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Crystal Structure

A crystal structure or system is the three-dimensional arrangement of atoms or molecules within a crystal lattice. It is highly ordered and repetitive, creating a characteristic pattern that defines the crystal’s shape and properties. Crystallography is the science that studies crystal structure.

Crystal Structure and Unit Cell

A unit cell can be defined as the smallest repeating unit within a crystal lattice that retains all the structural and symmetry features of the crystal. The basis of unit cells lies in their ability to represent the three-dimensional arrangement of atoms or ions within the crystal lattice. This arrangement is determined by considering both the size and shape of the unit cell, as well as its orientation to adjacent unit cells.

Crystal Structure and Bravais Lattice

A Bravais lattice can be defined as a three-dimensional array of points representing a crystal’s periodicity and symmetry. There are 14 distinct Bravais lattices classified into seven crystal families and four atom placement styles based on their symmetries. Each crystal family has a unique set of lattice parameters and symmetry operations. Primitive lattice vectors play a crucial role in defining each Bravais lattice type. These vectors connect adjacent lattice points within a unit cell and determine the overall shape and symmetry of the crystal system.

Crystal Structure

Crystal Structure Types

Crystal structures are classified into seven lattice families.

1. Cubic

The cubic crystal lattice family is the most fundamental of all the seven lattice families. It refers to the three main types of cubic crystals: face-centered cubic (FCC), body-centered cubic (BCC), and simple cubic (SC).

The FCC lattice is characterized by atoms at each corner of the cube and the center of each face. This arrangement results in a total of four atoms per unit cell. FCC structures are commonly found in aluminum, copper, and gold.

The BCC lattice consists of atoms located at each corner of the cube, with an additional atom positioned at the center of the cube. It gives a total of two atoms per unit cell. BCC structures can be observed in metals like iron, chromium, and tungsten.

The SC lattice features atoms only at each corner of the cube. This results in one atom per unit cell. SC structures are rare but can be found in certain elements, such as polonium.

Cubic Crystal System

2. Tetragonal

The tetragonal lattice structure can be distinguished by its four-sided unit cell and axes of different lengths perpendicular to each other. In this arrangement, two of the crystallographic axes are of equal length. The third axis is perpendicular to the other two but of a different length, creating a rectangular prism-shaped unit cell.

The tetragonal structure’s defining symmetry makes its properties unchanged when rotated or reflected within the lattice. Materials exhibiting a tetragonal lattice structure can be found in various substances, from metals like zirconium and titanium to minerals like rutile and cassiterite.

Tetragonal Crystal System

3. Orthorhombic

Orthorhombic crystals are known for their rectangular prism shape, with three unequal axes intersecting at right angles. These axes intersect at right angles, akin to the dimensions of a cuboid, giving orthorhombic crystals their distinct rectangular prism-like shape.

Orthorhombic symmetry differs from the cubic system due to unequal axis lengths. Within this family, crystals exhibit varying degrees of symmetry across the three axes, leading to a range of possible crystal forms.

Orthorhombic crystals can be found in numerous mineral formations, such as the stunning topaz and the shimmering aragonite.

Orthorhombic Crystal System

4. Rhombohedral

The defining characteristic of the rhombohedral crystal lattice family lies in the shape of its unit cell, a rhombohedron—a six-faced geometric figure where all sides are of equal length but angles are not necessarily right angles. This unique structure leads to a lattice system that diverges from the orthorhombic, cubic, and tetragonal systems, displaying its symmetrical properties.

Crystals exhibit a threefold rotational symmetry axis perpendicular to a unique fourfold rotational symmetry axis. This configuration results in crystals often appearing in shapes reminiscent of hexagons, demonstrating a threefold rotational symmetry around their vertical axis. The angles between the crystal faces are unequal but consistent within the same crystal system, typically measuring around 60 degrees.

Examples include carbonates like calcite and dolomite and certain metals and alloys such as aluminum and bismuth.

Rhombohedral Crystal System

5. Hexagonal

Each unit cell of the hexagonal crystal lattice family resembles a hexagonal prism with a regular hexagon at its base and parallelograms as side faces. The lattice is characterized by two lattice constants: the hexagonal “a” axis and the vertical “c” axis, which are unequal in length. This unique arrangement grants hexagonal crystals distinctive properties and behaviors compared to other crystal systems.

The lattice possesses a six-fold rotational symmetry around the c-axis, meaning that when rotated by 60 degrees repeatedly, the crystal appears identical at every sixth turn. Additionally, hexagonal structures often exhibit a high degree of planar symmetry, enabling them to pack efficiently in space, leading to their natural prevalence among minerals like quartz, calcite, and graphite.

Hexagonal Crystal System

6. Monoclinic

In a monoclinic lattice, the three axes are of different lengths, with two axes intersecting at oblique angles and the third axis perpendicular to the other two. This results in a parallelogram-shaped unit cell.

The unequal axes lengths and angles in monoclinic crystals give rise to anisotropic properties, meaning that their physical properties vary depending on the direction in which they are measured. It can include variations in electrical conductivity, thermal expansion, and optical behavior.

Monoclinic crystals can be found in minerals such as gypsum, orthoclase feldspar, and clinopyroxene.

Monoclinic Crystal System

7. Triclinic

Triclinic crystals exhibit the least symmetry among all seven crystal systems. The angles are not constrained to 90 degrees. Its three axes are all of different lengths and intersect at oblique angles, allowing maximum shape and structure variability among all crystal systems. This lack of symmetry creates a diverse range of crystal shapes and arrangements within this lattice family.

Minerals like microcline, plagioclase, and labradorite exhibit triclinic symmetry, showcasing the diverse nature of crystalline structures found in nature.

Triclinic Crystal System

FAQs

Q.1. What is the difference between atomic structure and crystal structure?

Ans. The primary difference between atomic structure and crystal structure lies in their definitions: atomic structure refers to how atoms are organized within a single molecule or a cluster of molecules, whereas crystal structure concerns the specific arrangement of atoms within a solid substance.

Q.2. Do all minerals have a crystal structure?

Ans. Yes, all minerals have a well-defined crystal structure.

Q.3. What determines the crystal structure of a mineral?

Ans. The crystal structure of a mineral is determined by the conditions under which the mineral forms, including chemical composition, temperature, pressure, and the presence of other substances.

Q.4. Does freezing involve the creation of a crystal structure?

Ans. Most liquids freeze by the formation of crystalline solids from the uniform liquid.

Article was last reviewed on Saturday, December 30, 2023

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