Soil

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

• Minerals • Soil fabric – spatial

• Organic Material arrangement of these

things

• 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?

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) log K = 18

• 2Cu+ + O2- = Cu2O 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



[ Zn 2 ] f S 2 [ H 2 O]

log K  9.57  log

[ H  ]2 f O 25 [ ZnS ]

0.







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

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



[ ZnCl  ] f S 2 [ H 2 O]

log K  16.6  log

[ H  ]2 f O 25 [ ZnS ][Cl  ]

0.









Compared to

[ Zn 2 ] f S 2 [ H 2 O]

log K  9.57  log

[ H  ]2 f O 25 [ ZnS ]

0.







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







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

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

• 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

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:

 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

1

  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