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Orthorhombic System

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Orthorhombic Crystal System

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In the study of crystals, there are classifications based on the basic geometry of the crystal. Scientists named these groups the primitive crystal systems. The orthorhombic crystal system is one of the categories into which we can classify our crystalline solids. The names of all the seven crystal systems are as follows: cubic, tetragonal, orthorhombic, hexagonal, triclinic, monoclinic, and rhombohedral. These systems work on the principle of the three axes on which we draw the crystals and the relation of the axes to each other. This article lays focus on the orthorhombic structure. 


Orthorhombic Crystal

In this category of crystal systems, we can place the crystal on three mutually perpendicular axes which are of unequal length. This property is the defining characteristic of this group. The orthorhombic crystal system has its own set of unit cells. To understand what a unit cell is, one must imagine the atoms or atom groups in the crystal structure as points. If we join these points, we obtain a structure known as the lattice. If we observe, this lattice consists of stacks of blocks, and these we call unit cells. 


Orthorhombic Unit Cell

The orthorhombic unit cell has its own set of unique characteristics. This unit cell has a set of three axes which we call the axes of twofold symmetry. If we rotate the crystal about these lines by an angle of 180°, the crystal will not change its appearance. To fulfill this property, the unit cell must have certain defined characteristics. In an orthorhombic unit cell, the angle between any two edges is always 90°. The edge lengths can, however, be unequal. 


Examples of Orthorhombic Structure 

Some examples of crystalline solids which take up the orthorhombic structure are as follows:

  • enargite 

  • marcasite 

  • cerussite 

  • barite 

  • staurolite

  • topaz

  • orthoenstatite 

  • aragonite 

  • olivine 

  • cementite 

  • alpha-sulfur 

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Variants of the Orthorhombic Crystal System

The orthorhombic system undergoes further classification to give rise to three subgroups of crystals. The common characteristic of these subgroups is that they consist of three mutually perpendicular axes which are unequal in length. These types are named the body-centered orthorhombic crystal system, base-centered orthorhombic crystal system, and face-centered orthorhombic crystal system respectively. The definitions of each of these crystal systems are as below: 

  • Body-centered Orthorhombic Crystal System: In this type of crystal structure, the lattice point is in the middle of the unit cell. 

  • Base-centred Orthorhombic Crystal System: In this type of crystal structure, there is a lattice point in the middle of each of the two ends. 

  • Face-centered Orthorhombic Crystal System: In this type of crystal structure, there exists a lattice point in the middle of each side. 

Solved Examples 

Q. Define an Orthorhombic Crystal

Answer: Orthorhombic system is one of the structural categories of crystals systems into which we can classify crystalline solids. In this type of crystal structure, the lattice consists of three axes that are perpendicular to each other and whose lengths may or may not be inconsistent. 


Q. State Whether this Statement is True or Not: In the Orthorhombic Crystal System, All Axial Angles are of the Same Value and are Identical. 

Answer: The statement is correct. The orthorhombic structure is such that all the crystallographic angles are of the same measure. This property is a fundamental characteristic of this type of crystal. 


Q. State How a Crystal Structure and a Crystal System are Different Terminologies.

Answer: When we talk about a crystal structure, we mean the arrangement of atoms and the geometry which exists within the unit cell of the crystal. However, when talking of crystal systems, only the unit cell geometry comes into consideration. For example, body-centered and face-centered are crystal structures that fall under the orthorhombic crystal system.


Q. Why is it So Essential to Know about Crystal Systems?

Answer: The importance of our awareness of crystal systems and crystal structures becomes clear when we obtain crystalline solids for use in our daily life. Two crystals having different structures will exhibit very different physical characteristics. The finest example of this is graphite and diamond. Due to their separate crystal systems and structures, one of these solids is exceptionally hard while the other is soft. These properties define their application, and thus, knowing how these crystals will behave under different physical conditions is essential. 

FAQ (Frequently Asked Questions)

Q. Explain the Unique Symmetric Characteristics of the Orthorhombic System and How it is Different from the Monoclinic System.

Answer: The characteristic feature of the orthorhombic system is that it contains a 2-fold axis of rotational symmetry. This is exceptionally unique as the 2-fold axis of symmetry also coincides with the crystal's c axis. In most other cases, when the axes of rotational symmetry coincide with the c axis, the axes are usually 3-fold and therefore, isometric. The orthorhombic system is, however, unique and has its 2-fold structure. Approximately every orthorhombic crystal contains a 2-fold axis of rotational symmetry. These axes lie in two perpendicular planes which mirror each other. This feature is often enough to classify a crystal as orthorhombic and distinguish it from the monoclinic category. 

Q. Briefly Discuss the Two-dimensional and Three-dimensional Bravais Lattices of the Orthorhombic System.

Answer: The following are brief notes on the two-dimensional and three-dimensional Bravais lattices which are a part of the orthorhombic crystal system:

  • Two-dimensional: 

In the two-dimensional system of Bravais lattices, we have two orthorhombic structures, namely: primitive rectangular lattice and centered rectangular lattice. These also have alternate names such as centered rhombic unit cell for the primitive rectangular type and primitive rhombic unit cell for the centered rectangular lattice type. 

  • Three-dimensional: 

In the three-dimensional Bravais lattice system, there are four types of orthorhombic crystals. These crystal structures are named as follows: primitive orthorhombic crystal structure, base-centered orthorhombic crystal structure, body-centered orthorhombic crystal structure, and face-centered orthorhombic crystal structure.