Rocks and Minerals

Rocks and Minerals

Geomorphology: Minerals and Rocks, Soils of India and the World

Rocks

• The Earth’s crust is composed of rocks.
• A rock is an aggregate of one or more minerals.
• Rock may be hard or soft and in varied colors. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white.
• Rocks do not have definite composition of mineral constituents.
• Feldspar and quartz are the most common minerals found in rocks.
• Petrology is science of rocks.
• A petrologist studies rocks in all their aspects viz., mineral composition, texture, structure, origin, occurrence, alteration and relationship with other rocks.

Type of Rocks

• There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. They are:
• Igneous Rocks — solidified from magma and lava;
• Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes;
• Metamorphic Rocks — formed out of existing rocks undergoing recrystallization.

Igneous Rocks

• As Igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.
• The Igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and solidifies and when magma in its upward movement cools and turns into solid form forming Igneous Rocks
• The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth. Igneous rocks are classified based on texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials.
• If molten material is cooled slowly at great depths, mineral grains may be very large. Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks.
• Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

Types of Igneous Rocks:

Based on place and time taken in cooling of the molten matter, igneous rocks can be divided into Plutonic and Volcanic rocks.

• • Plutonic Rocks or intrusive rocks:

• Sometimes, the molten matter is not able to reach the surface and instead cools down very slowly at great depths.
• Slow cooling allows big-sized crystals (large grains) to be formed.
• Granite is a typical example. These rocks appear on the surface only after being uplifted and denuded.
  • Lava or Volcanic Rocks or Extrusive rocks:
• These are formed by rapid cooling of the lava thrown out during volcanic eruptions.
• Rapid cooling prevents crystallization; as a result such rocks are fine-grained. Basalt is a typical example.
• The Deccan traps in the peninsular region are of basaltic origin.
• Basic rocks contain a greater proportion of basic oxides, e.g. of iron, aluminum or magnesium, and are thus denser and darker in color.

Based on the presence of acid forming radical, silicon, igneous rocks are divided into Acid Rocks and Basic Rocks.

• • Acid Rocks:
• These are characterized by high content of silica—up to 80 per cent, while the rest is divided among aluminum, alkalis, magnesium, iron oxide, lime etc.
• These rocks constitute the Sial portion of the crust.
• Due to the excess of silicon, acidic magma cools fast and it does not flow and spread far away.
• High mountains are formed of this type of rock.
• These rocks have a lesser content of heavier minerals like iron and magnesium and normally contain quartz and feldspar.
• Add rocks are hard, compact, massive and resistant to weathering.
  • Basic Rocks:
• These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent and the remaining 40 per cent is spread over iron oxide, lime, aluminum, alkalis, potassium etc.
• Due to low silica content, the parent material of such rocks cools slowly and thus, flows and spreads far away.
• This flow and cooling gives rise to plateaus. Presence of heavy elements imparts to these rocks a dark color.
• Basalt is a typical example, others being gabbro and dolerite. Not being very hard, these rocks are weathered relatively easily.

2. Sedimentary Rocks

• The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
• Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
  • Mechanically formed — sandstone, conglomerate, limestone, shale, loess etc. are examples;
  • Organically formed — geyserite, chalk, limestone, coal etc. are some examples;
  • Chemically formed — chert, limestone, halite, potash etc. are some examples.

3. Metamorphic Rocks

• The word metamorphic means ‘change of form’. These rocks form under the action of pressure, volume and temperature (PVT) changes.
• Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.
• Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
• Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.
• The materials of rocks chemically alter and recrystallize due to thermal metamorphism. There are two types of thermal metamorphism — contact metamorphism and regional metamorphism.
• In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallize under high temperatures. Quite often new materials form out of magma or lava are added to the rocks. In regional metamorphism, rocks undergo recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both.
• In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines.
• Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
• Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.
• Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism. 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.

Types of Metamorphic Rocks:

• Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals.
• Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz.
• Schist is metamorphic rock with well-developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces.
• Marble is a non-foliated metamorphic rock that is produced from the metamorphism of limestone. It is composed primarily of calcium carbonate.
• Slate is a foliated metamorphic rock that is formed through the metamorphism of shale. It is a low grade metamorphic rock that splits into thin pieces.
• Soapstone is a metamorphic rock that consists primarily of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, pyroxenes and carbonates. It is a soft, dense, heat-resistant rock that has a high specific heat capacity. These properties make it useful for a wide variety of architectural, practical and artistic uses.

Minerals

• The earth is composed of various kinds of elements.
• These elements are in solid form in the outer layer of the earth and in hot and molten form in the interior.
• About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium
• The rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.

What is a mineral?

• A mineral is a naturally occurring organic or inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
• A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite, etc. are also found.
• The basic source of all minerals is the hot magma in the interior of the earth.
• When magma cools, crystals of the minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.
• The minerals which contain metals are called as metallic minerals (eg: Haematite) and the metallic minerals which are profitably mined are called as the ores.
• The crust of the earth is made up of more than 2000 minerals, but out of these, only six are the most abundant and contribute the maximum.
• These six most abundant minerals are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Types of Mineral

Metallic Minerals

These minerals contain metal content and can be sub-divided into three types:

Precious metals: gold, silver, platinum etc.

Ferrous metals: iron and other metals often mixed with iron to form various kinds of steel.

Non-ferrous metals: include metals like copper, lead, zinc, tin, aluminum etc.

Non-Metallic Minerals

These minerals do not contain metal content. Sulphur, phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.

Physical Characteristics of Minerals

• External crystal form — determined by internal arrangement of the molecules — cubes, octahedrons, hexagonal prisms, etc.
• Cleavage — tendency to break in given directions producing relatively plane surfaces — result of internal arrangement of the molecules — may cleave in one or more directions and at any angle to each other.
• Fracture — internal molecular arrangement so complex there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.
• Lustre — appearance of a material without regard to color; each mineral has a distinctive lustre like metallic, silky, glossy etc.
• Color — some minerals have characteristic color determined by their molecular structure — malachite, azurite, chalcopyrite etc., and some minerals are colored by impurities. For example, because of impurities quartz may be white, green, red, yellow etc.
• Streak — color of the ground powder of any mineral. It may be of the same color as the mineral or may differ — malachite is green and gives green streak, fluorite is purple or green but gives a white streak.
• Transparency — transparent: light rays pass through so that objects can be seen plainly; translucent — light rays pass through but will get diff used so that objects cannot be seen; opaque — light will not pass at all.
• Structure— particular arrangement of the individual crystals; fine, medium or coarse grained; fibrous — separable, divergent, radiating.
• Hardness — relative resistance being scratched; ten minerals are selected to measure the degree of hardness from 1-10. They are:

1. Talc;
2. Gypsum;
3. Calcite;
4. fluorite;
5. Apatite;
6. Feldspar;
7. Quartz;
8. Topaz;
9. Corundum; and
10. Diamond. Compared to this for example, a fingernail is 2.5 and glass or knife blade is 5.5.

• Specific gravity — the ratio between the weight of a given object and the weight of an equal volume of water; object weighed in air and then weighed in water and divide weight in air by the difference of the two weights.