Minerals and Rocks
About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium.
The rest is constituted of titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.
The elements combine with other elements to form substances recognized as minerals.
A mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
Single element minerals are sulphur, copper, silver, gold, graphite etc.
The elements found in the lithosphere are limited and different ways to make up different varieties of minerals.
2000 minerals have been named and identified in the earth’s crust. But almost all the commonly occurring ones are related to the six major mineral groups known as rock-forming minerals.
All the minerals are the hot magma in the interior of the earth, when the magma cools, crystals of minerals appear.
A systematic series of minerals are formed in sequence to solidify to form rocks.
Physical characteristics
External crystal form –it is determined by its internal arrangement of molecules. It can be cubic, tetrahedral, tabular, hexagonal, etc.
Cleavage – The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice.
Fracture – When the internal molecular arrangement is so complex that there are no planes of molecules, the crystal will break irregularly.
Lustre –the appearance of material without regard to colour. Each mineral has a distinctive lustre like metallic, glossy, silky, etc.
Colour – The colour of a mineral depends on its molecular structure, Malachite, azurite, chalcopyrite, etc. Some minerals due to the presence of different impurities show different colours, such as quartz – it can be red, white, green, etc.
Streak – It is the colour of the ground powder of any mineral. It may be of the same colour as the mineral or different- Malachite is green and gives a green streak, Fluorite is purple or green but gives a white streak,
Transparency – Transparent- light rays pass through so that the objects can be seen plainly: Translucent- light rays pass through but get diffused and the objects cannot be seen. Opaque- light will not pass at all.
Structure –particular arrangement of the individual crystals. Fine, medium and coarse-grained: fibrous- separable, divergent and radiating.
Hardness – relative resistance being scratched, ten minerals are selected to measure hardness from 1 – 10, talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum and diamond. Compared to this, a fingernail is 2.5 and a glass or knife blade is 5.5.
Specific gravity – It is the ratio between the weight of a given object and the weight of an equal amount of water.
Some Major Minerals and Their Characteristics
Feldspar
Half of the earth’s crust is composed; Silicon and oxygen are common elements on it.
Sodium, potassium, calcium, aluminium etc. are found in specific feldspar varieties.
It has a light cream to salmon pink colour and it is used in ceramics and glassmaking.
Quartz
It is a component of sand and granite and consists of silica.
It is white or colourless and used in radio and radar.
Pyroxene
It consists of calcium, aluminium, magnesium, iron and silica. It forms 10 per cent of the earth’s crust.
It is commonly found in meteorites and it is in green or black colour.
Amphibole
It consists of aluminium, calcium, silica, iron and magnesium, It forms 7 per cent of the earth’s crust.
It is in green or black colour and is used in the asbestos industry.
Mica
It comprises potassium, aluminium, magnesium, iron, silica etc. used in electrical instruments.
It forms 4 per cent of the earth’s crust.
It is commonly found in igneous and metamorphic rocks.
Olivine
Magnesium, iron, and silica are major elements which are used in jewellery. It is usually a greenish crystal found in basaltic rocks.
Besides these main minerals, other minerals like chlorite, calcite, magnetite, haematite, bauxite and barite are also present in the rocks.
Classification of Minerals
Metallic Minerals
These are classified into three types.
- Precious metals: gold, silver, platinum etc.
- Ferrous metals: iron and other metals which are mixed with iron to form various kinds of steel.
- Non-ferrous metals: copper, lead, zinc, tin, aluminium etc.
Non-Metallic minerals
The minerals do not contain metal content, for example, sulphur, phosphates and nitrates.
Cement is a mixture of non-metallic minerals.
Rocks
The earth’s crust is composed of rocks, which may be hard or soft.
The rocks of the earth’s surface area are exposed to denudational agents, which are broken into various sizes of fragments.
Rocks contain one or more minerals, for example, granite is hard and soapstone is soft.
Varied colours of rocks Gabbro is black quartzite can be milky white.
Petrology is the science of rocks.
Different kinds of rocks
Igneous rocks
Solidified from magma and lava from the interior of the earth.
The igneous rocks are formed when magma cools and is modified.
The process of cooling and solidification can happen in the earth’s crust or the surface of the earth.
Magma in its upward movement cools and turns into a solid form called igneous rocks.
Igneous rocks are based on texture. The texture is based on the size and arrangement of gains or other physical conditions of the materials.
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some examples of igneous rocks.
Sedimentary rocks
The word sedimentary is derived from the Latin word ‘sediment, which means ‘settling’.
These rocks formed as a result of the deposition of fragments of rocks by exogenous processes.
Sedimentary rocks are classified into three major groups.
Mechanically formed- sandstone, conglomerate, limestone, shale, loess etc.
Organically formed- geyserite, chalk, limestone, coal etc.
Chemically formed- chert, limestone, halite, potash, etc.
Metamorphic rocks
The word metamorphic means ‘change of form’
It is a process by which already consolidated rocks undergo recrystallization and reorganisation of materials within original rocks.
The materials of the rocks chemically alter and recrystallize due to thermal metamorphism. These are two types.
In contact metamorphism, the rocks come in contact with hot intruding magma and lava and the rocks crystallise under high temperature.
Regional metamorphism rocks occur in areas that have undergone deformation during an organic event resulting in mountain belts that have since been eroded to expose the metamorphic rocks.
Banded rocks minerals or metals of the different groups are arranged into alternating thin to thick layers, altering the light and dark shades.
Metamorphic rocks are classified into two major groups Foliated rocks and non-foliated rocks.
Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.
Rock cycle
Rocks cycle is a continuous process in which old rocks are transformed into new ones.
Igneous rocks are primary rocks. These rocks can be changed into metamorphic rocks.
Sedimentary and metamorphic rocks form from these primary rocks.
The fragments evolved out of rocks igneous and metamorphic again form into sedimentary rocks.
Sedimentary rocks themselves can turn into fragments.
The crustal rocks -igneous, metamorphic and sedimentary-once formed may be carried down into the interior of the earth through subduction.
In this process, parts or entire crustal plates subduct under another plate and the same melts at a high temperature in the interior.
This results in the formation of molten magma, the source of igneous rocks.
