Soil Chem Exam 2 Studyingggg

HO BOI

Mineral abundance in the crust

O>Si>Al>Fe

radius ratio

Ratio of Cation radius/Oxygen radius, assuming all bond energies are the same. Oxygen is a ligand

CN=3

RR=.15-.22

trig planar, 120 deg

CN=4

RR=.22-.41

tetrahedral, 109.5 deg

CN=6

RR=.41-.73

octahedral, 90 and 180 deg

CN=8

RR=.73-1

cubic, 90 deg

CN=12

RR >1

cubo-octahedrons, 60, 90, 120, 180 deg

CN for Al

higher temp leads to lower CN (heat shakes bonds and formation is difficult) Al(OH)3 and Al(OH)6

Pauling's Rules for mineral structure

(Assumption: bonds in minerals are ionic)

difference in EN>=2 then ionic bonds form

electrostatic valency principle/ES bond strength (S):

S=Z(cation)/CN

Bonding Environment for Silicates

They bond point to point (share 1 O), prevents ES repulsion from high CD of Si4+, Al/Mg/Fe3+ octahedrals bond edge to edge (share 2 Os)

Long term stability of configurations

Structures simplify over time, more complex structures are less stable than simple structures (more potential energy), 2:1:1-->2:1-->1:1-->oxides, follow Bowen's Reaction Series

Bowen's Reaction Series

Discontinuous Series: more tetrahedral linkages= more stable, decrease isomorphic substitution (charge buildup)= inc. stability, inc CN of charge compensating cation (Ca-->Na-->K)= higher stability due to denser packing

Continuous Series: presence/more Fe2+ decreases stability

Empty cation positions=destabilized mineral (not common)

Unit Cell

Smallest unit of a mineral that has the properties of that mineral, 1/2 unit cell formula is often represented

2 Types of Octahedral Layers

Dioctahedral layer: 2/3 of cation positions are filled 1/3 are empty (Gibbsite: Al2(OH)6)

Trioctahedral layer: 3/3 cation positions are filled (Brucite Mg3(OH)6)

Phyllosilicate Classification Components

Di/Trioctahedral, layer charge/ 1/2 unit cell (X), tetrahedral/octahedral ratio

PZC

Point of zero charge, aluminol group is a weaker acid than silanol

eg: Al-OH S=Z/CN=3/6=1/2

Al-OH2(1/2+)

PZC=8

Smectites vs Vermiculite

Smectites: 2:1 weak field charge b/c charge is in the octahedral layer far from the surface, allows of large interlayer spcace and high shrink/swell

Vermiculites: 2:1 strong field charge b/c layer charge is close to the surface in the tetrahedral and octahedral layers, less shrink/swell, sheds some H2O of hydration to get closer to cations

HIS and HIV

Hydroxy interlayered smecitite: hydroxy interlayer binds 2 units of 2:1 together noncontinuously with H-bonds

Hydroxy interlayered vermiculite: higher layer charge than HIS and more isomorphic substitution in the tetra layer, also noncontinuous

Mica-like minerals (illite)

no interlayer space, cation chemisorbed in interlayer, X of .85 is neutralized by K+ which works b/c CN=12 and low charge

K+ in Di and Trioctahedrals

In Di: positively charged H+ in the OH groups are shifted over due to repultion of K+ interlayers

In Tri: this is not possible b/c all the space is filled

Chlorites

2:1:1 (label di,di or tri,di), X is variable and greater than 3, H-bonds cause no interlayer space and no shrink/swell

Oxides

All CN=6, pH variable charges, stronger shared charges make stronger acids

eg: MnO2 S=4/6=2/3

Mn-OH2(2/3+)

charge: 0 +1/3

PZC=3

Organic Matter/Humus

pH charge dependent (PZC=3-3.5) since pH is usually above that it is negatively charged

3 fractions: Fluvic acid (most simple and charged, soluble in acid and base), Humic acid (soluble in strong base), Humin (most complex and least reactive, insoluble)

Low pH COOH groups protonated and there is no charge so particles stick, high pH COOH groups are negatively charged so OM particles dissperse

SEM and SEM-EDS

Scanning electron microscope: 300,000x can id minerals

SEM -Energy Dispersive Spectroscopy: excites e's and when they return to ground state they release a photo at a certain wavelength which can be converted to an elemental map of the mineral

XRD

X-ray Defraction: works to id crystalline minerals, gives d-spacing (vertical repeat distance in angstroms)

TGA

Thermal Gravametric Analysis: weigh soil very accurately as temp rises to 500 or 1000C, maps weight loss and derivative of the weight loss, tells you how much and what it is using the temperature region

ECEC

Effective CEC: Gives 6 major cations and exchangeable acidity

Replace basic cations with NH4, KCl removes H+ and AL3+

UN-FAO CEC

Best with gypsum and calcite rich soils

Wash with NH4 to remove basic cations, wash with isopropanol to remove NH4 from pore space, wash with KCl which kicks NH4 off exchange sites, then measure solution with NH3 electrode

USDA CEC

measures cations and total acidity p.120

wash with NH4 to measure basic cations, wash with BaCl2 which neutralizes all residual acidity bound to clay and humus up to pH8, titrate with

F- (Al(OH)3 + 3F-

Neutral Salt Saturation CEC

Structural charge from isomorphic sub of central cations

variable charge from functional groups of OM, protonation or deprotonation of minerals (Al/Fe oxides) (PZC)

Types of Acidity

Residual: Al and H bound to clay and OM, measured using BaCl2

Salt replaceable (exchangeable): Al and H held near clay and OM surfaces

Active acidity: Al and H in solution

Kaolinite

Di

X=0

1:1

Serpentinite

Tri

X=0

1:1

Pyrophyllite

Di

X=0

2:1

Talc

Tri

X=0

2:1

Smectites

Di or tri

X=.25-.6

2:1

Montmorillonite

Di

X=.33

2:1

Vermiculite group

Di or tri

X=.6-.9 (.74)

2:1

K inter layer dehydrates

Strong field charge

HIS

Di (tri)

X= .25-.6

2:1

Small interlayer with h bonds. Subtract interlayer charge to calc cec

HIV

Di (tri)

X= .6-.9

2:1

Small interlayer and higher layer charge than HIS more isomorphism in the tetra layer

Mica like (Illite)

2:1

Di

X=0.6-0.9

No interlayer space

K is chemisorbed

Mica

Muscovite

X=1

Di

2:1

Mica biotite

X=1

Tri

2:1

Reduced iron makes it less stable

Chlorides

2:1:1

X variable,

How do you predict the CN# around a central cation?

Know the radius ratio = radius of cation/radius of oxygen

Ex: radius ratio = 0.22-0.41

CN# = 4

What happens to CN# if the radius ratio increases?

CN# increases with increasing cation radius

Why are CN# = 4/6 important?

They are found in phyllosilicates consisting of tetrahedral (4) and octahedral (6)

What causes Al^3+ to have different CN# (4/6)?

Crystallization temperatures where CN# = 4 is found in primary minerals and 6 is found in secondary minerals (less energy and vibrational motions allowing for the cations to easily hold onto the oxygens).

What is the arrangement for CN# = 8?

Cubic arrangement

Ex: Ca-feldspar

What is close/hexagonal packing?

Oxygen repels each other while attached to central cation. The layers of spheres are packed so that spheres in alternating layers overlie one another. Higher magnitude of close packing means more stable structure because there are less gaps in between. Higher CN# more close packing occurs.

Point-to-point Vs Edge-to-edge

Point-to-point: sharing one oxygen adjacent to another tetrahedral. This is a stable structure and the reason why this happens is because Si^4+ repels the other Silicon in the adjacent tetrahedral.

Edge-to-edge: two oxygens are shared with a cation that has a CN# = 6. The cations like Al can be closer to each other because the charge is not as great as Si. Occur in low temperature conditions.

Nesosilicates

One silicon tetrahedral linking to Mg/Fe that links to another tetrahedral thru point to point linkage

Sorosilicates

Two SiO4 tetrahedra sharing a single oxygen linking (point to point) with Ca, Fe, or Al

Cyclosilicates

One tetrahedral sharing two oxygens and forms a ring structure

Inosilicates (single chain)

One tetrahedral sharing 2 oxygens

Inosilicates (double chain)

Sharing 2-3 oxygens

Phyllosilicates

Sharing 3 oxygens

What are the 2 types of tectosilicates?

No layer charge like quartz and layer charge with isomorphous substitution like feldspar

Tectosilicates

Like quartz, chert, and opal

Every single oxygen is shared and has a neutral charge

Linus Pauling's rules to predict stability and resistance to weathering of silicate structures (4)

1) Coordination # based on size

2) Electrostatic Valency principle or electrostatic bond strength (s = z/CN#)

3)Bonding environment for silicates

4) Stability of configurations (complex structures like 2:1 Vs 1:1)

What happens as you go down the Bowen's Reaction Series on the continuous side? Think about the stability of the mineral at Earth surface conditions.

There is less isomorphous substitution leading to less charge build up and greater stability.

The other thing is that the CN# is increasing of the charge compensating cation which means that there is more close hexagonal packing.

What is empty cation position and how does it happen?

It means we are missing a positive charge leading to destabilization of a structure. Eg: when the Si is gone the mineral will break.

It happens when the mineral is forming the cation can go missing.

When does isomorphous substitution occur?

During mineral formation, it is the process where it replaces a cation of similar size for another within a structure.

What are the components of an octahedron?

The central cation is either Mg2+ or Al3+ and the OH- forms a double pyramid structure.

What are the components of a phyllosilicate?

1) Si tetrahedral

2) Dioctahedral (Al2(OH)6) or Trioctahedral (Mg3(OH)6)

What are the radius ratio ranges for each of the different CN#?

0.15-.22 -> 3

0.22-0.41 -> 4

0.41-0.73 ->6

0.73-1.00 -> 8

1.00 -> 12

What is the Electrostatic Valency Principle or the Electrostatic Bond Strength?

It is the 2nd rule by Linus Pauling to help predict the stability of a mineral. The s value tells us the right configuration of the central cation within a structure which involves calculating the leftover charge from oxygen.

S = z/CN# = positive charge extending out from each bond from the central cation

What is the structure of kaolinite?

1:1 or tetrahedral and octahedral layer connected to each other thru H-bonding which also means that there is no interlayer space. Wafer-like structure under a microscope.

This results in having a low SSA.

What is a halloysite?

A polymorph of kaolinite where there is water in the interlayer forming a tube-like structure.

How does kaolinite (1:1) that has a layer charge of zero develop charge?

At the layer silicate edge, charge can be developed via protonation/deprotonation of edge functional groups = pH dependent charge.

Silanol groups can lose a proton (more negatively charged) at high pH (above pH = 4.8) and the aluminol groups can do the same (Above pH = 8).

Apply the electrostatic valency principle (s = z/CN#)

What is the point of zero charge?

It is the pH where the net charge of the surface is 0 since there are equal amounts of positive and negative charge in the edge groups.

Ex: PZC = 8 for aluminol and PZC = 4.8 for silanols

What are the layer charges (x) for the following 2:1 phyllosilicates? Smectite, Vermiculite, and Micas?

Smectite: 0.25-0.60 eq/ ½ unit cell

Vermiculite: 0.6 - 0.9 eq/ ½ unit cell

Mica: 1 eq/ ½ unit cell

What is the typical layer charge for 2:1:1 phyllosilicates?

X = variable but < 3 eq/ ½ unit cell

Commonly 0.8-1.6

What is the difference between a weak field charge and a strong field charge?

A weak field charge is caused by the presence of IS in the octahedral layer and strong field charge has IS in the tetrahedral layer.

A weak field charge is present in smectites where there is interlayer space (partially attenuated).

A strong field charge like vermiculite is not attenuated because the field charge is right on the surface of the mineral. There is interlayer space, but won't shrink-swell to a great extent.

NOTE: the hydrate cation in the interlayer space will differ in size between the two. Strong field charge will not allow for more water to be in the interlayer due to a strong attraction to the cation itself.

How do other colloids besides clay minerals like oxides and SOM develop charge?

They have pH dependent charge similarly to kaolinite (1:1) where the layer charge starts off at zero.

What are some spectroscopic techniques?

Advanced techniques (require high radiation sources): x-ray microscopy, Nano-scale secondary ion mass spec., Atomic Force Microscopy (AFM), X-ray microtomography (like a CAT scan), and Tender x- rays

Traditional techniques: Scanning electron microscope (SEM), X-ray diffraction (XRD), and Thermal Gravimetric Analysis (TG)

What is thermodynamic equilibrium constant K?

It represents the energy changes (exothermic or endothermic) taking place when the reactants move towards products.

In the equilibrium expression, we assume that solids have an activity of 1.

Can relate the K to Gibbs free energy of a reaction.

What does delta G^0 represent?

It tells us if the reaction in standard condition is spontaneous (releases energy) or not (takes in energy). It does not however tell us how fast the reaction is or the kinetics of it.

What are the formulas for delta G^0?

Delta G^0 = -RTlnK =

Delta H^0 - (T * Delta S^0)

What makes a reaction spontaneous and fast?

The EA for forward reaction is less than EA for backward reaction.

What is IAP?

Ion activity product, or the product of ions in solution.

Can related it to K equilibrium constant to predict whether the system is undersaturated and supersaturated.

What are the mechanisms of sorption?

Ion exchange, chemisorption, co-precipitation, precipitation, and absorption.

As you go down the list the concentration of ions increases as well as the strength of retention.

What is ion exchange/adsorption?

Retention allows for the reactive species to retain its water of hydration making it a reversible.

How do you predict with reactive species is more preferentially going to be on the soil surface during ion exchange?

Looks at hydrated charge density = z/radius of hydrated ion

The trend is Al > Ca > Mg > K = H > Na (look at the charge alone)

How do you predict the activity of exchangeable ions that are adsorbed to the soil surface?

Instead of Keq (ion in solution) we look at Ks (selectivity)

What are the two types of selectivity (Ks) that help model ion exchange?

Kv = Vanselow

The quantity of adsorbed ions is almost equivalent to the mole fraction of adsorbed ions (M)

Kgt = Gaines-Thomas

The equilibrium of the expression or the selectivity for ion exchange is equal to the equivalent fraction of the adsorbed/exchangeable ion times the ion in solution divided by the same thing but the ions are switched.

How do you interpret Ks?

If Kv > 1 the cation on the product side is favored to be adsorbed to the soil.

If Kgt = 1, there is no selectivity

How do you predict the likelihood of chemisorption?

1) CN# of central cation

2)Types of surface (OH-; terminal or bridging)

3) Electronegativity of central cation (low X is favorable for chemisorption)

What parameters do you need to predict the tendency for cations to chemisorb?

1) Need high electronegativity to be strongly retained on soil surface. Typically transition metals.

2) Higher tendency of a cation to hydrolyze means they are more likely to chemisorb

2) Higher tendency for cation to precipitate as metal hydroxides (low Ksp value) mean more likely to chemisorb.

How do you know what the charge of the soil surface is?

If pH < PZC, the surface is positively charged

Can oxyanions chemisorb to SOM?

Not favorable electrostatic environment since OM is mostly negatively charged

What is precipitation/homogeneous nucleation?

At the beginning, IAP/Ksp > 100 (Lots of products)

Solids form out of solution, there are no catalyst that facilitates precipitation, but the concentration of ions in solution is a big contributing factor. Once the solid is formed, it is now the catalyst that will continue to grow and eventually IAP = Ksp

What is absorption?

Uptake of nutrients by plants

CEC increases from ECEC to UN-FAO CEC to USDA CEC, why?

The pH of the soil is less than PZC so it creates higher negative conditions on the soil surface.

Why is there permanent charge on 2:1 phyllosilicates?

Inherent charge due to isomorphous substitution

Ex: Vermiculite has IS in tetrahedral layer (Al/Si)

1:1 phyllosilicates like kaolinite, oxides, and SOM has variable charge and a PZC

What are the 2 types of charge systems?

1) Constant surface charge and Variable surface potential (Colloids with IS)

2) Variable surface charge and Constant surface potential (pH dependent charge)

What is surface potential?

Measured in J/C and it is a work term. Work/energy to move a point charge from one location to another. Decreases as you move from surface to bulk solution.

Why does the variable surface potential decrease with high salt concentrations?

The soil surface is going to be neutralized by the high amounts of cations which decreases the surface potential and decreases the diffuse layer thickness