Rocks are aggregates or physical mixture of one or more minerals. Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition. Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum to this uppermost part of the earth. These are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.
Rocks are of immense economic importance to us.
Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types:
(a) Igneous
(b) Sedimentary and
(c) Metamorphic
Igneous Rocks
Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types
Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth
Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.
Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and properties of a liquid.
On the basis of their mode of occurrence, igneous rocks can be classified as : extrusive or volcanic rocks and intrusive rocks.
(i) Extrusive igneous rocks are formed by cooling of lava on the earth’s surface. As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks. Gabbro and basalt are very common examples of such rocks. These rocks are found in volcanic areas. Deccan plateau’s regur soil in India is derived from lava.
(ii) Intrusive igneous rocks are formed when magma solidifies below the earth’s surface. The rate of cooling below the earth’ s surface is very slow which gives rise to formation of large crystals in the rocks. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite and dolerite are common examples of intru- sive rocks. From this point of view, therefore, igneous rocks can, in accor- dance with their mode of formation, be classified as (a) Plutonic, (b) Hyp- abyssal and (c) Volcanic rockmasses. The huge blocks of coarse granitic rocks are found both in the Himalaya and the Decean Plateau.
Sedimentary Rocks
Sedimentary rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks. Generally, these rocks have some type of fossil between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks. Sedimentary rocks are widely spread on the earth surface but to a shallow depth.
The formation of sedimentary rocks involves five processes:
- Weathering – The first step is transforming solid rock into smaller fragments or dissolved ions by physical and chemical weathering as discussed in the last lecture.
2. Erosion – Erosion is actually many process which act together to lower the surface of the earth. In terms of producing sediment, erosion begins the transpiration process by moving the weathered products from their original location. This can take place by gravity (mass wasting events like landslides or rock falls), by running water. by wind, or by moving ice. Erosion overlaps with transpiration.
3. Transportation – Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment that tell us something about the mode of transportation.
4. Deposition – Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.
5. Lithification (Diagenesis) – Lithification is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual .
Metamorphic Rocks
Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hard- ness, structure and composition of all types of pre-existing rocks. The process which bring about the change is known as Metamorphism and the ultimate products, formed due to operation of such processes are defined as the Metamrphic rocks.
Metamorphism refers to the changes in mineral assemblage and texture that result from subjecting a rock to pressures and temperatures different from those under which the rock originally formed.
The original rock that has undergone metamorphism is called the protolith. Protolith can be any type of rock .
Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids. Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism. Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana. In Kangra and Kumaun regions ]of Himalaya, slates of different colours are found.
Minerals
Minerals are defined as solid, inorganic, naturally occurring substances with a definite chemical formula and general structure. Almost all chemical elements in the Earth’s crust are associated with at least one mineral. They vary in color, hardness, density, crystal form, crystal size, transparency, composition, location, and abundance. Some minerals are radioactive (uranophane), while others are magnetic (magnetite). Some are uncombined elements, such as gold, silver, sulfur, bismuth, copper and platinum. Certain minerals, such as fluorite (calcium fluoride), glow with a vibrant luminescence after exposure to ultraviolet light. Feldspar (a potassium aluminum silicate) and quartz (silicon dioxide) are the most abundant minerals found in the Earth’s crust. Mankind has coveted many minerals and gemstones due to their striking, beautiful colors. Some specimens of opal exhibit iridescent, rainbow-like colors within their crystals, while some specimens of the gemstone tourmaline show progressions of watermelon-like green to pink color from one side of a crystal to another. Although interesting, a few minerals are deceiving; many amateur prospectors have been tricked into thinking they have found gold, but have instead found pyrite (iron disulfide), otherwise known as “Fool’s Gold” due to its uncanny resemblance to the precious metal. Diamond, the hardest natural substance found on Earth, has long been prized as the most desirable of all minerals due to its stunning interaction with light. Most diamonds, though, are impure and not suitable as gemstones; instead, they find industrial use as cutting tools, and are even made synthetically to meet this demand.
What Makes a Mineral a Mineral?
In order for something to be a mineral, it must first meet four criteria:
- First, all minerals are solid. So, while water may contain minerals, water itself can’t be a mineral because it’s liquid.
- Minerals are all naturally formed. This means they can’t be manufactured in a lab. Synthetic gems, like cubic zirconia, are therefore not minerals.
- All minerals have a unique and specific chemical composition. This is like the DNA of the mineral – it’s what makes the mineral different from other minerals.
- Lastly, all minerals have a crystalline structure. Minerals are some of the most beautiful substances on Earth, because they are always arranged in an orderly geometric pattern. Minerals of the same type always have the same geometric arrangement of their atoms.
Properties of Minerals
The Physical properties of minerals are used by Mineralogists to help determine the identity of a specimen. Some of the tests can be performed easily in the field, while others require laboratory equipment. For the beginning student of geology, there are a number of simple tests that can be used with a good degree of accuracy. The list of tests is in a suggested order, progressing from simple experimentation and observation to more complicate either in procedure or concept.
The following physical properties of minerals can be easily used to identify a mineral:
Color
Most minerals have a distinctive color that can be used for identification. In opaque minerals, the color tends to be more consistent, so learning the colors associated with these minerals can be very helpful in identification. Translucent to transparent minerals have a much more varied degree of color due to the presence of trace minerals. Therefore, color alone is not reliable as a single identifying characteristic.
Streak
Streak is the color of the mineral in powdered form. Streak shows the true color of the mineral. In large solid form, trace minerals can change the color appearance of a mineral by reflecting the light in a certain way. Trace minerals have little influence on the reflection of the small powdery particles of the streak.
The streak of metallic minerals tends to appear dark because the small particles of the streak absorb the light hitting them. Non-metallic particles tend to reflect most of the light so they appear lighter in color or almost white.
Because streak is a more accurate illustration of the mineral’s color, streak is a more reliable property of minerals than color for identification.
Hardness
Hardness is one of the better properties of minerals to use for identifying a mineral. Hardness is a measure of the mineral’s resistance to scratching. The Mohs scale is a set of 10 minerals whose hardness is known. The softest mineral, talc, has a Mohs scale rating of one. Diamond is the hardest mineral and has a rating of ten. Softer minerals can be scratched by harder minerals because the forces that hold the crystals together are weaker and can be broken by the harder mineral.
The following is a listing of the minerals of the Mohs scale and their rating:
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Orthoclase Feldspar
- Quartz
- Topaz
- Corundum
- Diamond
Cleavage & Fracture
Minerals tend to break along lines or smooth surfaces when hit sharply. Different minerals break in different ways showing different types of cleavage.
Cleavage is defined using two sets of criteria. The first set of criteria describes how easily the cleavage is obtained. Cleavage is considered perfect if it is easily obtained and the cleavage planes are easily distinguished. It is considered good if the cleavage is produced with some difficulty but has obvious cleavage planes. Finally it is considered imperfect if cleavage is obtained with difficulty and some of the planes are difficult to distinguish.
The second set of criteria is the direction of the cleavage surfaces. The names correspond to the shape formed by the cleavage surfaces: Cubic, rhombohedral, octahedral, dodecahedral, basal or prismatic. These criteria are defined specifically by the angles of the cleavage lines as indicated in the chart below:
Cleavage Type Angles
Cubic Cleaves in three directions @ 90o to one another Rhombohedral Cleaves in three directions but not @ 90o to one another Octahedral Cleaves in four directions Dodecahedral Cleaves in six directions Basal Cleaves in one direction Prismatic Cleaves in two directions
Fracture describes the quality of the cleavage surface. Most minerals display either uneven or grainy fracture, conchoidal (curved, shell-like lines) fracture, or hackly (rough, jagged) fracture.
Crystalline Structure
Mineral crystals occur in various shapes and sizes. The particular shape is determined by the arrangement of the atoms, molecules or ions that make up the crystal and how they are joined. This is called the crystal lattice. There are degrees of crystalline structure, in which the fibers of the crystal become increasingly difficult or impossible to see with the naked eye or the use of a hand lens. Microcrystalline and cryptocrystalline structures can only be viewed using high magnification. If there is no crystalline structure, it is called amorphous. However, there are very few amorphous crystals and these are only observed under extremely high magnification.
Transparency or Diaphaneity
Diaphaneity is a mineral’s degree of transparency or ability to allow light to pass through it. The degree of transparency may also depend on the thickness of the mineral.
Tenacity
Tenacity is the characteristic that describes how the particles of a mineral hold together or resist separation. The chart below gives the list of terms used to describe tenacity and a description of each term.
Magnetism
Magnetism is the characteristic that allows a mineral to attract or repel other magnetic materials. It can be difficult to determine the differences between the various types of magnetism, but it is worth knowing that there are distinctions made.
Luster
Luster is the property of minerals that indicates how much the surface of a mineral reflects light. The luster of a mineral is affected by the brilliance of the light used to observe the mineral surface. Luster of a mineral is described in the following terms:
Metallic The mineral is opaque and reflects light as a metal would.Submettalic The mineral is opaque and dull. The mineral is dark colored.Nonmettalic The mineral does not reflect light like a metal.
Nonmetallic minerals are described using modifiers that refer to commonly known qualities.
Waxy The mineral looks like paraffin or wax.Vitreous The mineral looks like broken glass.Pearly The mineral appears iridescent, like a pearl.Silky The mineral looks fibrous, like silk.Greasy The mineral looks like oil on water.Resinous The mineral looks like hardened tree sap (resin).Adamantine The mineral looks brilliant, like a diamond.
Odor
Most minerals have no odor unless they are acted upon in one of the following ways: moistened, heated, breathed upon, or rubbed.
Taste
Only soluble minerals have a taste, but it is very important that minerals not be placed in the mouth or on the tongue. You should not test for this property in the classroom.
Specific Gravity
Specific Gravity of a mineral is a comparison or ratio of the weight of the mineral to the weight of an equal amount of water. The weight of the equal amount of water is found by finding the difference between the weight of the mineral in air and the weight of the mineral in water.
Detail account of Mineral resources of India is as follows:-
| Iron Ore | Chhattisgarh 24 % | Dalli, Rajhra (Durg), Bailadila, Raoghat, Aridongri | |||
| Goa (21%) | Sanquelim, Sanguem, Quepem, Satari, Ponda, Bicholim | ||||
| Karnataka (20%) | Bellary, Hospet, Sandur | ||||
| Jharkhand (17 %) | Noamund, Gua | ||||
| Orissa (15 %) | Gurumahisani, Sulaipat, Badampahar (Mayurbhanj), | ||||
| Kiriburu, Meghahataburu, Bonai (Sundargarh). | |||||
| Maharashtra | Chandrapur, Ratnagiri, Bhandara | ||||
| Andhra Pr. | Karimnagar, Warangal, Kurnod, Cuddapah, Anantpur di | ||||
| Tamil Nadu | Tirthmalai, Yadapalli, Killimalai, kanjamalai, & Gondumalai | ||||
| Copper | M. P (46%) | Balghat (Malanjkhand) | |||
| Rajasthan (33%) | Khetri (Jhunjhunu & Alwar) | ||||
| Jharkhand (21%) | Singhbum | ||||
| Manganese | Karnataka (38%) | Bellary, North Kanara, Shimoga | |||
| Orissa (17%) | Kendujhar, Sundargarh, Koraput, Kalahandi, Bolangir | ||||
| M.P. (10 %) | Balghat | ||||
| Maharashtra (8%) | Nagpur & Bhandara | ||||
| Bauxite | Orissa (44 %) | Koraput, Kalahandi, Sundargarh | |||
| Jharkhand (18 %) | Gumla, Lohardaga, Ranchi, Palamau | ||||
| Maharashtra (13%) | Kolhapur, Ratnagir | ||||
| Chhattisgarh(11%) | Bastar, Bilaspur, Surguja district | ||||
| M.P. (11 %) | Mandla, Satna, Jabalpur, Shahdol | ||||
| Gujarat (8 %) | Jamnagar, Kachchh, Junagarh | ||||
| Tamil Nadu (4 %) | Salem, Nilgiri | ||||
| Chromite | Orissa (97 %) | Sukinda, Kendujhar, Dhenkanal ditricts | |||
| Karnataka (2.3 %) | Hassan | ||||
| Maharashtra (%) | Chandrapur | ||||
| Jharkhand (%) | Purbi & Paschmi Singhbum district | ||||
| Andhra Pr (%) | Khammam | ||||
| Lead | Rajasthan (80 %) | Zawar region (Udaipur), Dariba, Rajura | |||
| Orissa (11%) | |||||
| Andhra Pr (8 %) | |||||
| Zinc | Rajasthan (99 %) | Zawar –a. Pipli khan to Barla khan b. Mochia, Magra, Balaria | |||
| Sikkim | |||||
| Gold | Karnataka (67 %) | Kolar, Hutti gold fields (Raichur), Gulbarga | |||
| Jharkhand (26 %) | Subarnarekha, Sona, Sanjai, South koel, Garra rivers | ||||
| Andhra Pr (7 %) | Ramgiri Gold Fields (Anantapur district) | ||||
| Silver | Andhra Pr (42 %) | ||||
| Bihar Jharkhand32 | |||||
| Rajashtan (25 %) | Zawar | ||||
| Karnataka | |||||
| Nickel | Orissa | Cuttack, Kyonjhar, & Maiyurbhanj | |||
| Non Metallic Minerals | |||||
| Limestone | M.P, Chattisgarh, Andhra Pradesh, Rajasthan | ||||
| Dolomite | Orissa (Birmitrapur in Sundergarh District-largest in India), M.P & Chattisgarh | ||||
| Phosphate | Rajasthan (Udaipur) Uttaranchal (Dehradun), M.P. (Jhabua), U.P. (Lalitpur) | ||||
| Kaolin | Kerala is largest producer. | ||||
| Mica | Andhra Pradesh, Jharkhand (Kodarma-Large) & Rajasthan | ||||
| Gypsum | Rajasthan & J & K. | ||||
| Steatite | Rajasthan. It is also called soapstone/ Potstone. | ||||
| Magnesite | Tamil Nadu | ||||
| Pyrite | Bihar is sole producer | ||||
| Graphite | Orissa, Rajasthan | ||||
| Diamond | M.P. (Panna) | ||||
| Beryllium | Rajathan, Jharkhand | ||||
| Salt (NaCl) | Gujarat (60%), Tamil Nadu & Maharashtra | ||||
| Marble | Rajasthan | ||||
| Zircon | Beach Sand of Kerala | ||||
| Kyanite | Singhbum distict in Jharkhand-largest. Used as refractory material | ||||
| Antimony | Punjab | ||||
| Asbestos | Karnataka & Rajasthan | ||||
| Beryllium | Rajasthan | ||||
| Sulphur | Tamil Nadu | ||||
| Tin | Bihar, Jharkhand | ||||