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Formation of Crystal and Crystal systems

Formation of Crystal and Crystal Systems

The crystals of the substance are obtained by cooling the liquid (or the melt) of the solution of that substance. The size of the crystal depends upon the rate of cooling. If cooling is carried out slowly, crystals of large size are obtained because the particles (ions, atoms or molecules) get sufficient time to arrange themselves in proper positions.

Atoms of molecules cluster

dissolved embryo crystal

(If loosing units dissolves as embryo and if gaining unit grow as a crystals).

Bravais (1848) showed from geometrical considerations that there can be only 14 different ways in which similar points can be arranged. Thus, there can be only 14 different space lattices. These 14 types of lattices are known as Bravais Lattices. But on the other hand Bravais showed that there are only seven types of crystal systems.

Table: Bravais lattices corresponding to different crystal systems

Crystal system

Space lattice

Examples

Cubic

,

Simple: Lattice points at the eight corners of the unit cells. Body centered: Points at the eight corners and at the body centred.

Face centered:Points at the eight corners and at the six face centres.

, diamond, CsCl, NaCl, and alums. etc.

Tetragonal

,

Simple: Points at the eight corners of the unit cell. Body centered: Points at the eight corners and at the body centre ., white etc.

Orthorhombic (Rhombic)

,

Simple: Points at the eight corners of the unit cell.

End centered: Also called side centered or base centered. Points at the eight corners and at two face centres opposite to each other.

Body centered : Points at the eight corners and at the body centre

Face centered:Points at the eight corners and at the six face centres.

, , rhombic sulphur, etc.

Rhombohedral

or Trigonal

,

Simple: Points at the eight corners of the unit cell calcite, quartz, etc.

Hexagonal

,

Simple: Points at the twelve corners of the unit cell out lined by thick line.

or Points at the twelve corners of the hexagonal prism and at the centres of the two hexagonal faces. graphite, ice, etc.

Monoclinic

,

Simple: Points at the eight corners of the unit cell End centered: Point at the eight corners and at two face centres opposite to the each other.

monoclinic sulphur etc.

Triclinic

,

Simple: Points at the eight corners of the unit cell.

etc.

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