Crystal Lattice Structures

by Ritika

The various gemstones take their shape and structure based on their unique crystalline structure that each gemstone possesses. These crystalline structures of each gemstone is based and made from a mixture of different elemental compounds. Thus the atomic structure associated with elemental building blocks contributes towards the shape of a crystal or gemstone.

Crystal Structure and Technical Terms

The various attributes associated with a gemstone like its symmetry, geometric shape, optical properties and cleavage planes are all determines and based on the crystal structure of a gemstone. The technical term Crystal Habit is used to depict the growth pattern of crystal.

Crystal’s Lattice

The grouping of crystal structures constitutes a crystal system. These are divided and grouped based on the axial system which describe the crystal lattice. A lattice denotes regular array of lattice points in three dimensions. That is a crystal lattice is nothing but atoms arranged in a symmetrical pattern on a three dimensional network fashion. The structure formed by connecting the eight lattice points is a parallelepiped which is technically termed as a crystallographic unit cell.

The crystal is a repeating array and its structure is determined by the following two properties namely:

• pattern of repetition which is termed as the lattice type
• what is repeated which is termed as the unit cell

Types of Crystal Systems

Each crystal system is arranged in a set of three axes geometric system. Each crystal system takes a unique shape based on the unique geometrical arrangement of each crystal system on the three axes. There are seven types of crystal systems and they are as below:

• Isometric System
• Hexagonal System
• Tetragonal System
• Rhombohedric (Trigonal) System
• Orthorhombic System
• Monoclinic System
• Triclinic System

The above seven crystal systems are listed in order of their decreasing symmetry. The geometric arrangement of the individual lattice points within each of the seven crystal systems described above is termed as Bravais lattices and there are fourteen distinct Bravais lattices.

Crystal Lattice Structures

The popular crystal lattice structures are

• Simple Cubic
• Cubic Close Packed Crystals
• Body-Centered Cubic
• Hexagonal Close-Packed Crystals
• Carbon and Related Structures
• Manganese Structures
• The Laves Phase Structures
• Perovskite and Related Structures
• Quartz and Related Structures
• Sulfur and its compounds
• Structures of the Lanthanides, Actinides and Their Compounds
• IIIb-VIIb Structures
• Binary Pnma Alloy Structures
• and Other Crystal Structures

Cubic unit cell and Simple Cubic Unit Cell

The elements which constitute a cubic unit cell are six square faces and three equal non-coplanar edges. That is if the sides are denotes by x, y, z then a cubic unit cell has the following characteristics namely

x=y=z

The volume of the cubic unit cell is denoted as x3
Simple Cubic unit cell is also denoted as SC in short. The simple cubic unit cell has one lattice point present in each of the eight corners of a cube. If the lattice points are present only at the eight corners of the cube then it is termed technically as primitive.

Cubic Close Packed Crystals

Crystalline structures associated in a closed packed cubic lattice are termed as cubic close packed crystals.

Body-Centered Cubic

Body-Centered Cubic is also referred as BCC in short consist of a single host atom at each corner of the cubic unit cell and a single atom in the cell center. The arrangement in BCC is such that each of these atoms touches eight other host atoms along the body diagonal of the cube.

Hexagonal Close-Packed Crystals

Hexagonal Close-Packed Crystals is also termed as HCP in short. In this structure the arrangement of plane is in such a fashion such that atoms in successive planes associate in a triangular groove with the preceding plane.

Carbon and Related Structures

Crystalline lattice refers to the atoms in a crystal that are arranged in a regular repeating pattern. The property or attribute of a crystal is highly determined by its lattice. For instance both graphite and diamond constitute carbon as its major element or atom. But both graphite and diamond differs in its properties which is based on how the atom namely carbon is arranged in both. That is we can see that graphite which is black in color has the attribute of being soft and also acts as a good lubricant. This clearly depicts that atoms in graphite could be separated easily. In contrast the attribute of diamond are that it is strong and extremely hard and is also transparent at the same time. This clearly shows that the atoms in diamond could not be separated easily as it is strongly bounded.

Manganese Structures

There are several structures in which Manganese is available and the most rarest forms are termed as aMn and ßMn.

The Laves Phase Structures

The elements arranged in Laves Phase Structure are in the order as AB2. In this the atom A is arranged in the similar way as in crystal like diamond and the atom B is arranged in the shape of tetrahedra around the atom A.

Perovskite and Related Structures

The crystal structure like Perovskite structure have ferroelectric attribute. The structure of crystal along with this attribute enables these to become permanently polarized when these are subjected to electric field of sufficient magnitude.

Quartz and Related Structures

The chemical symbolic representation of Quartz is SiO2 and this exists in different forms. The crystal structure in which alpha Quartz is present is termed as Trigonal. The Trigonal crystal system has totally four axes. Of these four axes three axes are of equal length and they lie at an angle of 120¡ã from each other. The fourth axes are either longer or shorter in length but exist at a right angle toward the other corners.

Structures of the Lanthanides, Actinides and their Compounds

The Lanthanides, Actinides and their Compounds exist in various structures. Some of the rare forms among these are namely Cf, aPu, ßPu and so on.

Other Crystal Structures

There are other crystal structures as well like Trigonal omega phase referred to as C6, Cuprites referred to as C3 and so on which do not fit in any of the above described.