Mineral Weathering and Secondary Mineral
Formation
weathering: chemical alteration of minerals (in soils, involves water,
gases, acids, etc).
Parent material
soil
Desilication via weathering
Parent Material=primary silicates formed from
igneous/metamorphic processes
Soil= secondary silicates, oxides, carbonates, etc.formed
from weathering processes
Behavior of Elements During Chemical Weathering
•Soils are depleted in elements relative to parent material
•Element loss/depletion is determined by elements position on periodic table
(which column or group of columns) AND the element’s ionic potential
Z/R = ionic potential z=charge, r=radius
Classes:
Z/R= 0-3
ion surrounded by H2O shell, soluble in H2O (Na, Ca, etc)
Z/R=3-~9.5
ion so strongly attracts H2O that insoluble
oxides/hydroxides form (Al, Fe)
Z/R=>~9.5
soluble oxyanions form (S, C, etc.)
Ionic potential of important elements
•Red arrow indicates
decreasing attaction
to H2O within a
group of elements
•Decreasing
attraction is
reflected in
weathering
losses…..
Element loss varies with ionic potential
2
Ti group
N
1.5
C
I
log (soil Zr /c rust Zr )
Br
Alkali metals and
alkaline earths
1
0.5
S
Zr
0
Ti
Li
Si
A lP Cl
-0.5
Fe
Rb
Sr
Tm
Nd Dg YLu
b
uT b
Cs P r SEGd
Hf
m
Ba
Em
Ce
Ho
Th
U
KCa
Mg
F
-1
Na
Be
-1.5
0
20
40
60
atomic number
80
100
Mineral Particle Size and Mineralogy
Gravel > 2mm (primary)
Sand >= 0.05 to 2.0 (primary)
Silt <0.05 to 0.002 (primary + secondary)
Clay < 0.002 (secondary)
Most secondary mineral are silicates, and most
secondary silicates are phyllosilicates.
Mineral
Classification
Tetrahedral Sheet
Arrangement
Example
Chemical Formula of Specific
Minerals
Si/Al+Fe
Phyllosilicates
2 (tetra):1(oc ta)
sme ctite
group
vermi culite
group
kaolin
group
silica group
iron oxides
(montmorilli nite)
Mx(Al3.2Fe0.2Mg0.6)(Si 8)O20(OH)4 (1)
(trioctahedral vermiculite)
Mx(MgFe)6(Si8-xAlx)O20(OH)4(2)
(kaolinite)
(Al4)(Si 4)O10(OH)8
(opal) SiO2 •nH2O) (3)
(geothite) FeOOH
2
(hematite) Fe2O3
0
(ferrihydrite) Fe5(O4H3)3 (4)
0
(gibbsite) Al(OH)3
0
(calcite) CaCO 3
NA
NA
NA
2 (tetra):1(oc ta)
1 (tetra):1(oc ta)
Tectosilicates
Oxides
NA
NA
alumi num
oxides
Carbonates
Organic Matter
NA
NA
NA
2
1
infinity
0
CEC
(meq(+)/100g
mineral
110 (range 47162) (5)
150 (range 144207) (2)
1 (range 0-1)
(6)
0
~0 (pH
dependent) (4)
~0 (pH
dependent)
~0 (pH
dependent)
~0 (pH
dependent)
~0 (8)
100-900 (pH
dependent) (9)
(1) from G. Sposito, The Chemistry of Soils, Oxford University Press, New Yo rk (1989).
(2) from L.A. Do uglas, Vermiculites. In: J.B. Dixon and S.B. Weed, Minerals in Soil Environments, 2nd Ed., Soil Science
Society of America, Madison, WI (1989).
(3) amorphous or paracrystalline
(4) from U. Schwertman and R.M. Ta ylor, Iron Oxides, Chap. 8 In: J.B. Dixon and S.B. Weed (op. cit. 2).
(5) from G. Borchardt, Smectites, Chap. 14 In: J.B. Dixon and S.B. Weed (op. cit. 2).
(6) from J.B. Dixon, Kaolin and Serpentine Group Minerals, Chap. 10 In: J.B. Dixon and S.B. Weed (op. cit. 2).
(7) from P.H. Hsu, Aluminum Oxides and Hydroxides, Chap. 7 In: J.B. Dixon and S.B. Weed (op. cit. 2).
(8) from H.E. Doner and W.C. Ly nn, Carbonate, Halide, Sulfate, and Sulfide Minerals, Chap. 6, In: J.B. Dixon and S.B.
Weed (op. cit. 2).
(9) from J.M. Oades, An Introduction to Orga nic Matter in Soils, Chap. 3 In: J.B. Dixon and S.B. Weed (op. cit. 2).
Observed Silicate Mineral Weathering Pathways in Soils
PRIMARY SILICATES
SECONDARY MINERALS
NESOSILICATES
silicon
smectite
silicon
opal
kaolinite
iron
gibbsite
silicon
Si(OH)4
INOSILICATES
Fe oxides
calcium
calcite
calcium
PHYLLOSILICATES
biotite
muscovite
TECTOSILICATES
-K
trioctahedral
illite
-K
dioctaheral
illite
trioctahedral
vermiculite
dioctahedral
vermiculite
plagioclase
feldspars
quartz
Si(OH)4
INCREASING DEGREE OF DESILICATION
1:1 phyllosilicates: kaolinite
•One layer of Si tetrahedra
•One layer of Al octahedra
•Individual minerals are held to
another via H bonds
2:1 Phyllosilicates: di and trioctahedral
Dioctahedral (smectites)
•Substitution of +2 for +3 in
octahedral layer (called
isomorphous substitution)
•Creates a net negative
charge (and property of
cation exchange capacity)
•Results in expandable
layers
Trioctahedral (vermiculite)
•Substitution of +3 for +4 in
tetrahedral layer
•Also has CEC, but little or
no expansion
Other secondary mineral groups: oxides
Al oxides (gibbsite)
•Results of vigorous
chemical weathering
(desilication)
Non-silicate secondary minerals: oxides
Fe oxides
1. Geothite
• Yellowish brown
• Acidic, OM-rich envir.
2. Hematite
• Bright red
• Warm, dry environments
Non-silicate secondary minerals: carbonates
Calcite
•Ca is released from
some weathering source
•Forms in arid to semiarid environments when
soil solution becomes
saturated
•Presence in upper 1m
related to MAP
•Depth of carbonate
layer related to MAP
Geographical distribution related to climate
•Greater than
100cm/yr
removes
carbonate
•Below 100cm,
depth~MAP
Non-silicates: sulfates (gypsum)
•Presence of sulfates
in soils usually
occurs in hyperarid
climates (or sites
with high water table
and evaporative
enrichment of salts)
Secondary Minerals in California Soils: Sierra
Nevada
Soil Mineralogy vs. Elevation