Mineralogy Fundamentals

Landscapes, soils and surface environments - Workshop 2a

Raphael Viscarra Rossel, Lewis Walden

2026-02-25

Yesterday: Recap

Important

Soils vary horizontally AND vertically

  • Horizons (O-A-E-B-C) form through soil processes
  • CLORPT explains WHY soils differ (P + T dominated Scarp vs SCP)
  • Same climate → very different soils → different ecosystems

Today: What are soils made of?

We know soils differ — but what actually creates those differences?

CLORPT → minerals → soil properties → ecosystem function

Today we focus on the minerals:

  • What minerals form under different conditions?
  • Why does mineralogy matter for how soils behave?

Learning goals

By the end of this block:

  1. Explain why mineralogy matters for soil properties and ecosystem function
  2. Distinguish primary from secondary minerals
  3. Describe the structure and properties of 1:1 (kaolinite) and 2:1 (smectite, illite) clays
  4. Link clay type to weathering intensity, age, and landscape position in Australia

Why mineralogy matters

Minerals are the building blocks of soils, they control:

  • Cation exchange capacity (CEC) = ability to hold nutrient ions (Ca²⁺, K⁺, Mg²⁺)
  • Water-holding capacity
  • pH buffering
  • Nutrient retention and availability
  • Soil structure and workability

Note

Understanding minerals = predicting soil behavior

How rocks become soil: Weathering processes

Physical - abrasion, root wedging, thermal cycling

  • Breaks rock into smaller pieces
  • No chemical change

Granite → quartz + feldspar fragments

Chemical - hydrolysis, oxidation, dissolution

  • Transforms minerals into new ones
  • Releases nutrients

Feldspar → clay + dissolved K⁺, Ca²⁺

Important

Climate controls intensity: more water + warmth = faster chemical weathering

WA examples

Physical — Porongurups

Unloading, jointing, block disintegration

Chemical — Wave Rock

Hydrolysis of feldspar, deep weathering

Weathering connects to CLORPT

Which factors control weathering intensity?

  • Cl (Climate): Temperature + water drive chemical weathering
  • P (Parent material): Some minerals weather faster
  • O (Organisms): Organic acids accelerate weathering
  • R (Relief): Drainage affects water contact time

This is HOW CLORPT creates different minerals!

Chemical weathering reactions

⓵ Hydrolysis (for clay formation)

  • Water breaks mineral bonds

  • Feldspars → clays + dissolved ions

  • Example: Granite feldspars → kaolinite clay

⓶ Oxidation (creates soil colors)

  • Oxygen reacts with iron
  • Fe²⁺ (grey) → Fe³⁺ (red/yellow)

Example: Laterites’ red color from hematite

⓷ Dissolution (removes minerals)

  • Minerals dissolve in water
  • Carbonates and salts removed

Example: CO\(_3\) dissolve in acidic, wet climates

CLORPT controls these rates: Cl, P, T

Goldich stability series

Weatherability Minerals Found in
Fast Olivine, pyroxene Basalt
Moderate Feldspar, mica Granite
Resistant Quartz (SiO₂) Sandstone

Implication:

  • Basalt → fast weathering → smectite
  • Granite → slow weathering → kaolinite + oxides
  • Quartz sand → stable → minimal clay

Primary vs secondary minerals

Primary Secondary
Origin Inherited from rock Formed by weathering
Examples Quartz, feldspar, mica Clays, Fe/Al oxides
Size Sand and silt Clay (<0.002 mm)
Role Starting material Controls soil chemistry


The transformation:

Tip

Primary minerals ➡ (chemical weathering) ➡ Secondary minerals

Clay mineral building blocks (I)

Two basic sheets:

Tetrahedral sheet (T):

  • Silica (Si–O) structure
  • Provides the framework

Octahedral sheet (O):

  • Al or Mg (Al/Mg–OH)
  • Provides charge and bonding sites

Clay mineral building blocks (II)

How they stack:

  • 1:1 clays: → T–O (tightly bonded)

  • 2:1 clays: → T–O–T (space between layers)

Why structure matters

Structure Charge CEC Swelling
1:1 (T–O) Low Low None
2:1 (T–O–T) Higher Higher Possible

Kaolinite (1:1)

Structure: T–O, tightly bonded, non-expanding

Conditions: Extreme weathering, very old, high leaching

Properties: Low CEC (~3–15 cmol/kg), low nutrient retention, low water holding, stable structure

Distribution: Widespread, common clay in Australia’s oldest soils (e.g. Darling Scarp)

Smectite (2:1 expanding)

Structure: T–O–T, expanding interlayer

Conditions: Moderate weathering, younger, base-rich parent material, seasonal wet/dry

Properties: High CEC (~80–150 cmol/kg), high water-holding, shrink–swell

Distribution: Eastern Australian Vertosols — Central Lowlands and Murray Basin on basalt

Illite (2:1 K-fixed)

Structure: T–O–T, with K\(^+\) locking layers together

Conditions: Intermediate weathering, mixed parent materials (often sedimentary, aeolian), temperate climates

Properties: Moderate CEC (~20–40 cmol/kg), moderate water and nutrient retention, no swelling

Distribution: In temperate regions of SE Australia, often in mixed soils and aeolian sediments.

Activity pairs/small groups (5 min): Clay mineral identification

For each scenario, identify:

(i) the dominant clay mineral, (ii) CEC (low, med, hi) and (iii) a management implication.

Scenario A:

Ancient plateau, >50 million years old, granite parent material, high rainfall, red acidic soils, low natural fertility

Scenario B:

Lowland plain, <5 million years old, basalt parent material, seasonal rainfall, deep cracks in summer, dark fertile soils

Scenario C:

Temperate hills, mixed sedimentary rocks, moderate rainfall, moderately fertile soils, no major cracking or leaching problems

[3 min pair/group discussion, 2 min quick debrief]

Clay mineral weathering gradient

Weathering intensity Dominant clays CEC Swelling Soil age
Low (young, dry, resistant rock) Illite, mixed Moderate Low Young to intermediate
Moderate (base-rich rock, seasonal) Smectite High High Young to moderate
Extreme (old, wet, high leaching) Kaolinite Low None Ancient
  • As weathering intensity ↑ → CEC ↓, fertility ↓, water-holding ↓, structure stabilises
  • Eastern basalts (smectite) → SE temperate (illite/mixed) → WA laterites (kaolinite)

Iron and Aluminium oxides

Formed when Fe\(^{2+}\)/Al\(^{3+}\) oxidise during intense weathering

Oxide Colour Formula
Goethite Yellow-brown FeOOH
Hematite Red Fe₂O₃
Gibbsite White (Al oxide) Al(OH)₃

Where: Highly weathered, tropical, old well-drained soils (e.g. WA laterites)

Properties: Low CEC, stable, fix phosphorus → P unavailable to plants

Ferrosol

Kandosol

WA laterites = kaolinite + Fe/Al oxides (low CEC, low nutrient-holding)

Distribution of Australian clay minerals and CLORPT


  1. Western Australia: Kaolinite + Fe oxides (ancient, laterites)
  2. Eastern Australia: Smectite (younger, Vertosols)
  3. South East Australia: Illite + mixed clays (intermediate)

CLORPT in action: different formation factors → different minerals → different soil behavior

Western Australia: Kaolinite + Fe/Al oxides

CLORPT
P Granite (feldspar + quartz)
T 100+ million years
Cl Long-term leaching


Result:

  • Kaolinite + Fe oxides
  • Low CEC, acidic, P-fixing → very low fertility

WA laterite profile

Eastern Australia: Smectite (Vertosols)

CLORPT
P Basalt (mafic minerals)
T <10 million years
Cl Seasonal wet/dry


Result:

  • Smectite-dominant
  • High CEC, fertile, shrink–swell cracking

South Eastern Australia: Illite + mixed clays

CLORPT
P Mixed sedimentary
T Intermediate
Cl Temperate


Result:

  • Illite + kaolinite (or smectite)
  • Moderate CEC, moderate fertility, versatile

Brown sodosol

Scarp vs SCP mineralogy contrast

Scarp SCP
Parent Granite Quartz sand
Time >100 Myr <1 Myr
Clay Kaolinite + oxides Almost none
CEC Low (~12) Very low (<2)
Fertility Low Extremely low
Management Heavy fertilizer + lime Massive nutrient inputs

Why clay type matters for management

Kaolinite (low CEC)

  • Nutrients don’t stick → leach away
  • Need constant fertilizer
  • Low water-holding → drought-prone
  • Acidic (need lime)

e.g. Laterites, tropical soils

Smectite (high CEC)

  • Fertile
  • Waterlogging risk when wet
  • Shrink-swell → cracks foundations, roads
  • Sticky when wet, hard when dry

e.g. E. Australia wheat belt

Illite (moderate CEC)

  • Balanced properties, easier to manage
  • Versatile for diverse uses
  • Moderate fertility and water-holding

e.g.Temperate regions

Understanding clay mineralogy = predicting behavior = better management

Activity: Reading clays (10 min)

Three landscapes (6 min):

⓵ Old plateau, red soils, low fertility → Which clay? CEC?

⓶ Basalt plain, deep cracks, productive → Which clay? Implications?

⓷ Temperate hills, moderate fertility → Which clays?

Debrief (4 min)

Key takeaways

  1. Mineralogy bridges CLORPT → properties → ecosystem function
  2. Weathering (esp. hydrolysis) transforms primary minerals → secondary minerals
  3. Clay structure controls behavior: kaolinite (1:1), low CEC, Smectite/illite (2:1), higher CEC
  4. Australian gradient: WA kaolinite → S. East illite/mixed → EA smectite reflects weathering intensity and age
  5. Clay type predicts: Nutrient retention, water-holding, fertility, management needs

Next: How mineralogy and biology shapes soil properties

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