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Important Question And Answer: Civil - Engineering Geology - Mineralogy

Civil - Engineering Geology - Mineralogy

1) Define mineralogy.


It is defined as naturally occurring inorganic solid substance that is characterized with a definite chemical composition and very often with a definite atomic structure. It is a branch of geology, which deals with the various aspects related to minerals such as their individual properties their mode of formation and mode of occurrence.


2) How are minerals are identified and what are the symmetry elements of crystal?


Their colour, streak, hardness, cleavage, crystal form, specific gravity and lustre generally identify minerals. The symmetry elements are:


i)                        Plane of symmetry

ii)                        Axis of symmetry

iii)              Centre of Symmetry


3) What are the physical properties of minerals?


The physical properties are:


i)                    Colour

ii)                    Lustre

iii)              Streak

iv)                Hardness

v)                  Cleavage

vi)                  Fracture

vii)              Tenacity

viii)         Structure

ix)               Specific gravity

x)                Form

xi)              Miscellaneous


4) Define streak.


Streak is the colour of the finely powdered mineral as obtained by scratching or rubbing the mineral over a rough unglazed porcelain plate. The plate often named as streak plate. It is important and diagnostic property of many coloured minerals.


5) What is mean by fracture and types of fracture?

The appearance of the broken surface of a mineral in a direction other than that of cleavage is generally expressed by the term fracture. The types are:


i)                   Even

ii)                Uneven

iii)               Conchoidal

iv)              Splintry

v)                  Hackly

vi)              Earthy



7) Define Planes/axis/Centre of symmetry.


Plane of symmetry:


It divides a crystal into similar and similarly placed halves. This plane provides crystal so that one half is the mirror image of the other.


Axis of symmetry:


If is a crystal one being rotated, come to occupy the same position in space more than one in a complete turn, the axis about which rotation taken place is called an axis of symmetry.


Centre of symmetry:


A crystal has a centre of symmetry when like faces; edges are arranged in pairs in corresponding positions on opposite sides of a central point. The cube and bricks obviously have centre of symmetry.



9) What is mean by holohedral form?


The maximum symmetry elements of the system. The form is having all the faces, which have same position with regard to the crystallographic axis. The highest symmetry elements of the system. The forms present in normal class of the system are holohedral form since it has the maximum symmetry elements than the other classes.


10) Define hexagonal system.


All those crystals, which can be referred to four crystallographic axes of which three axes


are horizontal, equal interchangeable and intersecting each other at 1200 between the positive ends. The fourth axes are vertical and at right angles to the three horizontal axes, are grouped under hexagonal system.


11) Give the physical properties and uses of mica?


The mica group only muscovite and biotite are of common occurrence as rock forming minerals.

S.no    properties   muscovite    biotite

1          Colour           Transparent, gey or light brown            Brown to black

2          Streak            Uncoloured  Same as colour

3          Cleavage       perfect          perfect

4          Lustre            Vitreous        Vitreous

5          Hardness      2-2-5  2-2-5

6          Specific gravity       2.8-3.1          2.6-3.1

7          System          Monoclinic   Monoclinic

12) What are the chemical compositions in feldspar group?


The feldspars are chiefly aluminosilicates of Na, K and Ca with following general formula:




In which = Na, K, Ca ad Ba and Z = Si and Al


Some examples of chemical composition of feldspar minerals are


i) Na Al Sio3 O8                     ii) K Al Sio3 O8                       iii) Ca Al2 Si2O8





13) Mention the various types of forms?


Any group of similar faces showing identical mathematical relations with crystallographic axes makes a form. Forms are further distinguished into the following types:


a)       Holohedral form

b)    Hemihedral form

c)     Hemimorphic form

d)     Enantiomorphic form

e)     Fundamental form

f)      Open and closed form


14)           What are the five symmetry classes in Isometric system?


The types are:

a) Axes of symmetry: 13 in all


3 are axes of four-fold symmetry

4 are axes of three-fold symmetry

6 are axes two fold of symmetry

b) Planes of Symmetry: 9 in all


3 planes of symmetry are at right angles to each other and are termed the principal planes;


6  planes f symmetry are diagonal in position and bisect the angles between the principal planes.


15) Define tetragonal system?


The tetragonal system includes all those crystals, which can be referred to three crystallographic axes so that two axes are equal, interchangeable and horizontal in positions. The third axis is either longer or shorter than the other two and is vertical in position and all three axes are mutually at right angles to each other.


16)           Give the three types of pyramid.


Three types of pyramid:


i)                   Pyramid of first order: eight faces; each face cuts the two horizontal axis at equal length besides intersecting the vertical axis.


ii)                Pyramid of second order: Eight faces; each face cuts only one of the two horizontal axes.


iii)              Pyramid of third order: It is also called ditetragonal pyramid is a closed form of 16 faces in which each face cuts all the three crystallographic axes.


17)           Draw the axial diagram for orthorhombic system?


The three crystallographic axes are arbitrarily designated as a, b and c. The b axis is always longer of the two horizontal axes and is also designated as micro axis. The a axis being shorter is designated as brachy axis.


18) Define dome and types of domes?


A dome is defined as a form whose faces essentially meets the vertical axis and are parallel to one of the two horizontal axes. Two types of domes:


i)  Macrodome: An open form of two faces in which each face meets the vertical axis and the a-axis and is parallel to the macro axis.


ii) Brachycome: An open from of two faces each face is parallel to the brachy axis and meets the other two axes.


19) What are the forms allowed in monoclinic system?


The forms are:




a)     a- pinacoid

b)       b-pinacoid

c)     c-pinacoid

ii)                Domes

a)      Orthodome

b)    Clinodome


iii)              Prims


a)       Unit prism

b)      Orthoprism

c)     Clinoprism


iv)              Pyramids


a)        Unit  pyramid

b)       Orthopyramid

c)     Clino pyramid


20) Write short notes on:

i)                   Twin plane


ii)                Twin axis


Twin plane:


It is a common to both the halves of the crystal and across which one half may appear to be the reflection of the other. Any pale parallel to a crystal face may be a twin plane.


Twin axis:


It is a crystallographic direction along which a rotation of some degrees seems to have produced the resultant twins. In other words if a backward rotation to the same extent is possible to be given the two parts should take the form of a singe crystal.


21) What is the common twin law in Hexagonal system?


The laws are:


Brazillian Law: In this law the prism of IInd order is twin plane.


Dauphine law:  In this law c-axis is the twinning axis. Twins are generally intergrown


Japanese law:  It contact twins result on this law in which pyramid is a twinning plane.


22) Define peat.


It is a essentially a partly changed vegetable matter in the first stage of transformation to coal. The vegetable structure is easily visible and the evidence of its being in the process of transformation is also clearly seen. Peat is generally composed of remains of moss like plants but 23) What is mean by cannel coal?


It is a type of bituminous coal, which is exceptionally rich in spores, and resins of plants, which are believed to have been deposited by wind in subsequence condition. The cannel coal is sometimes further distinguished as bog head coal, if in its constitution the algal remains are dominating over the spores and resins.


24)           What are the possibilities may suggest in Inorganic theories?


The possibilities are:


i)                    Reaction of alkali metals

ii)                   Reaction of iron carbide

iii)              Concentration


iv)              Decomposition


25)           Give any one factor in controlling the accumulation?


Lithology: Certain types of rocks are better suited as reservoirs because of their inherent qualities of permeability and effective porosity. Rocks with effective porosity of greater than 15 percent and permeability greater than 1000 millidarcies are considered 'A' class reservoirs.


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