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Soil Powered By Docstoc
• soil - (i) The unconsolidated mineral or organic
  material on the immediate surface of the earth that
  serves as a natural medium for the growth of land
  plants. (ii) The unconsolidated mineral or organic
  matter on the surface of the earth that has been
  subjected to and shows effects of genetic and
  environmental factors of: climate (including water
  and temperature effects), and macro- and
  microorganisms, conditioned by relief, acting on
  parent material over a period of time. A product-soil
  differs from the material from which it is derived in
  many physical, chemical, biological, and
  morphological properties and characteristics.
            Soil Components
• Minerals               • Soil fabric – spatial
• Organic Material         arrangement of these
• Microorganisms
                         • Liquid – 1-35%
• Plants
                         • Chelates – organics
• Gas – CO2, CH4, H2S,     that bond metals
  etc.                     strongly, solubilizing
• Water                    them (bidentate or
                           polydentate = 2 or 2+
                           bonds to the metal)
                           – Equilibrium description?
• These are key organic compounds which
  SIGNIFICANTLY affect how much metals
  get into water and how they can be

Cu solubility:
• Cu2+ + EDTA  Cu(EDTA)        log K = 18
• 2Cu+ + O2- = Cu2O             log K = -15
              Soil stratigraphy
• Soil layers, or Horizons, lettered
• O = organic layer = plant fibers, high organics, leafy
• A = topsoil = minerals + organics
• E = leached layer = minerals leached, low organics
• B = accumulation zone = leached and carried down,
  lots of clays
• C = Parent material – partially weather original
• R = bedrock
• Process of turning sediments into sedimentary
  rocks  water-rock interactions precipitating
• Water is pores of sediments
  – ‘fresh’ muds can be >80% water…
  – Water can be trapped at time of deposition,
    transported in, or evolved from dehydration reactions
    of hydrous minerals
• Also can be significant organic matter
  – Drive redox reactions – reduce Fe3+, Mn4+, SO42-…
• Muds are compacted to shales – water is
  expelled, though up to 30% H2O can
  remain associated with clays even at 1 km
• Minerals from water and changing
  conditions  clays, sulfides, siicates,
         Clay Geochemistry
• Clays can have significant chemical
  substitution, they undergo phase
  transitions as diagenesis proceeds

• Illites  Smectites in shales for example
Al2Si4O10(OH)2*nH2O + KAlSi3O8 
  KAl2(AlSi3)O10(OH2) + 4 SiO2(aq) + n H2O
        Sandstone Diagenesis
• Sandy sediments have high permeability,
  meaning water flows through them faster
• The water brings ions, precipitation of calcite
  and silica occurs – WHY?
• These minerals cement the sediments
• Silica becomes a more important cementing
  material at high T
• Pressurized pockets can become more
  concentrated, when the pressure is released
  they are instantly supersaturated…
        Carbonate Diagenesis
• Aragonite and Mg-rich calcite are the major
  phases associated with shallow sedimentary
• Dolomite problem: Dolomite is not the first thing
  to form typically from a water, why are units of
  calcite so extensively dolomitized?
• Reaction requires a higher Mg/Ca ratio –
  occurring perhaps in sabka (supratidal pools)
  environments, or at seawater-meteoric water
  interfaces – where calcite is undersaturated but
  dolomite is supersaturated
• How do these ions get to these places and
  form cement?
• Transport through water…

• Diffusion and advection account for the
  movement of ions
          Economic Geology
• Understanding of how metalliferous minerals
  become concentrated key to ore deposits…
• Getting them out at a profit determines
  where/when they come out
                Ore Deposits
• Economic concentrations of materials are
  ores – combination of economics and
• Geochemically we are looking at processes
  that concentrate ores to a very high degree
  – Magmatic differentaition
  – Weathering processes
  – Hydrothermal water/rock interactions
  *water is especially important at causing this
                Gold  Au
• Distribution of Au in the crust = 3.1 ppb by
  weight  3.1 units gold / 1,000,000,000 units
  of total crust = 0.00000031% Au
• Concentration of Au needed to be
  economically viable as a deposit = few g/t 
  3 g / 1000kg = 3g/ 1,000,000 g = 0.00031%
• Need to concentrate Au at least 1000-fold to
  be a viable deposit
• Rare mines can be up to a few percent gold
  (extremely high grade)!
               Ore minerals
• Minerals with economic value are ore
• Minerals often associated with ore minerals
  but which do not have economic value are
  gangue minerals
• Key to economic deposits are geochemical
  traps  metals are transported and
  precipitated in a very concentrated fashion
  – Gold is almost 1,000,000 times less abundant
    than is iron
      Water-rock interactions
• To concentrate a material, water must:
  – Transport the ions
  – A ‘trap’ must cause precipitation in a spatially
    constrained manner
• Trace metals which do not go into igneous
  minerals easily get very concentrated in the
  last bit of melt
• Leaching can preferentially remove
  materials, enriching what is left or having
  the leachate precipitate something further
       Ore deposit environments
• Magmatic
  – Cumulate deposits – fractional crystallization processes can
    concentrate metals (Cr, Fe, Pt)
  – Pegmatites – late staged crystallization forms pegmatites
    and many residual elements are concentrated (Li, Ce, Be,
    Sn, and U)
• Hydrothermal
  – Magmatic fluid - directly associated with magma
  – Porphyries - Hot water heated by pluton
  – Skarn – hot water associated with contact metamorphisms
  – Exhalatives – hot water flowing to surface
  – Epigenetic – hot water not directly associated with pluton
    Hydrothermal Ore Deposits
• Thermal gradients induce convection of
  water – leaching, redox rxns, and cooling
  create economic mineralization
 Metal Sulfide Mineral Solubility
• Problem 1: Transport of Zn to ‘trap’:
  ZnS + 2 H+ + 0.5 O2 = Zn2+ + S2- + H2O

                                  [ Zn 2 ] f S 2 [ H 2 O]
            log K  9.57  log
                                  [ H  ]2 f O 25 [ ZnS ]

  Need to determine the redox state the Zn2+ would have
   been at equilibrium with…

  What other minerals are in the deposit that might
   indicate that?  define approximate fO2 and fS2-
   values and compute Zn2+ conc.  Pretty low Zn2+
• Must be careful to consider what the
  conditions of water transporting the metals
  might have been  how can we figure that

• What other things might be important in
  increasing the amount of metal a fluid could
  carry? More metal a fluid can hold the
  quicker a larger deposit can be formed…
• How about the following:
  ZnS + 2 H+ + 0.5 O2 + Cl- = ZnCl+ + S2- + H2O

                           [ ZnCl  ] f S 2 [ H 2 O]
     log K  16.6  log
                          [ H  ]2 f O 25 [ ZnS ][Cl  ]

  Compared to
                              [ Zn 2 ] f S 2 [ H 2 O]
      log K  9.57  log
                               [ H  ]2 f O 25 [ ZnS ]

  That is a BIG difference…
          Geochemical Traps
• Similar to chemical sedimentary rocks – must
  leach material into fluid, transport and deposit
  ions as minerals…
• pH, redox, T changes and mixing of different
  fluids results in ore mineralization
• Cause metals to go from soluble to insoluble
• Sulfide (reduced form of S) strongly binds
  metals  many important metal ore minerals
  are sulfides!
               Piquette Mine
• 1-5 nm particles of
  FeOOH and ZnS –
  biogenic precipitation

                           •Tami collecting

    Piquette Mine – SRB activity
• At low T,
  SO42- reduction is
  microbes are

• ‘Pure’ ZnS
  observed, buffering
  HS- concentration
  by ZnS precipitation
            Fluid Flow and Mineral
 • monomineralic if:
      – flux Zn2+ > HS- generation
      – i.e.  there is always enough Zn2+ transported to
        where the HS- is generated, if
 • sequential precipitation if:
      – Zn2+ runs out then HS- builds until PbS precipitates

                         ZnS                  ZnS
 y   Pb2+
 x Zn2+
z HS- generated          ZnS                  PbS
by SRB in time t
            Model Application
• Use these techniques
  to better understand ore
  deposit formation and
  metal remediation
Sequential Precipitation Experiments
• SRB cultured in a 125 ml septum flask
  containing equimolar Zn2+ and Fe2+
• Flask first develops a white precipitate (ZnS)
  and only develops FeS precipitates after
  most of the Zn2+ is consumed

• Upcoming work in my lab will investigate this
  process using microelectrodes  where
  observation of ZnS and FeS molecular
  clusters will be possible!
     Ore deposit environments
• Sedimentary
  – Placer – weathering of primary mineralization
    and transport by streams (Gold, diamonds,
  – Banded Iron Formations – 90%+ of world’s iron
    tied up in these (more later…)
  – Evaporite deposits – minerals like gypsum, halite
    deposited this way
  – Laterites – leaching of rock leaves residual
    materials behind (Al, Ni, Fe)
  – Supergene – reworking of primary ore deposits
    remobilizes metals (often over short distances)
                 Ore Deposit Types I
•   Placer uranium gold
•   Stratiform phosphate
•   Stratiform iron
•   Residually enriched deposit
•   Evaporites
•   Exhalative base metal sulphides
•   Unconfornity-associated uranium
•   Stratabound clastic-hosted uranium, lead, copper
•   Volcanic redbed copper
•   Mississippi Valley-type lead-zinc
•   Ultramafic-hosted asbestos
•   Vein uranium
•   Arsenide vein silver, uranium
•   Lode Gold
              Ore Deposit Types II
•   Clastic metasediment-hosted vein silver-lead-zinc
•   Vein Copper
•   Vein-stockwork tin, tungsten
•   Porphyry copper, gold, molybdenum, tungsten, tin, silver
•   Skarn deposits
•   Granitic pegmatites
•   Kiruna/Olympic Dam-type iron, copper, uranium, gold, silver
•   Peralkaline rock-associated rare metals
•   Carbonatite-associated deposits
•   Primary diamond deposits
•   Mafic intrusion-hosted titanium-iron
•   Magmatic nickel-copper-platinum group elements
•   Mafic/ultramafic-hosted chromite
• At temperatures greater than 200-300°C but
  less than melting, reactions changing the
  mineralogy and fabric of rock are
• P-T changes with burial, tectonic stresses,
  geothermal gradient differences, etc….
• Prograde – ‘forward’ direction – rxns
  occurring with increasing P-T
• Retrograde – ‘back’ direction – rxns
  occurring with decreasing P-T
              Phase Relations
• Rule: At equilibrium, reactants and products have
  the same Gibbs Energy
  – For 2+ things at equilibrium, can investigate the P-T
    relationships  different minerals change with T-P
• For DGR = DSRdT + DVRdP, at equilibrium,
     DG0, rearranging:
                 P         DS R
                          
                 T  DG 0 DVR
                Clausius-Clapeyron equation
 P         DS R
                                                V = Vº(1-bDP)
 T  DG 0 DVR
                                                         S 
                                                   P1                  P1

                                         S P  S 0     dP  S 0    VdP
           DSR change with T or P?                        P T
                                                     P2              P2

  DS R    DCP        DS    V                      b            
                   R    R 
                        DV              S 0  V 0 DP  ( P22  P 2 
    DT  P   T         R T  T  P
                                                         2            
   DV for solids stays nearly constant as P, T change,
    DV for liquids and gases DOES NOT
  • Solid-solid reactions linear  S and V nearly
    constant, DS/DV constant  + slope in diagram
  • For metamorphic reactions involving liquids or
    gases, volume changes are significant, DV terms
    large and a function of T and P (and often
    complex functions) – slope is not linear and can
    change sign (change slope + to –)
 P         DS R
          
 T  DG 0 DVR

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