🔘 AGY203. CRYSTALLOGRAPHY

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*CRYSTALLOGRAPHY & MINERAL CRYSTAL SYSTEM*

Crystallography is simple the study of crystals, study of growth, shape and geometric character of crystals. Each chemical element and compound tends to crystallize in a definite and characteristic firm when condition is favourable.

Crystallography is divided into 3 sections:-

(i) Geometrical

(ii) Physical and 

(iii) Chemical


 *CRYSTAL*

A Crystal is a regular polyhedral form, bounded by smooth face, which us assumed by a chemical compound, due to the action if its inter-atomic forces, when passing under suitable conditions, from state I'd a liquid or gas to that of a solid 


 *CRYSTAL FACE*

This is a polyhedral form that is bounded by flat planes.

 *Note:-* Crystal normally forms during the change of matter from liquid or gas to the solid state.


There are geometric forms which are subdivided into:-

A. Cubic / Isometric

B. Tetragonal

C. Orthorhombic

D. Monoclinic

E. Hexagonal

F. Triclinic


 *STENO'S LAW OF CONSTANT OF INTERFACIAL ANGLES*

In 1669, Nicolas Steno, a Danish physician and natural scientist, discovered by examination of numerous specimens of the same minerals, that when measured at same temperature, the angles between the similar crystal faces remain constant regardless of the size or the shape of the crystal


 *POLYMORPHS*

These are minerals with the sane chemistry, like diamond and graphite or sphalerite and wurtzite, but having different atomic structure and therefore crystallizing in different crystal systems.

Galena often forms equate shapes (cubes or octahedrons), quartz typically is prismatic, and barite tabular.


 *CRYSTALLOGRAPHIC AXES*

These are imaginary lines used to describe crystal faces. All crystals, with the exception of those belonging to the hexagonal system, referred to _3 crystallographic axes_ designated as a, b and c

In triclinic system, all the axes are of *different length* and at *oblique angles* to each other.

In orthorhombic system, the three axes are also different length but are mutually perpendicular.

 *a- axis* (horizontal and in front-back position)

 *b- axis* (horizontal and in left right position)

 *c- axis* (vertical)


 *NOTE:*

1. The front end of a, right hand end of b and the upper of c are positive and the opposite ends are negative.

2.  Alpha angle is designated between axial direction b and c, beta is between a, c and gamma is between a and b


 *CRYSTAL LATTICE AND UNIT CELL*

X-rqy studies, crystal structures are built up from simple structural units composed of a few atoms, ions or molecules

 *The Crystal Unit* is called the *Unit Cell* or *Space Lattice

 *Point Lattices* are the intersection of lines and planes which generate array of point in a crystal

 *Unit Cell:-* is the small unit that can be generated to form an entire lattice

To be continue.....

✍️ *FLASHPEE EDUCATIONAL TEAM*

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