INTRODUCTION

Mineral is a naturally occurring inorganic substance having definite chemical composition and with definite internal structure. The branch of geology which deals with the various aspects of minerals such as their individual properties, their mode of formation and mode of occurrence is called as mineralogy. A rock is defined as an aggregate of minerals and the study on rock is called Petrology. Based on their origin, geologically rocks are classified into igneous rocks, Sedimentary rocks and metamorphic rocks.

Igneous rocks:

They are derived from solidification of molten material ‘magma’ or ‘lava’. They subjected to different rate of crystallization. Thus, they exhibit different crystalline textures and structures. They are often connected with the interior of the earth’s crust. They are massive in form and free from fossil remains of ancient animals and plants. Ex: Granite from Magma, Basalt from Lava. These rocks are the fore most rocks to be formed on the earth’s crust. So they are known as ‘primary Rocks’. These rocks are characterized by vesicular structure, amygdaloidal structure and Aphanitic structure if they are volcanic. If they are Hypabyssal or plutonic, they are dense, compact and exhibit interlocking texture.

Sedimentary rocks:

These rocks are formed by the weathered, transported and deposited on the oceanic floor in the form of layers. They are also known as ‘Layered Rocks’. They are formed under water, they possess evidences of sedimentation viz, stratification, ripple marks, current bedding, graded bedding, mud/sun cracks, rain prints. Fossils etc., Example conglomerate, sand stone, limestone etc.

Metamorphic rocks:

These are derived from the pre-existing rocks due to intense temperature, pressure or both. These phenomena known as thermal metamorphism, dynamic metamorphism and dynamothermal metamorphism respectively. The rocks subjected to metamorphism, lose their original features and new structures are introduced. Example igneous rock Granite is metamorphosed to form Gneiss, sedimentary rock Limestone is metamorphosed to form Marble.

Some of the index properties of rocks are

1. Grain Size
2. Texture / Structure
3. Mineral Composition
4. Cementing Materials
5. Specific Gravity

1. Colour : The colour of rocks depends upon their constituent minerals or cementing material.

2. Texture: It is the mutual arrangement of component mineral grains size, shape. The texture will be different for igneous, metamorphic and sedimentary rocks.

Texture of Igneous Rocks
Texture	Description	Example
Equigranular	It consists of almost equidimensional mineral grains	Granite, Gabbro
Porphyritic	It consist of few large, well developed mineral grains	Granite porphery
Glassy texture	It consists of amorphous surface with rich in silica (SiO2 ) and rapid solidification of lava/magma in volcanic igneous rocks	Rhyolite, Obsidian
Vesicular/ Amygdaloidal texture	It consist of pores or vesicles due to release of gaseous substances
Texture of Sedimentary Rocks
Massive	Amorphous or very fine –grained and breaks with concentric curves	limestone
Fossiliferous	It consists of fossil remains burried, cemented together	Limestone
Concretionary	Hard and Soft clay with iron oxide or aluminium oxide	Laterite
Texture of Metamorphic Rocks
Granulose	It consists of interlocking of shapeless grains or crystalline minerals	Quartzite, Marble
Schistose	It consists of flaky, foliated grains or layers twisted or curved
Gneissose	It consists of alternate bands of light coloured and dark coloured mineral	Gneiss, Augen gneiss

3. Grain size:

Fine: Grain diameter 1 mm or less

Medium: Grain diameter 1 mm to 5 mm

Coarse: Grain diameter 5 mm and above

4. Mineral Composition:

The combination and proportion of the mineral component in Rock, Mineral compositions can be divided in two types:

1. Essential minerals: These are essential for description, identification and classification, of Rocks. Ex : Quartz, Feldspar, Mica. Augite, Calcite, Talc, Chlorite.
2. Accessary minerals: These may be present/obsent. Ex: Mica, Magnite, Iron pyrite, Garnet.

5. Cementing Material: (Exclusively for Sedimentary Rocks)

Natural binding material deposited in between component sedimentary pebbles, fragments, grains and/or fossil remains.

Calcareous	It imparts white colour and pale colour to sand stones and can be known by acid test
Feriginous	Imparts shades of brown, red, or yellow colour to sand stone
Argillaceous	It provides only weak cohesion for sand particles, which fall of rubbing the sand stone
Siliceous	Resembles calcareous cementing material but provides competence and durability to sand stone
Glaucontic	It provides green colour to sand stone