Occurrence and Formation of Minerals
Minerals are naturally occurring inorganic solids found in the earth's crust and they are either made up of one element or they are a combination of elements. Most minerals are highly valuable and their study, classification, and excavation all fall under the subject of geology. In this article, we will discuss the occurrence and formation of minerals. Several factors affect the formation of a mineral. Since minerals occur in a wide variety of environments, we have a wide array of these resources forming under various chemical and physical conditions. If we classify minerals based on their process of forming, we will have four types, namely: igneous, sedimentary, metamorphic, and hydrothermal.
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Minerals Found in Igneous Rocks
Igneous rocks form due to the cooling and solidification of lava. The minerals occurring in these rocks are igneous. Partial melts of previously formed rocks exist in the earth's crust and mantle and at times due to tectonic tensions, the molten mass travels towards the surface and starts cooling. If they solidify below the earth's surface, they form intrusive rocks and otherwise they form extrusive rocks. The mineral compounds which exist in the earth's mantle travel to the surface with the lava and then solidify as part of an igneous rock. This process leads to the formation of igneous minerals. Some igneous minerals are olivines, pyroxenes, and micas.
Minerals Found in Sedimentary Rocks
Minerals found in sedimentary rocks are a result of the process of sedimentation. In this process, rocks and soil undergo erosion by the various forces of nature, including wind and water. These eroded particles travel to different regions and settle layer by layer in certain places. In these regions, structures formed of sedimentary rocks come to life. Sedimentary rocks contain many valuable minerals such as quartz, feldspar, calcite, dolomite, etc.
Minerals Found in Metamorphic Rocks
Metamorphic rocks form due to the complete metamorphosis of existing rocks. Metamorphosis means transformation and this change occurs due to certain physical phenomena. Environmental changes such as an increase or decrease in temperature or pressure lead to chemical changes in the mineral compounds of rocks to give rise to new minerals. This process is the basis of the occurrence of metamorphic minerals. Some examples of such minerals are garnet, corundum, and kyanite.
Minerals Found in Hydrothermal Rocks
Hydrothermal minerals occur in the fissures of rocks and form as a result of the chemical precipitation of hot solutions erupting from the earth's core. In the previous three types, we observe that minerals are closely intergrown within the rocks in an interlocking pattern. However, hydrothermal solutions tend to flow into the fractures in a rock structure to gain a suitable space for chemical precipitation of the minerals in the solution. Geologists collect their samples from these very fissures. When a mineral undergoing precipitation grows without any external disturbance, beautifully shaped crystals come to life. This crystalline form gives an aesthetic appeal to the minerals.
Petroleum: Formation and Occurrence
This section is a short discussion on petroleum formation and occurrence. Petroleum is a very crucial industrial mineral. Petroleum is a result of the slow decomposition of organic matter due to chemical biochemical factors. This mineral naturally occurs in layers between sedimentary rocks. The trapped organic matter undergoes decomposition by bacterial action and changes in the chemical environment. This process turns the organic waste into globules of oil and gas.
Questions and Answers
Q1. Explain the term Minerals and State their Importance.
Answer: Minerals are solid inorganic compounds naturally occurring in rocks in the earth's crust. Minerals are highly valuable commercially and industrially. Coal, natural gas, and petroleum act as fuels. Several other minerals find application in medicines, fertilizers, etc.
Q2. Name the Softest and Hardest Minerals.
Answer: Talc is the softest naturally occurring mineral and diamond is the hardest one.
Q3. Explain the reason behind the Wide Variety in the Colour, Texture, Lustre and Hardness of Minerals.
Answer: The physical properties of minerals are widely spread when it comes to colour, density, hardness, lustre, etc. This variety yields from the fact that each mineral is formed from a particular set of elements under particular environmental conditions, making them unique.
FAQs on Mineral Chemical Compound - Occurrence and Formation
1. What exactly is a mineral, and what criteria must a substance meet to be classified as one?
A mineral is a naturally occurring, inorganic solid substance that has a distinct chemical composition and an ordered internal crystalline structure. For a substance to be classified as a mineral, it must satisfy five key conditions: it must be naturally formed (not man-made), solid at normal temperatures, inorganic (not derived from living organisms), have a specific chemical formula, and possess a regular, repeating atomic arrangement.
2. What are the main geological processes that lead to the formation of minerals?
Minerals are formed through several key geological processes. The primary methods of mineral formation include:
- Crystallisation from Magma or Lava: As molten rock cools, atoms arrange themselves into orderly crystal structures. Slow cooling deep inside the Earth results in large crystals, while rapid cooling on the surface creates smaller ones.
- Precipitation from Solutions: When water containing dissolved elements evaporates or cools, the elements can crystallise to form minerals. This is common in both surface water bodies (forming salt) and hot hydrothermal solutions deep underground.
- Metamorphism: Existing minerals are transformed into new ones when subjected to intense heat and pressure without melting. This recrystallisation changes the mineral's structure and stability.
- Weathering: The chemical breakdown of rocks on the Earth's surface can form new, stable minerals like clay from the decomposition of feldspar.
3. In what different forms or modes do minerals typically occur within rocks?
Minerals occur in various forms depending on how they were formed and concentrated. The main modes of occurrence are:
- Veins and Lodes: Minerals fill cracks and fissures in igneous and metamorphic rocks. Minor occurrences are called veins, while larger ones are known as lodes.
- Beds or Layers: In sedimentary rocks, minerals often accumulate in distinct horizontal layers through deposition and concentration. Coal and some forms of iron ore are examples.
- Residual Mass: When surface rocks are weathered and washed away, less soluble minerals like bauxite are left behind as a concentrated residual mass.
- Alluvial or Placer Deposits: Dense, durable minerals eroded from rocks are carried by water and deposited in valley floors or riverbeds. Gold, platinum, and tin are often found as placer deposits.
4. How does the formation of minerals in igneous rocks differ from that in metamorphic rocks?
In igneous rocks, minerals are formed directly from the cooling and solidification of molten magma or lava. The process is one of primary crystallisation. In metamorphic rocks, minerals are formed from pre-existing minerals that are altered due to intense heat and pressure. This process, known as recrystallisation, changes the texture and mineral composition of the rock without it melting, creating new minerals stable under the new conditions.
5. How is petroleum formed, and why is it found in sedimentary rocks?
Petroleum is a fossil fuel, not a mineral, formed from the remains of ancient marine organisms. Over millions of years, these organic remains were buried by layers of sediment. The immense pressure and heat from these layers cooked the organic matter, transforming it into petroleum (oil) and natural gas. It is typically found trapped within porous sedimentary rocks because these layers of sand and silt effectively buried the organic material and provided spaces for the oil and gas to accumulate.
6. What is the practical difference between a mineral occurrence and a mineral deposit?
A mineral occurrence refers to any location where a mineral is found, indicating its presence in the Earth's crust. However, a mineral deposit is a specific type of occurrence where the mineral is concentrated enough in quantity and quality to be considered for profitable extraction. Essentially, all deposits are occurrences, but only a tiny fraction of occurrences are valuable enough to be called deposits.
7. What is the fundamental distinction between a rock and a mineral?
The key distinction lies in their composition and structure. A mineral has a definite chemical formula and a specific, ordered crystalline structure (e.g., quartz is SiO₂). A rock is an aggregate or mixture of one or more minerals. For example, granite is a rock composed of minerals like quartz, feldspar, and mica. A rock does not have a single, fixed chemical composition because it is a mixture.
8. Why are valuable metallic minerals like copper and zinc often found concentrated in veins and lodes?
Veins and lodes are formed when superheated, mineral-rich water solutions (hydrothermal fluids) from deep within the earth push up through fractures in existing rock. As these hot fluids cool or react with the surrounding rock, the dissolved metals can no longer stay in solution and precipitate out. This process naturally concentrates the metallic minerals within the narrow fractures, making them economically viable to mine, as opposed to being sparsely distributed throughout the entire rock body.
