Answer: A crystal structure is described by both the geometry of, and atomic arrangements within, the unit cell, whereas a crystal system is described only in terms of the unit cell geometry. For example, face-centered cubic and body-centered cubic are crystal structures that belong to the cubic crystal system.

Table 1: Crystal Structure for some Metals (at room temperature)

Because FCC atoms are arranged more closely together than BCC atoms, FCC metals will tend to be more dense and more stable. This is a very broad rule, however! Tungsten, one of the densest metals, is BCC.

The seven crystal systems are: (1) Cubic (2) Tetragonal (3) Orthorhombic (4) Monoclinic (5) Triclinic (6) Trigonal (or Rhombohedral) and (7) Hexagonal. Space lattices are classified according to their symmetry.

There are six basic crystal systems.

• Isometric system.
• Tetragonal system.
• Hexagonal system.
• Orthorhombic system.
• Monoclinic system.
• Triclinic system.

One might suppose stretching face-centered cubic would result in face-centered tetragonal, but the face-centered tetragonal is equivalent to the body-centered tetragonal, BCT (with a smaller lattice spacing). Tetragonal system has total seven number of classes.

In the hexagonal crystal system we have a=b≠c and α=β=90∘,γ=120∘. In option C.), the triclinic crystal system all the lattice sites and all the bond angles are unequal. That is in the triclinic crystal system we have a≠b≠c and α≠β≠γ≠90∘. It is the most unsymmetrical crystal system.

The finest amethyst color is a strong reddish purple or purple with no visible color zoning. Dealers prefer strongly saturated reddish purple to dark purple, as long as the stone is not so dark that it reduces brightness.

The plane of symmetry (also called the ‘mirror plane’ or ‘symmetry plane’) is a plane by which the crystal may be divided into two halves which are mirror images of each other. …

The triclinic lattice is the least symmetric of the 14 three-dimensional Bravais lattices. It has (itself) the minimum symmetry all lattices have: points of inversion at each lattice point and at 7 more points for each lattice point: at the midpoints of the edges and the faces, and at the center points.

Thus, this crystal has the following symmetry elements: 1 – 4-fold rotation axis (A4) 4 – 2-fold rotation axes (A2), 2 cutting the faces & 2 cutting the edges. 5 mirror planes (m), 2 cutting across the faces, 2 cutting through the edges, and one cutting horizontally through the center.

Face-centered Cubic: Lattice points at corners plus one at the center of each face of the cube. The simplest crystal structures are those which have only one atom at each of these lattice points….