introduction to crystallography

Crystallography and Mineralogy

Dr.B.GURUGNANAMProfessor and HeadCentre for Applied Geology GRI-DU

Mr.K.KALAIVANANResearch ScholarCentre for Applied Geology GRI-DU.

Introduction to Crystallography

What is crystallography?

CRYSTALLOGRAPHY is simply a fancy word meaning "the study of crystals“

• The study of crystalline solids and the principles that govern their growth, external shape, and internal structure

• Crystallography is easily divided into 3 sections - geometrical, physical, and chemical.

Definition of crystal

A CRYSTAL is a regular polyhedral form, bounded by smooth faces, which is assumed by a chemical compound, due to the action of its inter atomic forces, when passing, under suitable conditions, from the state of a liquid or gas to that of a solid.

Crystalline and Amorphous

• (In Greek amorphous means “formless”) do not have overall regular internal structure; their constituent particles are arranged randomly; hence, they are isotropic, have no symmetry, and cannot be bounded by faces.

• Particles are arranged in them in the same way as in liquids; hence, they are sometimes referred to as super cooled liquids.

• Examples of amorphous substances are glass, plastics.

• In distinction to crystalline substances, amorphous ones have no clearly defined melting point.

Cont…

• Comparing curves of cooling (or heating) of crystalline substances and amorphous substances, one can see that the former has two sharp bend-points (a and b), corresponding to the beginning and end crystallization respectively, whereas the latter is smooth

Curve of cooling of a crystalline subsatnce

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Curve of cooling of amorphous substances

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ab

Symmetry

Symmetry map be defined in rotation to (i) Plane of symmetry

• Divided a crystal into two equal half • Mirror plane

(ii) Axis of symmetry • Parallel to the edge of intersection of

two crystal faces.• Binary, trigonal, tetragonal, hexagonal

(iii) Centre of Symmetry• Centre of symmetry if an imaginary

line passing the centre

Classification of crystals

• Classification of crystals according to the degree of crystallization

• Euhedral crystals• Subhedral crystals• Anhedral crystals

Euhedral Crystal Subhedral Crystal Anhedral

Crystal

Crystallographic forms

1- PedionIt is an open form made up of a

single face

Cont…

1- PinacoidIt is an open form made up of two

parallel faces

Basal Pinacoid

Side pinacoid

Front Pinacoid

Cont…

3- Dome

It is an open form made up of two nonparallel faces symmetrical with respect to a symmetry plane

4- Sphenoid

It is an open form made up of two nonparallel faces symmetrical with respect to a 2-fold or 4-fold symmetry axis

Cont…

5- Disphenoid

It is an closed form composed of a four-faced form in which two faces of the upper sphenoid alternate with two of the lower sphenoid.

Cont…

6 -Bipyramid

It is an closed form composed of 3, 4, 6, 8 or 12 nonparallel faces that meet at a point

Orthorhombic bipyramedDitetragonal bipyramid

Tetragonal bipyramid

Dihexagonal bipyramid

Hexagonal bipyramid

Cont…

7- PrismIt is an open form composed of 3, 4, 6, 8 or 12 faces, all of which are parallel to same axis.

Orthorhombic prism Tetragonal prism

Ditetragonal prism

Hexagonal prismDihexagonal prism

Cont…

8- Rhombohedron

It is an closed form composed of 6 rhombohedron faces,

9- Scalenohedron

It is an closed form composed of 12 faces, each face is a scalene triangle. There are three pairs of faces above and three pairs below in alternating positions

Crystal Habits

The term "crystal habit" is used to categorize the appearance, shape, and size of a crystal, and identify its unique growth characteristics that result from its crystalline structure and growth environment.

1. Crystal Twinning ( two individual crystals share some of the same crystal lattice points)

2. Growth Conditions (heat, pressure, and space)

3. Trace Impurities (present during crystal formation)

Crystal Habits

Crystal Habits

Crystal Habits

Crystal Habits

Crystal Habits

Crystal Habits

Crystal Habits

Crystal Habits