Transcript Soil chemistry, diagenesis

Soil
• 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
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Minerals
Organic Material
Microorganisms
Plants
Gas – CO2, CH4, H2S,
etc.
• Water
• Soil fabric – spatial
arrangement of these
things
• Liquid – 1-35%
• Chelates – organics
that bond metals
strongly, solubilizing
them (bidentate or
polydentate = 2 or 2+
bonds to the metal)
– Equilibrium description?
Chelators
• These are key organic compounds which
SIGNIFICANTLY affect how much metals get
into water and how they can be transported:
Cu solubility:
• Cu2+ + EDTA  Cu(EDTA)
• 2Cu+ + O2- = Cu2O
log K = 18
log K = -15
Soil stratigraphy
• Soil layers, or Horizons, lettered
OAEBCR
• 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
minerals
• R = bedrock
Diagenesis
• Process of turning sediments into sedimentary
rocks  water-rock interactions precipitating
minerals
• 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-…
Diagenesis
• Muds are compacted to shales – water is
expelled, though up to 30% H2O can
remain associated with clays even at 1 km
depth
• Minerals from water and changing
conditions  clays, sulfides, siicates,
carbonates
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
carbonates
• 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
geochemistry…
• 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
concentration!!
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%
Au
• 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
• 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
away
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
log K  9.57  log
[ Zn 2 ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ 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 fS2values 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
out??
• 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
log K  16.6  log
[ ZnCl  ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ ZnS ][Cl  ]
Compared to
log K  9.57  log
[ Zn 2 ] f S 2 [ H 2 O]
[ H  ]2 f O02.5 [ 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
samples
cells
ZnS
Piquette Mine – SRB activity
• At low T,
thermochemical
SO42- reduction is
WAY TOO SLOW –
microbes are
needed!
• ‘Pure’ ZnS
observed, buffering
HS- concentration
by ZnS precipitation
Fluid Flow and Mineral
Precipitation
• 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
y
Pb2+
ZnS
ZnS
ZnS
PbS
x Zn2+
z HS- generated
by SRB in time t
Model Application
• Use these techniques
to better understand ore
deposit formation and
metal remediation
schemes
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,
other)
– 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
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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
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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
Metamorphism
• At temperatures greater than 200-300°C but
less than melting, reactions changing the
mineralogy and fabric of rock are
metamorphic
• 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
differently…
• For DGR = DSRdT + DVRdP, at equilibrium,
DG0, rearranging:
DS R
 P 

 
 T  DG 0 DVR
Clausius-Clapeyron equation
DS R
 P 

 
 T  DG 0 DVR
V = Vº(1-bDP)
 S 
S P  S 0     dP  S 0    VdP
P T
P2 
P2
P1
DSR change with T or P?
DCP
 DS R 



D
T

 P T
 DS   V 
  R    R 
 DVR T  T  P
P1
b

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 S 0  V 0 DP  ( P22  P12 
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 –)
DS R
 P 

 
 T  DG 0 DVR