Section 1: Soil Minerals and Weathering Flashcards
Mineral Components in soil
Primary and secondary minerals
Primary mineral
rock-forming minerals; formed at high temp and pressures; unstable under current atmospheric conditions; weather and release their ionic constituents
Secondary mineral
mineral products of weathering; form under current atmospheric conditions; may be unstable if conditions change but change is very small; typically clay sized <2 micrometers
Rocks that form soils
lithophile elements; felsic rock (O, Si, Al, K, Na)- quartz and feldspar; Mafic rock (Mg, Fe, Ca)- olivine, pyroxene, amphiboles
What composition of soil reflects earth crust composition
young soils
building blocks of soil minerals
tetrahedra and octahedra
soil mineral structures
2:1 minerals - tetrahedra: octahedra: tetrahedra
lattice
an array of points in space represents the periodic nature of a mineral structure
Principles of ionic solid structures: the “hard sphere” model
only applies to ionic solid structure like Si-O or Al-O for example – oxygen sheets filled with cations that are needed for electrical neutrality
Principles of ionic solid structures: Oxyanion (O2-)
makes up 47% (by weight), 94% (by volume) of the earths crust
Principles of ionic solid structures: Aluminosilicates
most common primary and secondary mineral of the lithosphere and soil; made of oxyanion lattice with smaller metallic cations; stuffed in the interstices of a close-packed structure
close-packing of spheres
hexagonal, cubic, etc - cations are in specific places to make it more stable- can extract tetrahedral and octahedral structure from these
close-packed structures has 2n tetrahedral and n octahedral holes - what is n?
n is the number of anions
a polyhedron of anions is formed around…
each cation in the mineral structure
coordination number (CN)
tells you how many anions (ligands) are bonded to the central cation - determined by the radius ratio
Radius ratio
r cation/ r anion; dictates the SIZE of the metal cation that can Stably occupy the holes creates by close-packed anions
tetrahedral holes are
smaller; accommodate Si4+, Al3+, Fe3+
octahedral holes are
larger; accommodate Mg2+, Al3+, Fe3+, Fe2+
Can Ca+, Na+ and K+ fit into tetrahedral and octahedral structures?
no, they are too large
Expected Ion Coordination
*minimum value thus 0.15-0.224 is trigonal, etc
RR - coordination type - CN
- 15 - trigonal-3
- 224 - tetrahedron -4
- 414 - octahedron - 6
- 732 - cube - 8
- 00 dodecahedron - 12
What coordination numbers does Si4+ have? Mg2+ and Fe2+?
Si 4+ always has CN 4
Mg2+ and Fe2+ always has CN 6
Bond Valence (v) definition
charge balanced by each bond; formal charge observed btw the central cation and each coordinating atom (anion or ligand)
Bond valence (v) equation
v= z cation/CN
z= valence CN= coordination number
octahedral or tetrahedral cation occupancy
based on electrical charge neutrality; uses bond valence theory to determine their filling
cations coordinating each anion
anion charge/ v cation
v = bond valence
is there charge neutrality on the edges of structures?
no, because the edge sites have hydroxyls
main class of soil minerals: phyllosilicates
formed by: a sharing of 3 oxygens to form hexagonal rings (Si2O5)2-; Si tetrahedra only share corners - stable
What is the most common form of point sharing for soil minerals?
the most common is edge sharing or 2 point sharing but the most stable is corner or 1 point sharing
Pauling Rules for Mineral Structures #1
a coordinated polyhedron of anions is found about each cation, the cation-anion distance being determined by the radius sum, and the coordination number of the cation by the radius ratio
Pauling Rules for Mineral Structures #2
in a stable coordination structure, the total strength of the valency bonds which reach an anion from all the neighboring cations is equal to the charge of the anion
Electrostatic valency principle (bond valence)
Pauling Rules for Mineral Structures #3
the existence of edges and particular of faces, common to two anion polyhedra in a coordinated structure decreases its stability; this effect is large for cation with high valency and small coordination number and is especially large when the radius ratio approaches the lower limit of stability of the polyhedron
Pauling Rules for Mineral Structures #4
in a crystal containing different cations, those of high valency and small CN tend to not share polyhedral elements with each other *have to optimize spacing
Pauling Rules for Mineral Structures #5
the number of essentially different kinds of constituents (polyhedra forming a structure) in a crystal tends to be small
*not much variability in the type of coordination environment
Common soil mineral structures
2:1 and 1:1 layer silicate structures
2:1 layer silicates
smectite, vermicuites, etc. Illite is the MOST common
crystal planes and faces
miller indices, hkl
nomenclature; different mineral crystal faces will have very different properties; 3 main faces/planes -100 (toward on X), 010 (away on Y), 001 (up on z - this area is huge)
Properties of soil solids: charge
isomorphous substitution and terminal broken bonds
isomorphous substitution
developer a charge within a mineral layer
isomorphous substitution: permanent or structural charge
is pH INdependent; replacement of one ion with another having a different charge but with no change in mineral structure
isomorphous substitution: secondary minerals: charge within a layer implications
substitution results in a charge deficiency which is delocalized; no bonds are altered thus no chemical affinity for ions in solution; mostly in the 001 plane; ions bind through electrostatic forces; source of cation exchange capacity (CEC) sites
Terminal broken bonds
pH DEPENDENT (variable) charge
Terminal broken bonds: dissociation of pH-dependent functional groups
dissociate in response to shift in soil pH; addition or release of protons from the surface result in difference charges
Terminal broken bonds: implications
charge is localized at specific sites; surface sites are chemically reactive (ions retained by chemical bond or electrostatic forces); develop CEC and/or AEC
Terminal broken bonds: where do they develop?
on x and y axis (edges) of layer silicate clays; on OM - this is how it gets its charge
why’s quartz always found?
because it resists weathering due to covalent bonding and lack of charge
as the Si/O molar ratio increases, what else increases?
sharing of O, covalence and resistance to weathering
sheet aluminosilicates (phyllosilicates): Micas
continuous sheets of tetrahedra; each share 3 basal Os; bound by cations in octahedral coordination(2:1); Al3+ subs for Si4+ in tet sheets (isomorphous sub); K+ ions reside in the interlayer cavities or holes bc the balance charge, link mica layers and K+ is no exchangeable; non-expandable
Common micas in soils
muscovite (dioctahedral- 2 aluminum and one out there for neutrality) and biotite (triococtahedral - 3 magnesium to sum to 6 for neutrality)
framework silicates: feldspar
3-D framework(corner sharing tetrahedra); each tetrahedra shares all 4 Os; 1 or 2 in 4 of the Si4+ can be replaces by Al3+ (isomorphous sub); framework charge can be balanced by K+, Ca2+, Na+
framework silicates: Quartz
3-D framework (corner sharing tetrahedra); each tetrahedra shares all 4 Os; no substitution; not close-packed; strong Si-O-Si bonds
secondary minerals: layer silicates or phylosilicates: aluminosilicates: composed of:
Si, Al tetrahedra; Mg, Al, Fe octahedra
secondary minerals: layer silicates or phylosilicates: aluminosilicates: classification based on
number of tetrahedra and octahedra in a layer; octahedral site occupancy (octahedral composition: who and how many cations in octahedral positions)
General Classes of Phyllosilicate Minerals: charged
All of these have charge because of substitution; with a charge of 1, it is mica; with a fractional charge, it is illite, vermiculite, smectite, these are non-expandable clay minerals
Process leading to reduction in layer charge (relative to mica)
cation substitution in the octahedral sheet; exchange of Si4+ for Al3+ in the tetrahedral sheet; Fe2+ oxidation to Fe3+; H+ incorporation into the structure
permanent charge (layer charge per formula unit) - illite, chlorite, vermiculite, smectite, kaolinite, Fe/Al oxides
0.6-0.8 illite non-expanding min. swelling; 0.6-0.8 chlorite non-expanding min. swelling; 0.6-0.9 vermiculite expanding some swelling; 0.25-0.6 smectite max. swelling; 0 kaloninite and Fe/Al oxides non-expanding no swelling
Clay swelling in water depends on
magnitude of layer charge; location of charge in 2:1 layer; exchange cation charge (and hydration)
Na+ clays freely expand because they need more hydration whereas Ca2+ and Mg2+ have higher charge and hydrate more concentrated
smectite swelling
free swelling because of the range of substitution - tetrahedral and octahedral charge
mica swelling
no swelling - mostly tetrahedral charge
vermiculite swelling
limited expansion - mostly tetrahedral charge
Variable-charge soil minerals: secondary minerals: oxides and hydroxides of Fe, Al, Mn
hexagonal or cubic close-packed O2- and/or OH- anions with Fe3+, Al3+, Mn4+, Mn3+ in octahedral sites
Variable-charge soil minerals: secondary minerals: oxides and hydroxides of Fe, Al, Mn: classification
arrangement of octahedral (MO6) unites; share corner, edges, faces
Variable-charge soil minerals: secondary minerals: oxides and hydroxides of Fe, Al, Mn: some characteristics
have charge-unsatisfied structures anions/ligands on all surface planes
have no permanent (structural) charge
have pH-dependent (variable)charge (both + and -): amphoteric minerals
CEC varies but high SA for the amorphous (short-range ordered, SRO) oxides
retain metal cation and inorganic and organic anions strongly (chemical affinity- chemisorption)
young soil minerals
illite, vermiculite, smectite, allophone
old soil minerals
kaolinite, Fe oxides, Gibbsite
variable charge minerals in old soils
ultisols, oxisols
clay low in CEC
kaolinite, hematite, goethite, gibbsite
low native pH
variable charge minerals in young soils
andisols
highly pH-dependent CEC and AEC due to allophonic clays which lack permanent charge
weakly weathered
retain OM very strongly
Surface charge of variable-charge minerals: point of zero charge
(PZC) the pH at which magnitude of negative charge is equal to the magnitude of surface positive charge (CEC=AEC)
ability of the surface to adsorb ions of either charge is at a minimum
flocculation/aggregation is favored
is there a point of zero charge (PZC) for all minerals?
no, minerals that use isomorphous substitution do not have enough edge sites to stabilize it. Since isomorphous substitution is not pH dependent, there will be little to no change to the charge with a change in pH.
for example - smectite vs allophane
Clay mineral identification methods
X-ray diffraction; thermal methods; infrared absorption; electron microscopy; cation exchange
X-ray diffraction (XRD) why?
diffraction from more than one row of atoms; use braggs equation to determine d-spacing btw atomic planes
X-ray diffraction (XRD) how?
non-destructive technique; applicable to crystalline materials; more than 5% in the samples; prep sample by separating the clay fraction and clean it so that there is no OM or oxides on it
infrared absorption spectroscopy - vibrational
two types of bond vibration modes in simple molecules: stretching and bending
identification of functional groups
nature and identity of compounds that are amorphous to XRD
1% in sample for conventional FTIR
soil formation factors
parent material, climate, biota, topography, time
conversion of primary minerals to secondary minerals with the release of plant nutrient elements in soluble forms
primary mineral –weathering– secondary mineral + soluble elements
soil formation from parent material results in fundamental changes at 3 levels
physical (reduction in particle size, increase in mineral surface area)
mineralogical (a slow transition from primary to secondary minerals)
chemical (a long-term result of losses in base cations, silica– hydration, hydrolysis, dissolution, carbonation, redox, complexation)
weathering processes influence…?
the elemental composition, mineralogy, chemical characteristics and morphology of soils
mineral weathering occurs when…
water comes into contact with mineral particles
secondary mineral formation
alteration (weathering process) - part of the parent material structure is inherited by the weather product
neoformation - dissolved Si, Al, Fe, and base cations precipitate at low temp controlled by leaching intensity of local soil environment ; temp and moisture conditions
Main chemical mechanisms of weathering
exchange - displacement of base cations in structure by solution cations, most notably H+ dissociated from water to carbonic acid
hydration/hydrolysis - addition of water to the mineral structure by processes which hydrolyze metal-oxygen bonds
oxidation - electron removal from the mineral by molecular oxygen
why is olivine susceptible to weathering?
because there are no Si-O-Si bonding between tetrahedrons - island silicate- high temp formation - low stability
why is feldspar resistant to weathering?
high Si-O-Si bonding - point sharing, low temp formation, more stable
congruent dissolution
stoichiometric - whole mineral is dissolved
mineral weathering rates depend on?
intrinsic structural stability, mineral specific surface area, temperature, acidity, presence of complexing ligands, efficiency of product removal
What is this equation an example of:
2KAlSi3O8 (feldspar) +2H3O+ +7H2O — Al2Si2O5(OH)4 (kaolinite)+4H4SiO4 (silicic acid) +2K+
chemical weathering: example of hydrolysis
chelation (ligand-promoted dissolution)
facilitates detachment of a central metal ion and enhances its dissolution
what kind of dissolution does mica have and why?
it has incongruent dissolution. K+ decreases as well as particle size and charge and tetrahedral Al3+ while octahedral Al3+ and H2O expansion increases. some biotite is still kept after oxidation/reduction
what controls the release of K+ from mica
particle size because the K+ is so small it will be leached immediately
neoformation
formation of 1:1 layer silicate from a tectosilicate requires dissolution and re-precipitation
are primary and secondary mineral weathering reactions represented by assumed equilibrium expressions?
no because weathering is an irreversible process under the temperature/pressure conditions at the earths surface
idealized weathering series
rock - entisol- inceptisol - mollisol (–vertisol and spodosols) - alfisol - ultisol - oxisol
decreasing Si, increasing Al and Fe
size of particles
clay - <0.002
silt - <0.02<
sand - <2 mm
gravel - huge
Does clay or sand have more surface area?
Clay
dioctahedral Kaolinite
no charge, layers held together by H-bonding, does not swell, low SA and CEC, very widespread in warm and moist climates
smectite 2:1 layer silicate
di or tri- octahedral, isomorphous sub in tet or oct sheet, layer with neg charge is mostly in the oct sheet, free swelling clay (high water holding capacity)
vermiculite 2:1 layer silicate
di or tri octahedral, higher layer charge (than smectite) - localized in tet sheet, exchangeable cations pull layers together strongly, limited expansion (not much swelling)
Illite 2:1 layer silicate
clay-sized hydrous mica, derived from weathering of muscovite (dioctahedral mica), M2+ subs for Al3+ in octahedral layer, less K+ than mica but more structural OH groups, K+ prevents swelling
Chlorite 2:1 layer silicate
di or tri octahedral, charge localized in the tet sheet, metal hydroxide interlayer helps to balance the charge (Al3+ subs for Mg2+), non-swelling, no interlayer cation
hydroxy interlayers
when metals precipitate within the interlayers of 2:1 minerals with permanent charge
decrease CEC, increase AEC
occurs in vermiculites mostly but also smectites
