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!
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