MEV 012: Unit 04 - Rocks and Minerals

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 UNIT 4: ROCKS AND MINERALS

4.0 Introduction

This unit provides an in-depth understanding of minerals and rocks—the fundamental building blocks of the Earth. It explores their classification, composition, and behavior under various geological conditions. The unit also introduces the basics of geochemistry and weathering processes that transform rocks.

4.1 Objectives

After completing this unit, you will be able to:

  • Define minerals and classify them chemically and structurally.
  • Identify major rock-forming mineral groups.
  • Understand the formation and classification of rocks.
  • Explain types and factors of weathering.
  • Comprehend geochemical concepts including element mobility and classification.

4.2 Minerals

Minerals are naturally occurring, inorganic, crystalline solids with a definite chemical composition and physical properties. They form the basis of all rocks.

4.3 Chemical Classification of Minerals

Minerals are classified based on their chemical composition into:

  • Silicates (e.g., quartz, feldspar)
  • Oxides (e.g., hematite)
  • Sulfides (e.g., pyrite)
  • Carbonates (e.g., calcite)
  • Halides (e.g., halite)
  • Sulfates (e.g., gypsum)
  • Native elements (e.g., gold, copper)

4.4 Structural Classification of Silicates

Silicates are categorized by how the silica tetrahedra (SiO₄) are arranged:

  • Isolated (Nesosilicates) – Olivine
  • Single chain (Inosilicates) – Pyroxene
  • Double chain – Amphibole
  • Sheet (Phyllosilicates) – Mica
  • Framework (Tectosilicates) – Feldspar, Quartz

4.5 Common Rock-Forming Mineral Groups

4.5.1 Olivine Group

Green, high-temperature silicates found in mafic and ultramafic rocks.

4.5.2 Garnet Group

Hard, often red minerals found in metamorphic rocks.

4.5.3 Pyroxene Group

Dark-colored silicates common in igneous and metamorphic rocks.

4.5.4 Amphibole Group

Elongated, dark silicates found in metamorphic rocks (e.g., hornblende).

4.5.5 Mica Group

Sheet silicates with perfect cleavage—includes biotite and muscovite.

4.5.6 Feldspar Group

Most abundant group in Earth's crust, includes orthoclase and plagioclase.

4.5.7 Silica Group

Includes quartz, a common mineral in all rock types.

4.5.8 Carbonates

Includes calcite and dolomite, major components of sedimentary rocks.

4.6 Rocks

Rocks are naturally occurring aggregates of minerals. They are classified into three main types based on formation processes.

4.7 Classification of Rocks

4.7.1 Igneous Rocks

Formed by the cooling and solidification of magma or lava.

  • Intrusive (granite, gabbro) – formed beneath Earth's surface.
  • Extrusive (basalt, rhyolite) – formed on Earth's surface.

4.7.2 Sedimentary Rocks

Formed from the accumulation and lithification of sediments.

  • Clastic (sandstone, shale)
  • Chemical (limestone)
  • Organic (coal)

4.7.3 Metamorphic Rocks

Formed from existing rocks under heat and pressure.

  • Foliated (schist, gneiss)
  • Non-foliated (marble, quartzite)

4.8 Weathering

Weathering is the breakdown of rocks at the Earth’s surface due to exposure to atmosphere, water, and biological activity.

4.9 Factors Controlling Weathering

  • Climate (temperature, rainfall)
  • Mineral composition
  • Surface area
  • Time
  • Topography

4.10 Types of Weathering

4.10.1 Physical Weathering

Mechanical breakdown without chemical change (e.g., frost action, thermal expansion).

4.10.2 Chemical Weathering

Alters mineral composition through processes like oxidation, hydrolysis, and carbonation.

4.10.3 Biological Weathering

Involves plants, animals, and microbes—e.g., root expansion and organic acid production.

4.11 Basic Concepts of Geochemistry

Geochemistry deals with the chemical composition of Earth materials and processes.

4.11.1 Major Oxides

Constitute bulk of Earth's crust: SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O.

4.11.2 Trace Elements

Present in very small quantities but influence rock properties (e.g., Zn, Ni, Cr).

4.11.3 Rare Earth Elements (REEs)

Lanthanides + Y + Sc, used as geochemical tracers and in advanced technologies.

4.12 Geochemical Classification of Elements

  • Siderophile (iron-loving): Fe, Ni
  • Lithophile (rock-loving): Si, Al
  • Chalcophile (sulfur-loving): Pb, Cu
  • Atmophile (gas-loving): N, He

4.13 Mobility of Trace Elements

Refers to how easily elements move through water or molten material. Influenced by:

  • pH
  • Temperature
  • Redox potential
  • Bonding strength

4.14 Let Us Sum Up

This unit highlighted the core principles of mineral and rock science. Minerals are classified chemically and structurally, with silicates being the most abundant. Rocks form through igneous, sedimentary, and metamorphic processes. Weathering modifies rocks through physical, chemical, and biological means. The basics of geochemistry help understand element distribution and mobility within the Earth’s crust.

4.15 Keywords

·         Mineral-Naturally occurring, inorganic solid with a definite chemical formula and crystal structure.

·         Silicate-Mineral made of silica tetrahedra, forming the bulk of Earth’s crust.

·         Feldspar-Most common rock-forming mineral group in the crust.

·         Igneous Rock-Rock formed from the cooling and solidification of magma.

·         Sedimentary Rock-Rock formed from compacted and cemented sediment.

·         Metamorphic Rock-Rock altered by heat, pressure, or chemical processes.

·         Weathering-Breakdown of rocks at Earth’s surface due to environmental exposure.

·         Geochemistry-Study of Earth's chemical composition and processes.

·         Major Oxides-Main chemical components of rocks like SiO₂ and Al₂O₃.

·         Trace Elements-Elements present in minute amounts in rocks, often with significant effects.

·         Rare Earth Elements (REEs)-Group of 17 elements used in geochemical studies and technology.

·         Siderophile-Elements that associate with metallic iron (e.g., Fe, Ni).

·         Diagenesis-Changes occurring in sediments after deposition, before becoming rock.

 

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