24.2 Minerals, Rocks, the Rock Cycle, Soil & Earth Materials

Overview & Exam Relevance

Competency 016 (Cycles in Earth Systems) asks you to understand the rock cycle and how rocks, minerals, and soils form, along with their properties. It also asks you to describe the properties and uses of Earth materials such as rocks, soils, water, and atmospheric gases. This section covers mineral identification, the three rock families and the rock cycle, soil formation and texture, and the ways people use Earth materials. The water, carbon, and nitrogen cycles follow in the next section.

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Minerals: Defining Criteria & Diagnostic Identification

A mineral is defined by five mandatory geological criteria:

1. Naturally Occurring: Formed exclusively by natural geological processes, excluding laboratory-synthesized substances.
2. Inorganic: Not formed solely by organic biological processes and lacking carbon-hydrogen biological structures.
3. Solid: Must exist in solid form at standard Earth surface temperatures and pressures (liquid water is not a mineral, but glacial ice is).
4. Definite Chemical Composition: Expressible by a distinct chemical formula (e.g., quartz is strictly $\text{SiO}_2$; halite is strictly $\text{NaCl}$) or a defined range of stoichiometric elemental substitutions.
5. Ordered Internal Crystalline Structure: Constituent atoms are arranged in a regular, repeating, three-dimensional geometric lattice.

Mineral Diagnostic Properties

Elementary educators guide students through hands-on identification using standardized physical testing procedures:

• Color: The most immediately noticeable property, but scientifically the least reliable for identification because minor trace impurities dramatically alter appearance (e.g., pure quartz is colorless, but trace iron creates purple amethyst, while titanium impurities produce rose quartz).
• Streak: The color of a mineral's finely powdered residue obtained by scraping the specimen across an unglazed white porcelain streak plate (hardness $\approx 6.5$). Streak is consistent regardless of external color (e.g., brassy metallic pyrite leaves a diagnostic greenish-black streak; silvery-gray or red hematite consistently leaves a reddish-brown streak).
• Hardness: Resistance of a mineral's surface to scratching or mechanical abrasion, evaluated using Mohs Hardness Scale (a relative scratch scale from 1 to 10):

MOHS HARDNESS SCALE & CLASSROOM TESTING TOOLS

Mineral Standard         Hardness Rating        Common Reference Field Tools
─────────────────────────────────────────────────────────────────────────────
Talc                           1                
Gypsum                         2                ◄── Fingernail (Hardness ~2.5)
Calcite                        3                ◄── Copper Penny (Hardness ~3.5)
Fluorite                       4                
Apatite                        5                ◄── Steel Nail / Pocketknife (~5.5)
Orthoclase Feldspar            6                ◄── Glass Microscope Slide (~5.5)
Quartz                         7                ◄── Porcelain Streak Plate (~6.5)
Topaz                          8                
Corundum                       9                
Diamond                       10                

• Luster: The visual quality and intensity of light reflected from a mineral's fresh surface. Broadly categorized into metallic (opaque and highly reflective like polished metal, such as galena or pyrite) and non-metallic (vitreous/glassy like quartz, pearly like talc, silky like gypsum, adamantine/brilliant like diamond, or earthy/dull like kaolinite).
• Cleavage versus Fracture: Describes how a mineral breaks under applied mechanical stress:
  • Cleavage: The tendency of a mineral to break smoothly along flat, parallel planes corresponding to zones of atomic weakness in its crystal lattice. Cleavage is categorized by number of planes and angle of intersection: 1 direction (basal/sheet cleavage in mica), 3 directions at $90^\circ$ (cubic cleavage in halite and galena), or 3 directions not at $90^\circ$ (rhombohedral cleavage in calcite).
  • Fracture: Irregular, uneven breakage occurring when chemical bonds in all directions possess comparable strength. Types include uneven/jagged fracture or conchoidal fracture (smooth, curved, shell-like surfaces characteristic of quartz and obsidian).
• Acid Effervescence: When exposed to a drop of dilute hydrochloric acid ($\text{HCl}$), carbonate minerals—specifically calcite ($\text{CaCO}_3$)—react vigorously, releasing bubbling carbon dioxide gas: $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow$. This test distinguishes limestone and marble from quartz-based rocks.

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The Rock Cycle: Genetic Families & Transformations

A rock is a naturally occurring solid aggregate composed of one or more minerals, mineraloids, or organic materials. Rocks are categorized into three genetic families based on their mode of formation:

THE CONTINUOUS ROCK CYCLE

                   ┌───────────────────────────────┐
                   │         MAGMA / LAVA          │
                   └───────────────┬───────────────┘
                                   │ Cooling & Crystallization
                                   ▼
                   ┌───────────────────────────────┐
                   │         IGNEOUS ROCKS         │
                   └───────┬───────────────┬───────┘
                           │               │
     Weathering, Erosion,  │               │ Heat & Directed
     Transport, Deposition │               │ Pressure
                           ▼               ▼
┌─────────────────────────────┐         ┌─────────────────────────────┐
│         SEDIMENTS           │         │      METAMORPHIC ROCKS      │
└──────────────┬──────────────┘         └──────────────┬──────────────┘
               │ Compaction &                          │
               │ Cementation (Lithification)           │ Melting
               ▼                                       ▼
┌─────────────────────────────┐         ┌─────────────────────────────┐
│      SEDIMENTARY ROCKS      │────────►│         MAGMA / LAVA        │
└─────────────────────────────┘ Heat &  └─────────────────────────────┘
                                Pressure

1. Igneous Rocks

Igneous rocks crystallize from molten material (magma beneath the surface, or lava at Earth's surface):

• Intrusive (Plutonic) Rocks: Cool slowly deep underground insulated by overlying rock over thousands to millions of years. This slow thermal loss grants atoms ample time to migrate, producing coarse-grained, interlocking visible crystals (phaneritic texture). Examples: granite (felsic), diorite (intermediate), and gabbro (mafic).
• Extrusive (Volcanic) Rocks: Erupt onto Earth's surface or ocean floor, quenching rapidly upon contact with air or water. Rapid cooling prevents large crystal formation, resulting in fine-grained microcrystalline textures (aphanitic texture, such as basalt and rhyolite), glassy non-crystalline textures (obsidian), or gas-bubble perforated textures (vesicular texture, such as pumice and scoria).

2. Sedimentary Rocks

Formed from pre-existing rock fragments, dissolved chemical precipitates, or biological residues through the sequence: weathering $\rightarrow$ erosion $\rightarrow$ transport $\rightarrow$ deposition $\rightarrow$ compaction $\rightarrow$ cementation (lithification):

• Clastic Sedimentary Rocks: Built from accumulated mineral fragments and weathered rock debris (clasts) cemented together by silica or calcite. Classified strictly by grain size: shale (clay particles $< 0.004\text{ mm}$), siltstone ($0.004\text{ to }0.063\text{ mm}$), sandstone ($0.063\text{ to }2.0\text{ mm}$), and coarse conglomerate (rounded gravels $> 2\text{ mm}$) or breccia (angular gravels).
• Chemical Sedimentary Rocks: Precipitate directly from mineral-saturated aqueous solutions when water evaporates in arid basins. Examples: rock salt (halite) and gypsum.
• Organic / Biochemical Sedimentary Rocks: Formed from the accumulated shells, skeletal fragments, or carbonaceous remains of once-living organisms. Examples: fossiliferous limestone and chalk (calcium carbonate shells of marine organisms), and coal (compressed plant debris from ancient anoxic swamps).
• Crucial Exam Fact: Fossils are found almost exclusively in sedimentary rocks. The elevated temperatures and mechanical shearing associated with igneous and metamorphic processes destroy organic structures.

3. Metamorphic Rocks

Formed when any pre-existing parent rock (protolith) is subjected to intense heat ($>200^\circ\text{C}$), high lithostatic and directed pressure, or reactive hydrothermal fluids without melting:

• Foliated Metamorphic Rocks: Subjected to directed differential tectonic stress, causing platy, tabular minerals (such as micas) to rotate and recrystallize into parallel, banded, or layered alignments perpendicular to maximum stress. Foliation exhibits a diagnostic metamorphic grade progression from parent shale:

$\text{Shale (Sedimentary)} \xrightarrow{\Delta T, P} \text{Slate} \rightarrow \text{Phyllite} \rightarrow \text{Schist} \rightarrow \text{Gneiss (High Grade)}$

• Non-Foliated Metamorphic Rocks: Formed in environments where confining pressure is uniform in all directions (or through thermal contact metamorphism adjacent to magma intrusions), resulting in equidimensional, interlocking crystalline grains without banding. Examples: limestone metamorphoses into marble; quartz sandstone metamorphoses into quartzite.

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Soil Science: Composition, Horizons & Soil Texture Triangle

Soil is a dynamic, living natural terrestrial body composed of weathered mineral grains, organic matter, water, and gases supporting plant growth.

TYPICAL VOLUMETRIC SOIL COMPOSITION
┌────────────────────────────────────────────────────────┐
│ Mineral Matter (Weathered Rock Clasts): ~45%           │
├────────────────────────────────────────────────────────┤
│ Organic Matter (Decomposed Humus & Microbes): ~5%      │
├────────────────────────────────────────────────────────┤
│ Soil Water / Pore Fluid: ~25%                          │
├────────────────────────────────────────────────────────┤
│ Soil Air / Gases (Oxygen & Carbon Dioxide): ~25%       │
└────────────────────────────────────────────────────────┘

1. Soil Horizon Stratification (The Soil Profile)

Over prolonged pedogenic timescales, physical, chemical, and biological processes differentiate soil into distinct horizontal layers called horizons:

IDEALIZED MATURE SOIL PROFILE

[Surface] ─────────────────────────────────────────────────────────
  O HORIZON  │ Organic Layer: Fresh leaf litter, twigs, decomposing humus
─────────────┼─────────────────────────────────────────────────────
  A HORIZON  │ Topsoil: Dark mineral soil enriched with organic humus;
             │ high biological activity, root zones, seed germination
─────────────┼─────────────────────────────────────────────────────
  E HORIZON  │ Eluviation Zone: Light-colored, heavily leached zone
             │ depleted of clays and iron oxides (mature forest soils)
─────────────┼─────────────────────────────────────────────────────
  B HORIZON  │ Subsoil: Zone of accumulation (illuviation); dense clay,
             │ iron oxides, and minerals washed down from A and E
─────────────┼─────────────────────────────────────────────────────
  C HORIZON  │ Substratum: Partially weathered, fractured parent rock
             │ (regolith); zero organic matter
─────────────┼─────────────────────────────────────────────────────
  R HORIZON  │ Bedrock: Unweathered, consolidated parent rock foundation
[Bedrock] ─────────────────────────────────────────────────────────

2. Soil Texture Triangle & Physical Properties

Soil texture is determined by the relative mass percentages of three basic mineral particle sizes:

• Sand: Largest particles ($0.05\text{ to }2.0\text{ mm}$); feels coarse and gritty. Possesses large macro-pore spaces, resulting in high permeability (rapid water drainage) and very low nutrient and water retention capacity.
• Silt: Medium-sized particles ($0.002\text{ to }0.05\text{ mm}$); feels smooth, silky, and floury when dry, and creamy when moist. Holds moderate moisture and nutrients.
• Clay: Smallest particles ($< 0.002\text{ mm}$); microscopic sheet-like mineral lattices that feel highly sticky and plastic when wet, and hard when dry. Displays dense micro-pores resulting in low permeability (poor drainage), high water-holding capacity, and high susceptibility to compaction.
• Loam: An ideal agricultural soil containing a balanced mixture of approximately 40% sand, 40% silt, and 20% clay. Loam balances high nutrient retention and adequate moisture holding with necessary aeration and drainage.

SOIL TEXTURE TRIANGLE INTERPRETATION

                       100% Clay
                          ▲
                         / \
                        /   \
                       /     \
                      / Clay  \
                     /         \
                    / Silty     \
                   /   Clay      \
                  / Sandy         \
                 /  Clay   Clay    \
                /          Loam     \
               / Sandy Clay          \
              /     Loam      Silty   \
             /                 Clay    \
            /  Sandy           Loam     \
           /    Loam   Loam  Silt Loam   \
          / Loamy                         \
         /   Sand             Silt         \
100% Sand ──────────────────────────────────► 100% Silt

To determine soil texture on the triangle, locate the measured percentages of clay along the left horizontal axis, silt along the right diagonal axis, and sand along the bottom axis. Follow the grid lines inward; their intersection identifies the soil classification (e.g., a sample with 20% clay, 40% silt, and 40% sand is classified as loam).

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Properties and Uses of Earth Materials

Renewable vs. nonrenewable: Soil forms very slowly (often centuries for a few centimeters), so eroded topsoil is effectively nonrenewable on a human timescale. Minerals and rocks are nonrenewable, while water is renewed by the water cycle but can be overused or polluted. Recycling metals, conserving water, and preventing soil erosion protect these resources.