. Crystals in this system have no symmetry planes or axes.

b) Monoclinic: The monoclinic crystal system has three unequal axes, with one axis perpendicular to the other two. The two non-perpendicular axes intersect at an oblique angle, while the third axis is perpendicular to the other two. Crystals in this system have one twofold axis of rotational symmetry.

c) Rhombohedral: The rhombohedral crystal system is characterized by three equal axes that intersect at oblique angles. The angles between the axes are all the same. Crystals in this system have a threefold rotational symmetry axis.

d) Isotropical: The term "isotropical" does not represent a type of crystal system. In fact, it is not a term commonly used in crystallography. It seems to be a combination of the words "isotropic" and "tropical," but it does not have a specific meaning in relation to crystal systems.

Conclusion

The correct answer is option 'D' because "isotropical" does not represent a type of crystal system. The other options, triclinic, monoclinic, and rhombohedral, are all valid crystal systems with distinct characteristics.