 ## Important question of Geology for Sem 2 / Chapter 1 and 3

some Important questions of geology sem2 are given below….

## Important questions of geology sem2

1. Define Tetragonal system? Give its axial daiagram, forms, and symmetry?

Ans- A crystal system characterized by three axis at right angles of which only the two lateral axis are equal. All the crystals in the tetragonal system are referred to three mutually perpendicular axis.

1. 3 Axis, 2 horizontal are equal but the 1 vertical axis is of different length.
2. a1 and a2 is of same length but c1 is either longer or shorter.
3. All 3 axis are at right angle.

The following Forms are describe below:-

1. Basal Pinacoid (open form of two faces) :- It is an open form of 2 similar faces each face is parallel to 2 horizontal axis and meets the vertical axis at unit length.
2. Prism :- Open form of 4 or 8 faces in which each dace is parallel to vertical crystallography axis, depending upon its relationship with other 2 axis ,

They are divided into 3 types are described below :-

Prism of First Order :- This form has four rectangular vertical faces. Each face intersect the two horizontal crystallographic axis are at equal length and lies parallel to the vertical axis

Prism of second order :- This form has four rectangular vertical faces. Each face intersects one horizontal crystallographic axis are at equal length and lies parallel to the other side.

Prism of third order :- This form has eight rectangular vertical faces. Each face intersect the two horizontal crystallographic axis at unequal distances and parallel to the vertical axis.

Pyramid :- Closed form of 8 to 16 faces. Each face touches vertical crystallographic axis depending on its right angle.

Pyramid of 1st order :- This form have 8 faces. (HHL)

Pyramid of 2nd order :- This have form of 8 faces. (h0l)

Pyramid of 3rd order :-This form is composed of 16 faces. (hkl)

### 2. Define Mohs scale of hardness through a table ?

Ans :- A numerical value is obtained bu using the “Mohs scale of hardness”. In this scale there are ten minerals which are arranged in the order of their increasing hardness.

### 3. Define Plagioclapse Felspars and its series ?

Ans :- The plsgioclasper flespars from a complete solution series from pure “albite”, NaAlSi3O8 to pure “anorthite”, CaAl2Si2O8. The important intermediate members are “obligoclase”, “andesine”, “labradorite” and “bytownite”. The plagioclasper flesper crystallize in the triclinic system.

The properties of various members of the plsgioclase series vary in a uniform manner, with the change in the chemical composition.

Physical properties :-

1. Colour – green or grey.

2. Lusture – Vitreous

3. Specific gravity – 2.6 – 2.7

4. Hardness – 7

5. Form (Habbit) – Tabular

6. Cleavage – At an angle of 90 degree

Use :- Use in the manufacture of porcelain

## 4. Define the index system of miller ?

Ans :- In the index system of Miller, the notations of crystal faces are called “Miller indices”. The Miller indices of a face consist of a series od whole number that have been derived from the parameter by taking their reciprocals and then by cleaning of fractions.

## 6. Define axis, plane and centre of symmetry ?

Ans :- An optic axis of a crystal is a direction in which a ray of transmitted light suffers no birefringence (double refraction). An optical axis is a direction rather than a single line: all rays that are parallel to that direction exhibit the same lack of birefringence.

Crystal planes are defined as some imaginary planes inside a crystal in which large concentration of atoms are present. … Crystal planes and directions can be represented by a set of three small integers called Miller indices [because Miller derived a method of representing crystal planes].

Centre of symmetry: It is an imaginary point in thecrystal that any line drawn through it intersects the surface of the crystal at equal distance on either side.

## 7. Define Granet group ?

Ans :- Garnet id not a single mineral , but a group contain closely related, isomorphic mineral that form a series with each other. The granet members form intermediary minerals between each member, and may even intergrow within a single crystal. Its general formula is X3Y2 (SiO4)3 where x may be Ca, Mg, or Fe 2+ and y

The common granets are 1) “Pyrope ” , Mg3Al2Si3O12

2) ” Almandite” ,Fe3,Al2si3O12,

3) Spessaritite Mn3Al2Si3O12 and

4) Graossularite Ca3Al2Si3O12

CRYSTAL SYSTEM – Cubic

Colour – red brown yellow and green

Habbit – dodecahedral crystals common, also granular.

Cleavage – absent

Fracture – uneven

lusture – vitrous

Steak – white

Occurance – Occur abundantly in some metamorphic rocks such as mica-schists, hornblende- schists, and gneisses, also occur as an accesory mineral in some igneous rocks.

Uses – Transparent varieties are used as gern stones. Garnets are also used as an adrasive.

## 10. Define crystal and unit cell?

Ans :- A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

Unit cell :- The “lengths” of the various crystallographic axes are defined on the basis of the unit cell.  When arrays of atoms or molecules are laid out in a space lattice we define a group of such atoms as the unit cell.  This unit cell contains all the necessary points on the lattice that can be translated to repeat itself in an infinite array.  In other words, the unit cell defines the basic building blocks of the crystal, and the entire crystal is made up of repeatedly translated unit cells.
In defining a unit cell for a crystal the choice is somewhat arbitrary.  But, the best choice is one where:
1) The edges of the unit cell should coincide with the symmetry of the lattice.
2) The edges of the unit cell should be related by the symmetry of the lattice.
3) The smallest possible cell that contains all elements should be chosen.

these are some Important questions of geology sem2 give above. tho most Important questions of geology….

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