## Crystal Structure and Tetragonal System

A crystal structure contains atoms, and a crystal lattice is made of points. A crystal system is a set of axes, meaning it is a structure with an ordered array of atoms, ions, or molecules. Crystal structures occur due to the intrinsic nature of the particles for producing symmetric patterns.

Here is a brief description of unit cells, Bravais lattice, and various crystal systems, including the tetragonal pyramidal system.

### Unit Cell

Different atoms give various signals with varying strengths and dependence on electron density distribution in the closed shells. If the atom is lighter, the released signal is weaker, and vice versa. The mutual arrangement of these atoms is called the crystal structure, and they are derived from the chemical formulas and physical density of the solids.

A unit is the smallest part of the crystal component. A group of atoms, ions or molecules, arranged together purely builds up the crystal. Unit cells have a structure in 3D space, describing the bulk arrangement of the crystal's atoms.

### Bravais Lattice

Bravais Lattice means 14 different 3D configurations into which the atoms of a crystal can be arranged. There are various ways for describing a lattice, and the most fundamental one is known as Bravais Lattice.

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It is also referred to as an array of discrete points with an orientation and arrangement looking precisely the same as any other discrete points; that is, lattice points are indistinguishable. 7 main ones in 3D space are listed here.

### Triclinic System

In this system, all three axes incline towards each other and are of the same length. Depending on the three inclination angles, various forms of crystals are in paired faces. There is only a primitive cell in the triclinic Bravais Lattice. Some standard triclinic structures are Kyanite, Amazonite, Labradorite, Turquoise, Aventurine Feldspar, and Rhodonite. This structure is also found in Potassium Dichromate.

### Monoclinic Structure

There are three axes with two at right angles in this structure, and the third one is inclined. The length of all three axes is different based on the monocyclic system's inner structure, and it includes prisms and Basal pinacoids with inclined end faces. There can be primitive or base-centred monoclinic cells. Some common examples of monoclinic structure are Vivianite, Petalite, Gypsum, Howlite, etc. It is found in Sodium Sulfate and Monoclinic Sulfur.

### Orthorhombic System

It has three axes, all at right angles to each other. They are of different lengths and based on the rhombic structure, the orthorhombic system contains several crystal shapes, like pyramids, double pyramids, pinacoids, and rhombic pyramids. There are four types of Orthorhombic systems in Bravais Lattice: simple, base-centred, face-centred, and body-centred. Some common examples of it are Topaz, Zoisite, Tanzanite, Iolite, etc.

### Trigonal System

In the trigonal system, angles and axes are similar to the hexagonal systems. There are six sides at the system's base, and in all, there are three sides in this system. In the trigonal system, the crystal shape includes 3-sided pyramids, Rhombohedra, and Scalenohedral. For this Bravais Lattice, only the primitive unit cell exists. Some common examples of trigonal system are Quartz, Ruby, Jasper, Agate, etc. This system can be found in Sodium Nitrate.

### Hexagonal System

This system has four axes; 3 are of equal length and lies on the same plane. They intersect at 60 degrees, and the 4th axis intersects the other at right angles. The shapes for the system include double pyramids, double-sided pyramids and 4-sided pyramids. Hexagonal Bravias Lattice is only available as a simple hexagonal cell. Common examples of hexagonal system are Apatite, Sugilite, Beryl, etc. It is found in Zinc Oxide and Beryllium oxide.

### Tetragonal Crystal Shape System

The tetragonal crystalline structure contains three axes, and the central axis has a different length (either shorter or longer than others). The other two axes are in the same plane and have the same lengths. The tetragonal crystal shape includes double and 8-sided pyramids, 4-sided prism, pyrite, and trapezohedrons. A tetragonal system has simple and body-centred tetragonal cells, and the Bravais lattice follows the given relation:

a, b are equal but not equal to c

α, β and γ equals to 90 degree

Tetragonal crystal system examples are for the simple cells and body-centred cells structures. The typical examples of tetragonal crystal system are Titanium dioxide and Stannic Oxide.

Note: Here, a, b, and c denotes the dimensions of unit cells and α, β, and γ denotes the angles corresponding in the unit cells.

### Cubic System

In this system, all three axes intersect at right angles and have equal lengths. Cubic crystal systems include cube, octahedral, and hexaciscohedron. Common examples of this system include Garnet, Silver, Diamond, and Gold. The Cubic Bravais Lattices are of 3 types, including primitive cubic cell, Body-centered cubic cell, and face-centred cubic cell.

Q1: What is the Importance of a Crystal Structure? Differentiate Between Crystal Minerals and Rocks and Tell Whether Rocks Have Crystal Structures or Not.

Ans: If we consider distinguishing the two popular metals, diamond and graphite, it is perfect for the crystal structures' value. This is a clear example of why it is essential to know the composition of mineral elements and know-how the elements get stalked together.

A mineral refers to the inorganic substance or compound occurring naturally and having an ordered internal structure and chemical composition. It has different crystal shape and physical properties. In contrast, a rock of undistinguished mineral matter refers to the accumulation of one or more minerals.

A crystal composition is dependent on the condition of mineral formation. The minerals having the same chemical formula and different shapes are called polymorphs. Various conditions, including temperature and pressure, define the crystallisation in a rock structure.

Q2: Define the Factors Determining the Shape of the Crystal. What are the Basic Details of the Tetragonal System that One Must Remember?

Ans: there are various factors like size and length of the surface and faces, angles between them, etc., that define the crystal's shape. They are mainly for their geometry like the cubic crystal group belongs to the crystals centred on cubes.

The tetragonal crystalline structure contains three axes, and the central axis is either shorter or longer than the others. The other two axes are in the same plane with equal length. The tetragonal crystal shape includes double and 8-sided pyramids, 4-sided prism, pyrite, and trapezohedrons. A tetragonal system has simple and body-centred tetragonal cells, and the Bravais lattice follows the given relation:

a, b are equal but not equal to c

α, β, and γ equals to 90 degree

Here, a, b, and c are the dimensions of unit cells and α, β, and γ are the unit cells' angles.