Minerals and Rocks

Minerals and rocks


Minerals and rocks are the most common material of the earth.

Mineral

                  It is a natural inorganic substance of definite chemical composition and generally with a definite crystal form. All minerals except water and mercury are solid at normal temperature.
Primary Minerals are the minerals that have not been altered chemically since deposition and crystallization from molten lava are called primary minerals.
 e.g. quartz, feldspar and mica etc.
Secondary Minerals are the minerals resulting from the decomposition of a primary or from the re-precipitation of the products of primary minerals are called secondary minerals.
e.g. calcite, gypsum, and dolomite etc.

Rocks

It is defined as a natural aggregate of one or more minerals to form an appreciable part of the solid portion of the earth.
Rocks are classified into three types according to their origin
Igneous rocks
Metamorphic rocks
Sedimentary rocks


Igneous rocks
                               Rocks formed by the cooling and solidification of extremely hot molten material (lava) are called igneous rocks.
Geologists consider that igneous rocks were the first rocks formed on the earth whereas sedimentary and metamorphic rocks originated from igneous rocks.
The word igneous is derived from Latin word meaning fire, which provides a clue to the origin of the rocks. Many areas of the earth contain points of weakness in the crust, and sometimes the molten material within the earth forces its way through these points of weakness and appears on the surface. Once the molten material has escaped from the conditions of high temperature and pressure which exists under the surface, it begins to cool down. Eventually the flowing material slows down, comes to a stop and solidifies and becomes a solid igneous rock.
The characteristics of igneous rocks are determined by the composition of the original material from which it was formed.

Examples:
                 granite, gabbro, basalt.


Sedimentary Rocks
                                       Sedimentary rocks are formed from the layers of accumulated sediments.
Pre-existing rocks of any group (igneous, metamorphic and sedimentary) are broken down by the continuous process of weathering to form debris known as sediments. The process continues over millions of years and the early sediments are gradually buried and more sediment is finally compacted through the weight of the sediment layers one above the other, the water squeeze off and the minerals become cemented.
In the process of sedimentary rocks formation, layers of sediments are built up layers by layer. Layering is therefore a fundamental characteristic of these rocks. Each layer is separated from the one above and the one below by a line of demarcation known as bedding plane. This generally represents the sudden change in the grain size or in the composition of the sediment being laid down.


Example: 

                 shale, mudstone and limestone.


Metamorphic Rocks
                                          Meta means change and Morph means shape or form. Therefore, metamorphic rocks are those which are formed due to change in shape of pre-existing igneous or sedimentary rocks under very high temperature and pressure. The process of change is called metamorphism. Under extremes of and pressure, physical and chemical changes occur in pre-existing rocks and this process of metamorphism generally occurs deep within the earth’s outer layer. Because of this, most metamorphic rocks are revealed after a considerable amount of erosion has taken place.
Both sedimentary and igneous rocks can be changed into new rock types by the process of metamorphism.


Example: 
                slate and marble.


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