Chapter 5 Minerals And Rocks

The Earth is composed of various chemical elements. These elements exist in solid form in the outer part of the Earth (crust) and in a hot, molten state deeper within. Approximately 98% of the Earth's crust is made up of eight common elements: Oxygen, Silicon, Aluminium, Iron, Calcium, Sodium, Potassium, and Magnesium. Other elements like Titanium, Hydrogen, Phosphorus, Manganese, Sulfur, Carbon, and Nickel make up the remaining percentage.

These elements rarely exist in pure form in the crust; instead, they commonly combine with other elements to form various substances. These naturally occurring substances, with a specific chemical composition and an ordered internal atomic structure, are recognized as minerals.

Minerals are generally composed of two or more elements chemically bonded together. However, some minerals consist of a single element, such as Sulfur, Copper, Silver, Gold, and Graphite.

Despite the limited number of main elements in the lithosphere, they combine in diverse ways to create a wide variety of minerals. Over 2,000 minerals have been identified, but most rocks in the Earth's crust are formed from just six major mineral groups, often called the major rock-forming minerals.

The ultimate origin of most minerals is the hot, molten material (magma) found deep within the Earth. As magma cools and solidifies, mineral crystals begin to form, often in a specific sequence based on cooling temperature and composition. Organic substances like coal, petroleum, and natural gas, while often discussed with minerals, are formed from organic matter in solid, liquid, and gaseous states, respectively.

Physical Characteristics

Minerals possess distinct physical properties that help in their identification. Some key characteristics include:

1. External Crystal Form: This refers to the shape a mineral crystal takes when it grows freely, determined by the orderly internal arrangement of its atoms. Common forms include cubes, octahedrons, hexagonal prisms, etc.
2. Cleavage: This is the tendency of a mineral to break smoothly along specific flat surfaces or planes. Cleavage occurs along planes of weakness in the mineral's internal atomic structure. Minerals can have cleavage in one or more directions and at various angles.
3. Fracture: When a mineral breaks irregularly, not along cleavage planes, it exhibits fracture. This happens when the internal arrangement of atoms is too complex to create preferred breaking directions. Fracture can be conchoidal (shell-like), uneven, fibrous, etc.
4. Lustre: This describes the appearance of a mineral's surface in reflected light, regardless of color. Common types of lustre include metallic, vitreous (glassy), silky, resinous, pearly, or dull.
5. Colour: While sometimes a useful characteristic, color can be misleading as many minerals have variable colors due to impurities. Some minerals have inherent color based on their chemical composition (e.g., Malachite is green, Azurite is blue). However, minerals like Quartz can be white, green, red, or yellow depending on the presence of trace elements.
6. Streak: This is the color of a mineral's powder, obtained by rubbing the mineral across an unglazed porcelain plate (streak plate). The streak color is often more consistent than the mineral's visible color. For example, the mineral Hematite can be metallic grey or reddish-brown, but its streak is always reddish-brown. Malachite yields a green streak, while Fluorite (which can be various colors) gives a white streak.
7. Transparency: This describes how light passes through a mineral. Minerals can be:
   • Transparent: Light passes through clearly, allowing objects to be seen distinctly on the other side (e.g., clear Quartz).
   • Translucent: Light passes through but is scattered, so objects cannot be seen clearly (e.g., milky Quartz).
   • Opaque: Light does not pass through at all (e.g., metals, most Sulfides).
8. Structure: This refers to the physical arrangement of individual mineral crystals within a sample. It can be described as fine-grained, medium-grained, coarse-grained, fibrous, divergent (spreading out), or radiating.
9. Hardness: This is a measure of a mineral's resistance to scratching. Mineral hardness is typically measured using the Mohs Hardness Scale, a relative scale ranging from 1 (softest) to 10 (hardest). The scale consists of ten index minerals:
   1. Talc (softest)
   2. Gypsum
   3. Calcite
   4. Fluorite
   5. Apatite
   6. Feldspar (like Orthoclase)
   7. Quartz
   8. Topaz
   9. Corundum
   10. Diamond (hardest)
   Common objects can be used for comparison; for instance, a fingernail has a hardness of about 2.5, and a knife blade or piece of glass is around 5.5.
10. Specific Gravity: This is a measure of a mineral's density relative to the density of water. It is calculated as the ratio of the weight of a mineral sample to the weight of an equal volume of water. Higher specific gravity indicates a denser mineral.

Metallic Minerals

Metallic minerals are those that contain metal elements in their composition. They are a source of metals and often have a shiny luster and can conduct heat and electricity. They are broadly classified into three types based on the type of metal they contain:

• Precious Metals: Highly valuable metals like Gold, Silver, and Platinum.
• Ferrous Metals: Metals that contain iron and are used, often in alloys, to produce steel. Examples include iron ore itself and metals like Manganese, Chromium, and Nickel that are added to steel.
• Non-Ferrous Metals: All metallic minerals that do not contain iron. This category includes metals such as Copper, Lead, Zinc, Tin, and Aluminium.

Non-Metallic Minerals

Non-metallic minerals do not contain metal content. They are generally poor conductors of heat and electricity and lack the metallic luster characteristic of metallic minerals. Examples include Sulfur, Phosphates, Nitrates, Mica, Gypsum, and Limestone. Materials like Cement are produced from a combination of non-metallic minerals.

Rocks

The Earth's crust is primarily made up of rocks. A rock is defined as a naturally occurring aggregate of one or more minerals. Some rocks might be composed predominantly of a single mineral (like Limestone, which is mostly Calcite), while others are made up of a mixture of several minerals (like Granite, which contains Quartz, Feldspar, Mica, etc.).

Rocks vary greatly in hardness, softness, texture, and color. For example, Granite is very hard, while Soapstone (composed mainly of talc) is soft. Gabbro is typically dark or black, whereas Quartzite (metamorphosed sandstone) can be pure white if made from pure Quartz sand.

Unlike minerals, rocks generally do not have a definite or uniform chemical composition or internal structure; their composition depends on the minerals they contain and the proportions of those minerals.

The scientific study of rocks is called Petrology. A petrologist investigates rocks comprehensively, including their mineral composition, texture (size and arrangement of mineral grains), structure, origin, location, how they change over time, and their relationship with other rock types.

For geographers, understanding rocks is essential due to their close relationship with landforms and soils. Rocks are fundamental building blocks of the landscape, and soils are formed from the weathering and breakdown of rocks.

Based on their mode of formation, rocks are broadly classified into three major families:

Igneous Rocks

The term "igneous" comes from the Latin word "ignis", meaning fire. Igneous rocks are also known as primary rocks because they are the first rocks to form from the Earth's molten interior and are the source from which other rock types are ultimately derived.

They are formed when molten rock material (magma or lava) cools and solidifies. Magma is molten rock found beneath the Earth's surface, while lava is magma that has erupted onto the surface.

The process of cooling and solidification determines the texture of the igneous rock, specifically the size and arrangement of its mineral crystals. If magma cools slowly at great depths within the crust (plutonic or intrusive igneous rocks), the minerals have time to grow large, resulting in a coarse-grained texture (e.g., Granite, Gabbro).

If lava erupts onto the Earth's surface or cools rapidly near the surface (volcanic or extrusive igneous rocks), the rapid cooling prevents large crystal growth, resulting in fine-grained or even glassy textures (e.g., Basalt, Rhyolite). Intermediate cooling rates can result in intermediate crystal sizes (e.g., Diorite, Andesite). Igneous rocks can also incorporate fragments of existing rock or solidified lava, forming textures like volcanic breccia or tuff.

Sedimentary Rocks

The word "sedimentary" originates from the Latin word "sedimentum", meaning settling. Sedimentary rocks are formed from the accumulation and cementation of sediments, which are fragments of pre-existing rocks (igneous, metamorphic, or other sedimentary rocks), mineral grains, or organic matter.

Rocks exposed at the Earth's surface are weathered and eroded by various agents (like wind, water, ice, gravity) into smaller particles or dissolved substances. These fragments and dissolved materials are transported by external agents (like rivers, glaciers, wind) and eventually deposited in layers in basins, such as oceans, lakes, or riverbeds.

Over time, as more sediment accumulates, the weight of the overlying layers compacts the buried sediments, squeezing out water. Minerals dissolved in pore water precipitate and act as a natural cement, binding the sediment grains together. This process of compaction and cementation, which transforms loose sediment into solid rock, is called lithification.

A characteristic feature of many sedimentary rocks is their layered appearance, reflecting the successive deposition of sediments. These layers, or strata, can vary in thickness and composition (e.g., Sandstone, Shale).

Sedimentary rocks are classified based on the nature of their sediments and how they were formed:

• Mechanically Formed (Clastic): Formed from fragments of pre-existing rocks and minerals that were transported and deposited as solid particles. Examples include Sandstone (sand grains), Conglomerate (rounded pebbles), Shale (mud and clay), and Loess (wind-blown silt).
• Organically Formed (Biogenic): Formed from the accumulation and lithification of organic materials or the remains of organisms. Examples include Coal (accumulated plant matter), some types of Limestone (from shells or coral fragments), and Chalk (from microscopic marine organisms).
• Chemically Formed: Formed by the precipitation of minerals from solution. This happens when water containing dissolved minerals evaporates, leaving the minerals behind. Examples include rock salt (Halite), Gypsum, some types of Limestone (formed by inorganic precipitation), and Chert (microcrystalline quartz).

Metamorphic Rocks

The term "metamorphic" means "change of form". Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to significant changes in pressure, volume, and temperature (PVT) conditions. These changes occur deep within the Earth's crust due to processes like tectonic plate movements, burial under thick sediment piles, or contact with hot magma.

Metamorphism is a process where the minerals within the original rock (protolith) are transformed without melting. This transformation involves recrystallization (minerals growing larger or changing form) and reorganization of mineral grains and chemical components.

Types of Metamorphism:

• Dynamic Metamorphism: Occurs primarily due to mechanical forces like intense pressure and shearing associated with fault zones or plate boundaries. It involves crushing, grinding, and recrystallization of minerals without significant chemical change (e.g., forming Mylonite).
• Thermal Metamorphism: Primarily driven by heat.
  • Contact Metamorphism: Occurs when rocks are heated by direct contact with intruding hot magma or lava. The heat causes recrystallization in the surrounding 'contact aureole'. Sometimes, new minerals are formed as chemical components from the magma are added to the existing rock.
  • Regional Metamorphism: Affects large areas of rock and is associated with mountain building and tectonic plate collisions. Rocks are subjected to both high pressure (from burial and tectonic compression) and high temperature over vast regions, leading to widespread recrystallization and deformation.

During metamorphism, minerals in some rocks may align themselves in layers or lines due to directed pressure. This texture is called foliation or lineation. In some foliated rocks, different mineral groups separate into alternating light and dark bands, creating a distinctive layered appearance known as banding. Rocks exhibiting this banding are called banded rocks (e.g., Gneiss).

Metamorphic rocks are generally classified based on the presence or absence of foliation:

• Foliated Rocks: Show layering or alignment of minerals (e.g., Slate, Schist, Gneiss).
• Non-Foliated Rocks: Lack layered or banded textures, often consisting of a mosaic of intergrown crystals (e.g., Marble - metamorphosed limestone, Quartzite - metamorphosed sandstone).

Examples of metamorphic rocks include Gneiss (from granite or other rocks), Slate (from shale), Schist (from shale or basalt), Marble (from limestone), and Quartzite (from sandstone).

Rock Cycle

Rocks are not permanent structures; they undergo continuous transformation over geological time through a dynamic process called the Rock Cycle. This cycle describes how the three main rock types (igneous, sedimentary, and metamorphic) are interconnected and can change from one form to another.

Igneous rocks are considered the primary rocks, formed from the cooling and solidification of magma or lava. All other rock types can ultimately be derived from igneous rocks.

• Igneous rocks exposed at the surface can be weathered and eroded into sediments. These sediments are then transported, deposited, and lithified to form sedimentary rocks.
• Igneous rocks can also be subjected to high heat and pressure deep within the Earth, transforming them into metamorphic rocks.

Sedimentary rocks can also participate in the cycle:

• Sedimentary rocks can be weathered and eroded, and the resulting sediments can form new sedimentary rocks.
• If sedimentary rocks are buried deep or subjected to tectonic forces, they can be transformed into metamorphic rocks under increased pressure and temperature.

Metamorphic rocks complete the cycle:

• Metamorphic rocks exposed at the surface can be weathered and eroded into sediments, which can then form sedimentary rocks.
• If metamorphic rocks are subjected to sufficiently high temperatures, they will melt, forming magma. This magma can then cool and solidify to form new igneous rocks.

Furthermore, crustal rocks (igneous, sedimentary, or metamorphic) can be pulled down into the mantle in subduction zones (areas where tectonic plates converge and one plate slides beneath another). Once deep in the mantle, these rocks melt due to the high temperatures, returning to their molten state as magma, ready to begin the cycle anew by solidifying into igneous rocks.

Simplified diagram showing the interrelationships between igneous, sedimentary, and metamorphic rocks and the processes (melting, cooling, weathering, erosion, deposition, lithification, metamorphism) that transform them from one type to another.

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