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Regolith Minerals. Types, Nature & Significance. Mehrooz F Aspandiar CRC 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
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Regolith Minerals Types, Nature & Significance Mehrooz F Aspandiar CRC 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!
? Hem 2 mic kaolinite 5nm 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 EnvironmentspH vs Anions • The presence of specific minerals provides information on pH and type of dominant anion present in the system
Regolith EnvironmentspH 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 soils Acid Mine Drainage Salt Lakes Wetlands Modified from Taylor & Eggleton (2001)
Mineralogy and pH Perth wetlands acidification Inland acid sulfate soils Reddish-orange ferrihydrite + schwertmannite
Regolith EnvironmentsProfileDrainage • 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 EnvironmentsWet 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
Regolith EnvironmentsDrainage & Landsacpe • 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 EnvironmentsDrainage, landsacpe & substrate 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 EnvironmentsLandscape 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 EnvironmentsTemperature • 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)
