Elements of the crust and what a mineral is

Detail

1. What the Earth's crust is made of

• The crust is the thin, solid outer layer of the Earth. It is made of a small number of elements. An element is a basic substance that cannot be broken into simpler substances, such as oxygen or iron.
• The list below gives the share of each element by weight: how much of the crust's mass each one makes up.

Rank	Element	Share of crust by weight (%)	Note
1	Oxygen (O)	46.6	Almost half the crust; a gas, but here locked inside rocks
2	Silicon (Si)	27.7	Combines with oxygen to form silicates
3	Aluminium (Al)	8.1	The most common metal in the crust
4	Iron (Fe)	5.0	The second most common metal in the crust
5	Calcium (Ca)	3.6
6	Sodium (Na)	2.8
7	Potassium (K)	2.6
8	Magnesium (Mg)	2.1
—	The eight together	about 98	All other elements make up the last ~2%

• Standard abundance tables give the same order: oxygen, silicon, aluminium, iron [4]. Each of these eight elements is above 1% of the crust, and oxygen alone is close to 50% by weight [3]. Together the first eight make up more than 98% of crustal material [2].
• Memory trick: "Oh See All India Can See Pink Mangoes" gives O–Si–Al–Fe–Ca–Na–K–Mg.

2. Elements are combined, not free

• Most crustal elements do not occur alone as pure elements. They are chemically joined to each other.
• How silicates form:
• Oxygen and silicon are the two most common elements in the crust.
• They bond easily with each other.

• The result is silicate minerals: minerals built on a silicon–oxygen unit, often with aluminium, iron, calcium, sodium, potassium or magnesium added. Feldspar and quartz are examples.

• So the crust is mostly silicate rock. Of the roughly 6,000 mineral species known today, most are inorganic and most are silicates [5].

3. Crust vs the whole Earth

Feature	Earth's crust	Whole Earth
Most abundant element by weight	Oxygen	Iron
Most abundant metal	Aluminium	Iron
Why	Light elements rose upward	Heavy iron sank to the centre

• Why the two lists differ (differentiation):
• Differentiation means the sorting of Earth's material by density (how heavy a substance is for its size) when the young Earth was hot and partly molten.
• Heavy metals, mainly iron and nickel, sank inward and formed the core. The core is therefore called nife (Nickel + Fe, the chemical symbol for iron).
• Lighter silicates, rich in oxygen, silicon and aluminium, floated up and formed the crust.
• Result: iron leads for the planet as a whole, but oxygen and aluminium lead in the crust.

4. What a mineral is

• Mineral (NCERT definition): "a homogenous, naturally occurring substance with a definable internal structure".
• Homogenous: the same all the way through.
• Naturally occurring: formed by nature, not made in a factory.

• Definable internal structure: its atoms are arranged in a fixed, repeating pattern.

• The wider scientific definition is similar. A mineral is a naturally occurring solid with a definite chemical composition and an ordered, usually crystalline, atomic structure [6].

• The International Mineralogical Association (IMA) is the generally accepted world body that decides which substances count as mineral species and what they are named [6].

5. How many minerals are there

• NCERT says that over 2,000 minerals had been identified.
• About 6,000 mineral species are recognised today [5].
• By 2021, the IMA's Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC) had approved 5,744 standard short symbols, one for every mineral on the IMA list [7]. New minerals are added every year, so the total keeps rising.
• Even so, only a few minerals are common in most rocks. Most of the rest are rare.

6. Why minerals differ: properties used to classify them

• Cause → process → result:
• Minerals form under different physical conditions (heat and pressure) and chemical conditions (which elements are present).
• Different conditions lead to different atomic arrangements and different compositions.
• So minerals differ in the properties below, and these properties are used to identify and classify them.

Property	Simple meaning
Colour	How the mineral looks to the eye
Hardness	How well it resists scratching
Crystal form	The natural shape its crystals grow into
Lustre	How its surface reflects light (metallic, glassy, dull)
Density	How heavy it is for its size

7. Mineral vs rock vs ore

• Rock: a natural mixture of minerals.
• Some rocks contain only one mineral. Limestone is an example: it is mostly calcium carbonate (calcite).

• Most rocks contain several minerals. Granite, for example, has quartz, feldspar and mica.

• Ore: a mineral that has collected in one place together with other elements, and is concentrated enough that mining it makes a profit. Iron ore (haematite, magnetite) and bauxite (aluminium ore) are examples.

Term	What it is	Example
Mineral	A natural, homogenous substance with a fixed internal structure	Quartz, calcite, haematite
Rock	A combination of one or more minerals	Limestone (one mineral), granite (several)
Ore	A mineral deposit rich enough to mine for profit	Bauxite, haematite

• Key idea: every ore is a mineral deposit, but not every mineral deposit is an ore. Whether it counts as an ore depends on concentration and cost, not on the chemistry alone.

8. Minerals are scarce and non-renewable

• Workable deposits are only about 1% of the crust. Aluminium makes up 8.1% of the crust, but very little of it is concentrated enough to mine.
• Minerals are non-renewable. They take millions of years to form, and once mined they are not replaced on any human time scale.
• India's mineral base: India produces as many as 95 minerals. These are 4 fuel, 10 metallic, 23 non-metallic, 3 atomic and 55 minor minerals [8].
• India 2023-24 (provisional):
• Metallic minerals were worth about ₹1,27,599 crore, or 90.3% of the value of metallic plus non-metallic minerals. Non-metallic minerals were worth about ₹13,640 crore, or 9.7% [9].
• There were 795 metallic and 1,241 non-metallic mineral mines [9].
• Limestone, the single-mineral rock above, gave 90.5% of the value of non-metallic minerals. Its output was 451 million tonnes, up 11% from the year before [9].

9. Who studies minerals, and why

Geographers	Geologists
Study minerals as part of the crust, to explain landforms	Study how minerals formed
Map the distribution of mineral resources	Find out their age
Link minerals to economic activity: mining belts, industries, settlements	Study their physical and chemical composition

Prelims Hooks

• Order of crustal elements by weight: O (46.6) > Si (27.7) > Al (8.1) > Fe (5.0) > Ca (3.6) > Na (2.8) > K (2.6) > Mg (2.1). These eight make up about 98%.
• Most abundant metal in the crust: aluminium. It is not iron, which is a common trap.
• Most abundant element in the whole Earth (by weight): iron. This is because the core is nife (nickel + iron).
• Oxygen and silicon together make up about 74% of the crust. This is why silicates are the main mineral group.
• Calcium comes before sodium, potassium and magnesium in crustal abundance. Magnesium is the last of the "big eight".
• Limestone is a rock made of a single mineral. Most rocks contain several minerals.
• Ore = a mineral deposit that is commercially viable to mine. Workable deposits are only about 1% of the crust.
• Minerals known: over 2,000 (NCERT). About 6,000 species are recognised today, with the IMA as the naming body [5][6].
• India produces 95 minerals: 4 fuel, 10 metallic, 23 non-metallic, 3 atomic and 55 minor [8].
• Properties used to identify minerals: colour, hardness, crystal form, lustre, density.

Mains Points

• Crust vs whole Earth (GS-I): early differentiation by density sent heavy iron and nickel into the core (nife), while light silicates of oxygen, silicon and aluminium rose to form the crust. This explains why aluminium leads the metals in the crust while iron leads in the whole planet. It also explains why silicate rocks make up most landforms.
• Abundance is not availability (GS-III): aluminium is 8.1% of the crust, yet only ore-grade bauxite can be mined. Since workable deposits are only about 1% of the crust and minerals are non-renewable, India needs careful exploration (GSI), recycling and planned use of reserves.
• India's mineral economy (GS-III): India has a wide base of 95 minerals [8]. Metallic minerals carry about 90% of the value of metallic plus non-metallic output (2023-24) [9]. Limestone gives over 90% of the value of non-metallic minerals and feeds the cement and steel industries [9]. This shows why geographers map where minerals are, since that decides where industries, transport links and mining towns develop.
• Uneven spread (GS-I): different physical and chemical conditions form different minerals. As a result, mineral resources are spread unevenly across the world and within India, which shapes trade, regional development and resource disputes.