Physical Properties of Minerals (PDF)

SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) UNIT II MINERALOGY 1) Explain briefly about physical properties of minerals? (Nov 2005) The...

SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) UNIT II MINERALOGY 1) Explain briefly about physical properties of minerals? (Nov 2005) The following are the important physical properties: i) Color ii) Streak iii) Lustre iv) Structure v) Hardness vi) Specific gravity vii) Cleavage viii) Fracture ix) Tenacity x) Form Color: Color is not constant in most of the minerals and commonly the color is due to stain or impurities in the minerals some minerals show peculiar phenomena connected with color. Streak: The streak, which is the color of the mineral powder, is more nearly constant than the color. The streak is determined by marking unglazed porcelain or simply by scratching it with a knife and observing the color of the powder. Lustre: It is the appearance of a fresh surface of a mineral in ordinary reflected light Structure: This is a term used to denote the shape and form of minerals. Hardness: It is the resistance of mineral offers to abrasion or scratching and is measured relative to a standard scale of ten minerals known as Moh’s scale of hardness. Specific gravity: It may be defined as the density of the mineral compared to the density of water and as such represents a ratio.ie specific gravity of a mineral is the ratio of its weight of an equal volume of water. 1 CE 6301 ENGG GEOLOGY/UNIT II SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) Specific gravity of a mineral depends upon the weight and spacing of its atoms. Cleavage: It is defined as the tendency of a crystallized mineral to break along certain definite planes yielding more or less smooth surfaces. Fracture: The fractures of a mineral may be defined as the appearance of its broken surface. Tenacity: Important properties related to tenacity of the minerals are expressed by the terms like balances, flexibility, elasticity, sectility and mellability etc. When a mineral can be cut with a knife it is termed “sectile” and if the slice cut out from it can be flattened under a hammer. It is also said “mellable” “brittle” minerals. Term elastic is used if it regains its former shape as the pressure is released. Form: The internal atomic arrangement of a mineral is manifested outwardly by development if geometrical shapes or crystal characters. 2) Write short notes on i) Symmetry ii) Crystallographic axis Symmetry: Symmetry is understood a sort of regularity in the arrangement of faces on the body of a crystal. Symmetry is a property of fundamental importance for a crystal. It can be studied with reference to three different characters, commonly called elements of symmetry. These are: A plane of symmetry An axis of symmetry Centre of symmetry ii) CRYSTALLOGRAPHIC AXES These are also termed as axes of reference and are simply certain imaginary lines arbitrarily selected in such a way that all of them pass though centre of an ideal crystal. The concept of axes of reference is based on the fact that exact mathematical relations exist between all the faces on a given crystal with reference to its centre. 2 CE 6301 ENGG GEOLOGY/UNIT II SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) In crystallography following general assumptions have been universally agreed upon regarding these crystallographic lines: a) Three Straight Lines, essentially passing though a common centre and varying in mutual relationships with respect to their lengths and angular inclinations from: all equal, Interchangable and it right to all unequal and inclined with each other. b) Four straight lines, essentially passing through a common centre; one vertical, being unequal to the other three but at right angles to them. The three horizontal axes are separated from each other at 1200 3) Define the following terms i) Parameter ii) Indices iii) Symbols iv) Forms Parameters The relative intercepts made by a crystal face on the three crystallographic axes are known as its parameters. For instance in the three crystallographic axes are represented by XOX’,YOY’ and ZOZ’. With their relative lengths as Ox=a, Oy=b and Oz= c Indices In common practice the relationship of a crystal face with the crystallographic axes is expressed in simple whole numbers, which are called indices. In the miller system the indices for a given crystal face are derived from its parameters in two simple steps: i) By taking the reciprocals of the parameters actually obtained for the given face. ii) By clearing fractions if any by simplifications and omitting the letters for the axes in the final expression. Symbol It is the simplest and most representative of the indices for a set of similar faces that constitute a crystallographic form. For instance in this fig there are six exactly identical crystal faces, which have same mathematical relationship with all the three crystallographic axes. Forms Any group of similar faces showing identical mathematical relations with crystallographic axes makes a form. Forms are further distinguished into following types: 3 CE 6301 ENGG GEOLOGY/UNIT II SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) Holohedral form, hemihedral form, and hemomorphic form, enantiomorphic form, and fundamental form, open and closed form. 4) Give a detailed account of the elements of the symmetry and minerals crystallizing in any two of the following systems. i) Isometric system ii) Hexagonal system iii) Monoclinic system Isometric system: Definition: All those crystals that can be referred to three crystallographic axes, which are essentially equal in length at right angles to each other, and mutually interchangeable, are said to belong to the isomeric or cubic system. Axial diagram Since all the three axes are equal and interchangeable these are represented by the letter a. In the study position however the axes may be designated as a1,a2 and a3 the last being vertical. Classes Five symmetry classes fall in the Isometric system by virtue of their country The normal class is known as galena type. It has got the following symmetry. Forms Following are the forms that commonly develop in the crystals belonging to isometric system. 1. Cube 2. Octahedron 3. Dodecahdraon 4. Trisoctahedron 5. Trapezohedran 6. Hexaoctahedran 7. Tetra hexahedron Hexagonal system i) Three axes are horizontal, equal, interchangeable and intersecting each other at 1200 between the positive ends. ii) The fourth axes are vertical and at right angles to the three horizontal axes are grouped under the hexagonal system. 4 CE 6301 ENGG GEOLOGY/UNIT II SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) Axial diagram The horizontal axes all being equal are designated by the letter a(a1,a2,a3)and the vertical axis by the letter ‘c’ as usual. Forms Forms of hexagonal system differ in character from forms of all the other systems in that their parameters, indices and symbols are determined with respect to four crystallographic axes. Thus the general form expresses the relation of any hexagonal form. Monoclinic system The monoclinic system includes all those forms that can be referred to three crystallographic axes which are essentially unequal in length and further that can be of these is always inclined. Axial diagram All the three axes are unequal, they are designated by the letters a, b and c. The c axis is always vertical. The inclined axis is a- axis. It is inclined towards the observer and is also referred as clino axis Forms The common forms of this system are 1) Pinacoid 2. Domes 3. Prisms 4. Pyramid 6) Describe the classification and formation of coal? The term coal is generally applied to a sedimentary formation of highly carbonaceous character that is derived from vegetable matter involving set of process such as burial, compaction and biochemical transformation. Peat It is essentially a partly changed vegetable mater in the first stage of transformation to coal. The vegetable structure is easily visible and the evidence of its being in the processes of transformation. to coal. Peat is generally composed of remains of moss like plants but occasionally may contain reeds and partially altered portions of trees of higher order. 5 CE 6301 ENGG GEOLOGY/UNIT II SRI VIDYA COLLEGE OF ENGG & TECHNOLOGY QUESTION BANK (16 MARKS) Uses: Peat is a low value fuel in its application.It finds uses where available in abundance as i) Domestic fuel ii) gas purifier iii) For steam raising. Lignite It is a variously coloured variety of coal of lowest rank. In lignite transformation of vegetable matter to coal like material is almost complete. Fibrous texture is also shown by some lignite’s. Uses: These are used as domestic fuels and also in industry for distillation and gasification. This variety of coal has also been used in steam locomotives and for producing gas. Bituminous It is also known as the common coal,someties as coking coal and is ,in fact, the most common and important variety commercial coals. These are commonly black in colour, compact in structure breaking into almost cubical fragments when struck with hammer. They have a black streak. Bituminous coals burn freely leaving only a small mineral residue. Anthracite It is a coal highest rank in which original organic source has been completely transformed into carbonaceous substance. It is very hard, jet black in colour, compact in structure and showing an almost metallic luster. Uses: Anthracite is a favourite domestic fuel where available. It is also used for steam raising and other heating purposes. However because of its very low volatile matter content it is not suitable for making coke. Formation of coal There is absolutely no doubt regarding the nature of the source material form which coal is derived it is certainly always vegetable matter of one type or another. The two types of sources yielded vegetable material for the formation of the coal. i) The higher vegetation ii) The lower vegetation 6 CE 6301 ENGG GEOLOGY/UNIT II