Study Notes on Minerals by Jac Genis

MINERALS - Comprehensive Study Notes

Definition of a Mineral

  • A mineral is defined as:

    • Naturally occurring: Formed by natural processes without human interference.

    • Inorganic substance: Does not originate from living organisms.

    • Definite chemical composition: This can be expressed as a chemical formula.

    • Orderly arrangement of atoms: Atoms are organized in a three-dimensional lattice structure.

Physical Properties of Minerals

  • The key physical properties of minerals include:

    • Crystal form or habit: The external shape or growth pattern of a mineral crystal.

    • Colour: The visual perception of the mineral, which can vary.

    • Streak: The colour of the fine powder obtained by rubbing the mineral across an unglazed porcelain plate (streak plate).

    • Cleavage: The tendency of a mineral to break along flat planes.

    • Fracture: The tendency to break with an irregular or uneven surface.

    • Hardness: The resistance to being scratched by other substances.

    • Tenacity: The mineral’s resistance to breaking, bending, or deforming.

    • Magnetism: The ability of a mineral to be attracted to a magnetic field.

    • Smell/Taste/Touch: Other sensory properties that may aid in identification.

Chemistry and Properties

  • The chemistry of a mineral plays a significant role in determining its properties:

    • Isomorphs: Different minerals that have similar chemical compositions but the same crystal structure and habits.

    • Polymorphs: Minerals that share the same chemical composition but differ in their crystalline structure.

Diagnostic Properties

  • Colour as a Diagnostic Property:

    • While colour can be useful, it is not always reliable for identification due to variations in mineral composition and impurities. For example, calcite can appear in many different colours.

Streak

  • The streak of a mineral is determined by rubbing the mineral on an unglazed porcelain plate, producing a fine powder that reveals its true colour.

  • Example: The streak of hematite is a crucial characteristic for identification.

Crystal Systems

  • There are several types of crystal systems that categorize minerals based on their symmetry and structure:

    • Cubic: Example minerals include pyrite and galena.

    • Orthorhombic: Represented by minerals like topaz and olivine.

    • Tetragonal: Known through minerals such as rutile.

    • Monoclinic: Characterized by minerals including gypsum.

    • Hexagonal: Notable minerals include quartz and calcite.

    • Triclinic: Includes minerals such as plagioclase Feldspar.

Crystal Forms

  • Notable crystal forms mentioned include:

    • Garnet forms a dodecahedron shape.

    • Calcite exhibits a rhombohedron shape, significant for its cleavage.

    • Prismatic and Fibrous: Crystal habits seen in other minerals, with distinctive growth patterns.

Cleavage and Fracture

  • Cleavage: The tendency of a mineral to break along specific planes of weakness, resulting in pieces that resemble the original mineral shape.

  • Fracture: The process by which minerals break unevenly, leading to irregular surfaces.

  • Both properties are diagnostic for mineral identification:

    • Example: Galena exhibits cubic cleavage whereas calcite shows rhombohedral cleavage.

Lustre

  • Lustre refers to the shine or sheen of a mineral, which describes how light interacts with its surface. It can be categorized into:

    • Adamantine: Diamond-like brilliance.

    • Vitreous: Glass-like appearance.

    • Metallic: A shiny, reflective surface.

    • Earthy/Dull: Lacking shine, resembling dirt or soil.

Hardness

  • The hardness of a mineral gauges its resistance to being scratched, measured via Mohs Hardness Scale which ranks ten common minerals from 1 (softest) to 10 (hardest):

    1. Talc

    2. Gypsum

    3. Calcite

    4. Fluorite

    5. Apatite

    6. Orthoclase

    7. Quartz

    8. Topaz

    9. Corundum

    10. Diamond

Rock-Forming Minerals

  • A rock is an aggregate made of one or more minerals. Rock-forming minerals primarily consist of:

    • Silicates: Those containing silicon [Si] and oxygen [O].

Classification of Rock-Forming Minerals

  • A simple classification includes:

    • Salic Minerals (Low specific gravity, typically light): Example families include quartz, feldspars.

    • Femic (Mafic) Minerals (High specific gravity, typically dark): Example families include pyroxenes, amphiboles.

Accessory Minerals

  • Additional minerals that may occur in rocks include:

    • Native Metals: e.g., gold.

    • Sulphides: e.g., pyrite.

    • Oxides: e.g., magnetite.

Periodic Table of Chemical Elements

  • The periodic table serves as a foundation for understanding mineral composition. Elements present in the continental crust that are equal to or greater than 0.1% by weight include:

    • Major elements include: Hydrogen (H), Helium (He), Oxygen (O), Silicon (Si), Aluminium (Al), Iron (Fe), etc.