Minerals of Earth's Crust - Week 4 Notes

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Course: Senior High School - Earth Science

• Week 4: Minerals of Earth’s Crust

• Prepared for S.Y. 2025-2026 | 1st Semester

• Focus: Building foundational understanding of minerals, their formation, properties, and the classification of mineral groups.

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• Figure 1: The Cave of Crystals, Chihuahua, Mexico contains giant gypsum crystals — among the largest natural crystals found.

• Significance: Illustrates extraordinary crystal size and growth conditions in natural environments.

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• MINERALS: BUILDING BLOCKS OF ROCKS

  • Mineralogy: the study of minerals.

  • Minerals are the fundamental building blocks of rocks.

  • Humans have used minerals for thousands of years for practical and decorative purposes.

  • The formal study of minerals gained momentum during the Middle Ages due to increased mining.

  • Early examples hinting at mineral use: Computer chips (Quartz, Flint & Chert) used for early tools and weapons; Gold, silver, and copper mined by Egyptians (approximately 3700 BCE).

  • Bronze (copper + tin alloy) discovered by 2200 BCE.

  • Iron extracted from minerals like hematite, marking the decline of the Bronze Age.

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What is a Mineral?

A mineral is defined as any naturally occurring inorganic solid that possesses an orderly crystalline structure and a definite chemical composition that allows for some variation.

Criteria (3 main parts)

1) Naturally Occurring

• Formed by natural geologic processes.

• Synthetic materials produced in laboratories are not considered minerals.
  2) Generally Inorganic

• Inorganic crystalline solids found in nature (e.g., halite/table salt) are minerals.

• Organic compounds (e.g., sugars from plants) are generally not minerals.

• Inorganic compounds secreted by marine animals (e.g., calcium carbonate in shells, coral reefs) can be considered minerals if they become part of the rock record.
  3) Solid Substance

• Only solid crystalline substances are minerals.

• Ice is a mineral; liquid water and water vapor are not.

• Exception: Mercury (Hg) occurs naturally as a liquid.

• Figure 2: Quartz crystals (example of well-developed crystals)

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What is a Mineral? (continued)

• Orderly Crystalline Structure: Minerals are crystalline solids with atoms (or ions) arranged in a repetitive, orderly manner, resulting in regularly shaped crystals. Substances lacking a repetitive atomic structure (e.g., volcanic glass/obsidian) are not minerals.

  • Figure 3: Ordered (A) vs. unordered (B) arrangement of atoms. Minerals (A) have a repetitive arrangement; glass (B) is unordered.

• Definite Chemical Composition (with some variation): Minerals are chemical compounds with a specific chemical formula (e.g., quartz = $ext{SiO}_2$). Proportions of elements are constant for a pure mineral, though some variation can occur when similarly sized elements substitute for each other without altering the internal structure.

  • Example: $ext{SiO}_2$ for quartz.

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What is a Rock?

• A rock is any solid mass of mineral- or mineral-like matter that occurs naturally as part of Earth.

• Most rocks are aggregates of several different minerals (e.g., granite).

  • Aggregate means minerals are joined but retain their individual properties.

• Some rocks are composed almost entirely of one mineral (e.g., limestone, mostly calcite).

• Some rocks are composed of non-mineral matter:

  • Non-crystalline glassy substances (e.g., obsidian, pumice).

  • Solid organic debris (e.g., coal).

• The properties of rocks are largely determined by the chemical composition and crystalline structure of their constituent minerals.

• Figure 3: Hand sample of the igneous rock granite and three major constituent minerals.

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How Do Minerals Form?

• Minerals form through diverse processes and environments.

• Three Primary Mechanisms:
  1) Precipitation of mineral matter from a solution.
  2) Crystallization of molten rock by cooling.
  3) Mineral matter deposition as a result of biological processes.

• These processes also form most rocks.

• Note: Metamorphism alters existing minerals to form new ones.

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1) Precipitation from a Solution

• Minerals grow from an aqueous solution containing dissolved ions.

• Ions initially move freely.

• Saturation occurs due to:

  • Drop in temperature

  • Water loss through evaporation

• Once saturated, ions bond and form crystalline solids (salts) that precipitate.

• Examples:

  • Evaporite deposits: Great Salt Lake (Utah), Salar de Uyuni (Bolivia) — precipitate halite (NaCl), sylvite (KCl), gypsum (CaSO₄·2H₂O).

  • Groundwater deposits: Can fill fractures and voids.

  • Geodes: Spherical objects with inward-projecting crystals deposited by groundwater (e.g., quartz, calcite).

  • Key formulae: chemical reactions depend on dissolved ions and subsequent precipitation; main point is saturation leading to crystal growth.

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2) Crystallization of Molten Rock by Cooling

• Similar to water freezing.

• When magma (underground) or lava (on surface) is hot, atoms are mobile.

• As molten material cools, atoms slow and chemically combine to form crystals.

• Result: igneous rocks — a mosaic of intergrown crystals, often lacking distinct faces.

• Figure 4: Minerals can form when molten rock solidifies.

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3) Mineral Matter Deposition as a Result of Biological Processes

• Water-dwelling organisms transform dissolved materials into mineral matter for their hard parts.

• When organisms die, their mineralized remains accumulate and form rocks.

• Examples:

  • Corals: Use calcium ions from seawater to secrete calcium carbonate (calcite) skeletons, forming massive limestone reefs.

  • Mollusks (clams) and other invertebrates: Secrete shells of calcite and aragonite. Remains form a major component of limestone.

  • Diatoms & Radiolarians: Produce glass-like silica skeletons. Burial forms microscopic quartz crystals found in rocks like chert and flint.

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PROPERTIES OF MINERALS

• Minerals have unique physical and chemical properties.

• These properties are consistent for all samples of a given mineral.

  • Example: All quartz samples have the same hardness and density.

• Variation can occur due to:

  • Ionic substitutions

  • Impurities (foreign elements)

  • Defects in the crystal structure

• Diagnostic properties: Very useful for identification (e.g., halite’s salty taste).

• Ambiguous properties: Can vary; less reliable on their own (e.g., color).

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1) Optical Properties

• How minerals interact with light; frequently used for identification: Luster, Ability to Transmit Light, Color, and Streak.

• Luster types:

  • Metallic Luster: Looks like metal (e.g., native copper, galena).

  • Submetallic: Dull coating/tarnish on a metallic mineral.

  • Nonmetallic Luster: Described with terms such as:

  • Vitreous / Glassy (e.g., quartz)

  • Dull / Earthy

  • Pearly

  • Silky

  • Greasy

• Figure 5: Metallic versus submetallic luster.

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1a) Ability to Transmit Light

• Opaque: No light transmitted.

• Translucent: Light transmitted, but no image visible.

• Transparent: Light and image visible.

• Figure 6: Color variations in minerals.

1b) Color

• Generally the most conspicuous property but often not diagnostic.

• Ambiguities: Impurities can cause varied tints (e.g., quartz comes in pink, purple, yellow, etc.).

• Some minerals naturally exhibit many colors (e.g., tourmaline).

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1c) Streak

• Rub mineral across an unglazed porcelain streak plate.

• Key features:

  • Often consistent in color even if the mineral’s visible color varies.

  • Not all minerals produce a streak (e.g., harder than the streak plate like quartz).

• Distinguishing Luster:

  • Metallic minerals: Generally have a dense, dark streak.

  • Nonmetallic minerals: Typically have a light-colored streak.

• Definition: The color of a mineral in powdered form.

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2) Crystal Shape/Habit

• The common or characteristic shape of individual crystals or aggregates of crystals.

• Determined by how atoms grow in specific dimensions.

• Examples of common habits:

  • Equant (equidimensional): grows equally in all dimensions (e.g., magnetite as octahedrons, garnet as dodecahedrons, halite/fluorite as cubes).

  • Bladed

  • Fibrous

  • Tabular

  • Prismatic

  • Platy

  • Blocky

  • Thin, rounded crystals that break into fibers

  • Elongated crystals that are flattened in one direction

  • Minerals that have stripes or bands of different color/texture

  • Group of crystals that are cube-shaped

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3) Mineral Strength

• Describes how easily minerals break or deform under stress.

• Determined by the type and strength of chemical bonds.

3a) Hardness

• Resistance of a mineral to abrasion or scratching.

• How measured: rub unknown mineral against a material of known hardness.

• Mohs Scale of Hardness: relative ranking of 10 minerals (1 = softest, 10 = hardest).

• Gypsum (2) is only slightly harder than Talc (1).

• Common objects used for testing:

  • Fingernail (~2.5)

  • Copper Penny (~3.5)

  • Glass (~5.5)

• Figure 7: Mohs hardness scale (A) with common objects; (B) relationship to an absolute scale.

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3b) Cleavage

• Tendency of a mineral to break along planes of weak bonding.

• Appearance: produces relatively smooth, flat surfaces when broken.

• Key points:

  • Not all minerals have cleavage.

  • Cleavage directions can be excellent, fair, or poor; 1, 2, 3, or more directions.

  • Described by the number of cleavage directions and the angles at which they meet (e.g., 3 directions at 90 degrees in a cube).

• Example: Micas have excellent cleavage in one direction, forming thin sheets.

• Figure 8: Micas exhibit perfect cleavage (thin sheets).

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Cleavage vs Crystal Shape

• Cleavage: mineral breaks into pieces with the same geometry as the cleavage planes.

• Crystal Shape: refers to the original external shape of the unbroken crystal.

• Quartz crystals are smooth, but if broken, they fracture (not cleavage).

• Figure 2: Quartz crystals (example of crystal form vs cleavage).

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3c) Fracture

• Occurs in minerals where atomic bonds are equally strong in all directions.

• Produces uneven surfaces when broken.

• Types of fracture:

  • Irregular fracture: most common, uneven surfaces.

  • Conchoidal fracture: smooth, curved surfaces (e.g., broken glass; quartz).

  • Splintery fracture: produces splinters.

  • Fibrous fracture: produces fibers.

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3d) Tenacity

• A mineral’s resistance to breaking, bending, cutting, or other deformation.

• Types of tenacity:

  • Brittle: Shatters into small pieces when struck (e.g., quartz, halite).

  • Malleable: Can be hammered into thin sheets without breaking (e.g., native copper, gold).

  • Sectile: Can be cut into thin shavings (e.g., gypsum, talc).

  • Elastic: Bends and snaps back to original shape after stress is released (e.g., micas).

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4 Density & Specific Gravity

• Density: mass per unit volume.

• Specific Gravity (SG): a common measure used for minerals; ratio of a mineral’s weight to the weight of an equal volume of water.

• Most common minerals have SG between 2 and 3 (e.g., quartz ≈ $2.65$).

• Dense metallic minerals have much higher SG (e.g., galena ≈ $7.5$, gold ≈ $20$).

• Practical use: can be estimated by “hefting” a mineral in your hand (how heavy it feels compared to size).

• Key formula: $SG = rac{W<em>{ ext{mineral}}}{W</em>{ ext{water}}} = rac{<br>ho<em>{ ext{mineral}}}{ ho</em>{ ext{water}}}$

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5 Other Properties of Minerals

• Some minerals have distinctive, unique properties:

  • Taste: Halite (NaCl) is salty.

  • Feel: Talc is soapy; Graphite is greasy.

  • Smell: Some sulfur-bearing minerals have rotten-egg odors.

  • Magnetism: Minerals with high iron content (e.g., magnetite) are attracted to magnets; lodestone is a natural magnet.

  • Special Optical Properties: Double Refraction (calcite shows double images).

  • Chemical Reactions: Effervescence with acids (e.g., calcite reacting with dilute hydrochloric acid) releasing CO₂.

• Figure 9: Double refraction in calcite.

• Figure 10: Calcite reacting with weak acid.

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MINERAL GROUPS

• Over 4000 minerals identified; new ones found annually.

• Only a few dozen are abundant in Earth’s crust; these abundant minerals are called "Rock-Forming Minerals" as they make up most of the crust.

• "Economic Minerals": less abundant but used extensively in manufacturing.

• Note: Some rock-forming minerals are also economically significant (e.g., calcite in limestone for concrete).

• Classification: Mineral Species

  • Collection of specimens with similar internal structures & chemical compositions.

  • Examples: Quartz, Calcite, Galena, Pyrite.

  • Individual specimens within a species can vary slightly.

• Mineral Varieties

  • Some mineral species are further subdivided.

  • Example: Quartz (SiO₂) varieties:

  • Smoky Quartz: dark color due to trace aluminum.

  • Amethyst: violet color due to trace iron.

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Mineral Groups (continued)

• Mineral species are grouped into Mineral Classes.

• Minerals within each class share:

  • Similar internal structures.

  • Similar properties.

• Important Mineral Classes:

  • Silicates (SiO₄^{2-})

  • Carbonates (CO₃^{2-})

  • Halides (Cl⁻, F⁻, Br⁻)

  • Sulfates (SO₄^{2-})

• Examples:

  • Carbonates react with acid.

  • Halite (NaCl) and Sylvite (KCl) commonly found together in evaporite deposits.

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Silicate vs Nonsilicate Minerals

• Earth's crust composition: Only 8 elements make up >98% by weight of the continental crust:

  • Oxygen (O)

  • Silicon (Si)

  • Aluminum (Al)

  • Iron (Fe)

  • Calcium (Ca)

  • Sodium (Na)

  • Potassium (K)

  • Magnesium (Mg)

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Silicate Minerals

• Formed by: Oxygen and Silicon readily combining.

• Abundance: More than 800 known silicate minerals.

• They account for more than 90% of Earth’s crust.

• Therefore, the most abundant mineral group in Earth’s crust is the SILICATES.

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Silicate Minerals (reiteration)

• Formed by: Oxygen and Silicon readily combining.

• Abundance: More than 800 known silicate minerals.

• They account for more than 90% of Earth’s crust.

• Therefore, the most abundant mineral group in Earth’s crust is the SILICATES.

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Nonsilicate Minerals

• All other mineral groups are less abundant in Earth’s crust than silicates.

• Common Nonsilicate Groups:

  • Carbonates

  • Sulfates

  • Halides

• Economic Importance:

  • Iron & Aluminum: Used for automobiles.

  • Gypsum: Used for plaster and drywall.

  • Copper: Used for electrical wire and internet.

• Geological Importance: Major constituents in sediments and sedimentary rocks.

Summary of Key Concepts

• Minerals are inorganic, solid, crystalline substances with defined chemical compositions (with some variation).

• Rocks are aggregates of minerals; their properties reflect the minerals they contain.

• Minerals form via precipitation, cooling/crystallization of melts, and biological processes; metamorphism can alter minerals to form new ones.

• Minerals have diagnostic physical properties: luster, color, streak, hardness, cleavage, fracture, tenacity, density, specific gravity, and special properties (taste, magnetism, effervescence, double refraction).

• Silicate minerals dominate Earth’s crust; non-silicate minerals include carbonates, sulfates, and halides, among others.

• Geology connects mineral properties to real-world uses: construction, industry, technology, and environmental sciences.

Important Formulas and Reactions

• Quartz chemical formula: $ext{SiO}_2$

• Calcite chemical formula: $ext{CaCO}_3$

• Halite chemical formula: $ext{NaCl}$

• Specific Gravity (SG) relation: $SG = rac{W<em>{ ext{mineral}}}{W</em>{ ext{water}}} = rac{<br>ho<em>{ ext{mineral}}}{ ho</em>{ ext{water}}}$

• Calcite reacting with acid (effervescence):
  ${ ext{CaCO}<em>3 + 2HCl ightarrow CaCl</em>2 + CO<em>2 + H</em>2O}$

• Evaporite minerals include halite (NaCl), sylvite (KCl), and gypsum (CaSO₄·2H₂O).