Elements of crystal

presentation on elements of crystal including figures.

1. SNPIT & RC Presentation on : Elements of crystal. Subject : Geotechnics and Applied Geology.(2130606)

2. Prepared By: Sindhvad Kathan = 160490106087 Upadhyay Nishank = 160490106092 Sharma Vishal = 160490106085 Shukla Malhar = 160490106086

3. CRYSTAL A crystal may be defined as a natural solid body bounded by smooth and plain surface. Arranged in an orderly (i.e. geometrical) pattern , which is an outward expression of a regular internal atomic structure.

4. ELEMENTS OF CRYSTAL Parts of crystal 1. Angle 2. Edge 3. Face

5. 1. Crystal Faces The crystal are bounded by flat surfaces which are known as faces. A crystal may have only two faces of the same geometrical shape or it may have up to forty-eight faces of a combination of geometrical shapes.

6. 2.Edge The line of intersection formed by any two adjacent faces in a crystal is call an e dge.

7. 3. Solid Angle The points of intersection formed by three or more adjacent faces in a crystal are called solid angle.

8. 4. Interfacial Angle I n a crystal, the angle between normal of two adjacent faces is called ‘interfacial angle’ . Interfacial angles are measured either with a contact goniometer or a reflecting goniometer.

9. Goniometer

10. 5. Distortion Inspite of perfect internal atomic arrangement, sometimes crystals develop faces of different sizes shapes. This kind of geometrical irregularity in the shape of crystal is called ‘distortion’.

11. 6.Simple form If a crystal is bounded by all similar or like faces, is called a simple form.

12. Combination If a crystal is bounded by dissimilar or unlike faces, it is called a combination. ( i.e. combination of different simple forms)

13. 7.Zone and zone axis In many crystals, a group of faces are arranged in such a manner that their intersection edges are parallel to each other. Such faces constitude a zone. A line which passes through the centre of the crystal and lie parallel to the line of the faces intersections is called the zone axis.

14. 8.Crystallographic Axes In order to describe the faces and symmetry of crystal, a set of three or four reference axes are established. These imaginary reference lines are called “crystallographic axes”. While fixing a crystal in its study position with reference to the crystallographic axes, it is important to align it in such a way that one of the axes runs front to back, from the observer, another runs right to left, and one is passing from top to bottom of the crystal.

15. Crystallographic Axes

16. Axial Ratio It is the ratio between the lengths of different crystallographic axes in a given crystal system and is always constant for that crystal. Ex.: a=10.47 A* a:b=10.47/12.87=0.813 b=12.87 A* b:c=12.87/24.49=0.525 c=24.49 A* a:c=10.47/24.49=0.4275

17. Unit Cell Unit cell is a tiny building block of crystal. Each crystal is composed of number of a such unit cells. In each unit cell there is a group of atoms linked in a fixed spatial relationship to one another. Unit cell represent this smallest volume which possesses all the chemical, phisical and geometrical properties of the crystal different mineral species have different types of unit cell.

18. 9.Symmetry The faces ,edges and solid angles in crystals occur with some regularity. This is known as symmetry. Symmetry is property of fundamental importance for a crystal. I t can be studied with reference to three different characters, commonly called elephants of symmetry. These are: (i) Plane of symmetry (ii) Axis of symmetry (iii) centre of symmetry

19. (i) Plane of symmetry A plane of symmetry is an imaginary plane which divide a crystal into two halves, each of which is the mirror image of the other.

20. (ii) Axis of symmetry It is imaginary line through crystal about which if the crystal is Rotated, it gives the observer exactly the same view more than once in a single rotation.

21. (iii)Centre of symmetry A crystal is said to possess a centre of symmetry if on passing an imaginary line from some definite face, edge or corner on one side of crystal through its centre, another exactly similar face, edge or corner is found on the other side at an equal distance from the centre. The cube and octahedron possess a centre of symmetry, whereas a tetrahedron does not. Many crystals have no planes or axis of symmetry but do possess centre of symmetry.

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