Regolith MineralsRegolith Minerals

Types, Nature & Types, Nature & SignificanceSignificance

Mehrooz F AspandiarCRC LEME

WASM, Department of Applied Geology,

Curtin University of Technology

Types of regolith minerals• Phyllosilicates or Clay Minerals Smectites, Kaolinite, Illite, Vermiculite & interstratified

varieties of these

• Framework Silicates – opal A & opal-CT, quartz

• Oxides & hydroxides – Fe, Mn, Al & Ti

Geothite, hematite, maghemite, gibbsite, lithophorite

• Sulphates - gypsum, jarosite, alunite

• Carbonates – calcite, dolomite, magnesite

• Chlorides - halite

• Phosphates – crandalite, florencite

Nature of regolith minerals

• Most are very fine-grained (high surface areas), generally having grain sizes < 100 nm, some even < 10 nm

• They occur

– Intimately mixed with each other & organics

– Within microbes (magnetotactic bacteria)

• Large amount of defects in their crystal structure

• Fine size and crystal-chemical properties impart a dynamic nature to them

– Some change structural properties in real time!

kaolinite

Hem

5nm2 mic

?

How many minerals in a 100 nm or 0.01 micron?

What grain size?

Why study and understand regolith minerals?

• They regulate trace and major element

mobility in the regolith and therefore

impact on geochemistry of regolith,

groundwaters and surface waters

• They provide clues to the physio-

chemical conditions of the regolith such

as pH, Eh, temperature, drainage,

presence of anions

Mineral Hosts

Current and past environments and processes

How regolith minerals regulate element mobility

Regolith minerals regulate element mobility in the environment by •Incorporating elements in precipitating minerals

Fe in goethite & hematite•Adsorbing/Desorbing elements

Pb or Cu on goethite

If minerals dissolve or pH conditions change, elements become mobile

Every Atom needs a home!Where do elements reside in the regolith?

• Essential element e.g. Fe in goethite

• Accidental element e.g. Cr in geothite

• Adsorbed element e.g. Pb on goethite

• Surface precipitate e.g. PbCO3 on goethite

• Polymeric entities/clusters e.g. AlO6 dimers

• Dissolved entities e.g Ca(H2O)6

• Gaseous e.g. CO2

The dominant are mineral hosts (solid phase)

What can minerals tell us about regolith environments?

• pH and Eh

• Main anions (SO4, Cl-, SiO4)

• Wet (poorly drained) or dry (well drained)

• Landscape position (crest or valley bottom)

• Temperature

Regolith Environments

pH vs Anions

• The presence of specific minerals provides information on pH and type of dominant anion present in the system

Regolith Environments

pH vs Eh (redox condition)

Iron oxides, iron-sulphates, iron sulphides and carbonate minerals are indicators of pH and redox (oxidizing or reducing) conditions

Acid sulphate soilsAcid Mine DrainageSalt LakesWetlands

Modified from Taylor & Eggleton (2001)

Mineralogy and pHPerth wetlands acidification

Inland acid sulfate soils

Reddish-orange ferrihydrite + schwertmannite

• Impeded or slow groundwater flow results in smectite-carbonate-goethite association

• Free flow (well-drained) results in most stable assemblage kaolinite-goethite/hematite association

• However, need to consider climate and drainage together

For example, smectites may indicate aridity and/or impeded drainage

Regolith Environments

Profile Drainage

Regolith Environments

Wet vs Dry

• Use iron oxides as indicators • Goethite (yellow-brown)– high water activity• Hematite (red to dark red) – low water activity• This association provides wet vs dry

environmental information on • Micro – fine mottles• Profile – brown mottles below red soil• Landscape – red crests versus yellow valleys• Regional scale - red areas versus yellow

• Local regolith environment is dictated by landscape position because landscape position controls drainage

• Generally, hill crests are drier because they are well drained

• Valleys are wetter because poorly drained

Regolith Environments

Drainage & Landsacpe

Drainage in parts of regolith profiles can be controlled by parent material and substrate in addition to landscape position

If substrate impermeable, poor drainage results in goethite and/or smectite in particular units.

If substrate or media allow free drainage, hematite dominates

Regolith Environments

Drainage, landsacpe & substrate

Regolith Environments

Landscape and pH

• Silica-silicrete (opal, micro-quartz), carbonates (calcite, dolomite) and gypsum are indicators of pH and landscape in arid and semi-arid climates

After Taylor & Eggleton 2001

Regolith Environments

Temperature

• Few reliable indicators of temperature

• However, on continental scale goethite to hematite ratio shows an increase from equatorial regions to polar regions, ie the redder hematite predominates over browner goethite with cooler temperatures, drainage remaining same!

• Maghemite can be an indicator of forest fires

Mineralogical Analysis Techniques

• X-ray diffraction – tried and tested, best for > 4% mineral identification and other structural analysis – slow – new variants such as Synchrotron are powerful methods but more time consuming

• SEM – only surface morphology but with EDAX powerful

• Spectroscopy – – Transmission mode (IR)– Reflectance mode – rapid and robust but not always unique

to minerals– NMR – probes local crystal-chemical (in depth)

• TEM & AEM – if all else fails – with diffraction, chemical analysis and imaging – the ultimate

References

• Bingham,J., Fitzpatrick, R.W., Schulze, D.G (2002) Iron oxides. In Soil Mineralogy with Environmental Applications, 323-366.

• Dixon, J & Schulze (2002) (eds) Soil Mineralogy and Environmental Applications

• Taylor & Eggleton (2001) – Regolith Geology and Geomorphology (chapter 3)