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Orthorhombic System

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Last updated date: 23rd Apr 2024
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Introduction to Orthorhombic System

This article contains an explanation of the orthorhombic system, examples, and some solved examples. If you incur any doubts while reading the article you can refer to the frequently asked questions at the end.


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. 

 

The orthorhombic is a three-axis System each of which are mutually at 90° angles and x axis is vertical and the shortest with the a axis which is intermediate in length and is from front to back and the B axis is taken as left to right with the longest.


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-centred orthorhombic crystal system, base-centred orthorhombic crystal system, and face-centred orthorhombic crystal system respectively. The definitions of each of these crystal systems are as below: 

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


Solved Examples 

1. 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. 

 

2. 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. 

 

3. 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-centred and face-centred are crystal structures that fall under the orthorhombic crystal system.

 

4. 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.

 

5. What is orthorhombic and monoclinic?

Answer. Monoclinic is (crystallography) has three unequal axes with two perpendiculars and one oblique intersection while orthorhombic is (crystallography) has three unequal axes at right angles. Monoclinic and orthorhombic are two different kinds of crystal systems. Molecules or atoms in the case of a monoclinic crystal system are different to that of an orthorhombic Crystal system.


6. How many atoms are in orthorhombic?

Answer.  Although there are many crystal structures that fit with “orthorhombic” symmetry, the simple orthorhombic crystal structure has exactly 1 atom per lattice point in the simple orthorhombic Bravais lattice. Primitive orthorhombic crystals based on the Bravais lattice that exist with multiple atoms overall displaying primitive orthorhombic symmetry.


7. What is orthorhombic Sulphur?

Answer. Orthorhombic sulphur, Sα: The most significant form of sulphur is orthorhombic, Sα.  This is described as orthorhombic Sulphur,α Sulphur, Muthmann sulphur and rhombic sulphur by many. It is stable at room temperature and also at atmospheric pressure. Rhombic sulphur has a molecular structure. In the neighbouring molecules, the closest distance of approach of atoms is about 3.3A.

FAQs on Orthorhombic System

1. How many minerals are there in the orthorhombic crystal system?

There are 970  minerals in the orthorhombic crystal system. That is about 18% of all minerals which makes it the second only to the monoclinic system and also so the second largest crystal system.Of the orthorhombic minerals, 577 are in the Orthorhombic dipyramidal class, 88 are in the Orthorhombic disphenoidal Class, and 172 are considered to be orthorhombic but the crystallographic class to which they belong is unknown.

2. What is meant by general form in the orthorhombic system?

The general form intersects all three axes, each at a different number of unit lengths (but never 0). It is not always expressed on the crystal. General form shall contain Miller Indices {123}, {235}.It is generalized as {hkl}. In the Dipyramidal  class the orthorhombic  dipyramidal is the general form. In the pyramidal class orthorhombic pyramid is the general form and in the sphenoidal class orthorhombic disphenoidal is the general form.

3. How many faces does the dipyramid consist of?

Composed of eight triangular faces. each of which intercepts all three axes. An orthorhombic dipyramid is a closed-form (It can totally enclose space). The symmetry of the dipyramidal Class (3 mutually perpendicular 2-fold axes of rotation, each perpendicular to a mirror plane, and a centre of symmetry) requires the dipyramid.

4. How many faces does the pyramid consist of in an orthorhombic system?

The four triangular faces in the orthorhombic pyramid intercepts all the three axes either on the negative and on the positive end of the c- axis. There is a negative pyramid with the intercept at the C axis and the positive pyramid with all four faces intersecting the C axis.It is an open forum. The orthorhombic pyramid can not exist if there is a centre of symmetry.The two pyramids are separate forms. The Pyramid is the general form for the Pyramidal Class.

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

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. 

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