mineral weathering and secondary mineral formation weathering: chemical alteration of minerals (in...
Post on 22-Dec-2015
221 views
TRANSCRIPT
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
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 20 40 60 80 100
Li
Na
K
RbCs
Be
MgCa
SrBa
F
Cl
BrI
CePrNd
SmEuGdTbDg
HoEm
TmYbLu
TiZr
Hf
C
N
S
SiPAl
Fe
ThU
log (soil
Zr
/crust
Zr)
atomic number
Alkali metals and alkaline earths
Ti group
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.
MineralClassification
Tetrahedral SheetArrangement
Example Chemical Formula of SpecificMinerals
Si/Al+Fe CEC(meq(+)/100gmineral
Phyllosilicates 2 (tetra):1(octa) smectitegroup
(montmorillinite)Mx(Al3.2Fe0.2Mg0.6)(Si8)O20(OH)4 (1)
2 110 (range 47-162) (5)
2 (tetra):1(octa) vermiculitegroup
(trioctahedral vermiculite)Mx(MgFe)6(Si8-xAlx)O20(OH)4(2)
2 150 (range 144-207) (2)
1 (tetra):1(octa) kaolingroup
(kaolinite)(Al4)(Si4)O10(OH)8
1 1 (range 0-1)(6)
Tectosilicates NA silica group (opal) SiO2 •nH2O) (3) infinity 0Oxides NA iron oxides (geothite) FeOOH 0 ~0 (pH
dependent) (4)(hematite) Fe2O3 0 ~0 (pH
dependent)
(ferrihydrite) Fe5(O4H3)3 (4) 0 ~0 (pHdependent)
aluminumoxides
(gibbsite) Al(OH)3 0 ~0 (pHdependent)
Carbonates NA (calcite) CaCO3 NA ~0 (8)Organic Matter NA NA NA NA 100-900 (pH
dependent) (9)(1) from G. Sposito, The Chemistry of Soils, Oxford University Press, New York (1989).
(2) from L.A. Douglas, Vermiculites. In: J.B. Dixon and S.B. Weed, Minerals in Soil Environments, 2nd Ed., Soil ScienceSociety of America, Madison, WI (1989).
(3) amorphous or paracrystalline
(4) from U. Schwertman and R.M. Taylor, 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. Lynn, 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 Organic 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 smectite kaolinite gibbsitesilicon silicon silicon
opal iron Si(OH)4
INOSILICATES Fe oxides calcium
calcitecalcium
- KPHYLLOSILICATES biotite trioctahedral trioctahedral
illite vermiculite-K
muscovite dioctaheral dioctahedralillite vermiculite
TECTOSILICATES plagioclase
feldspars
quartzSi(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 semi-arid 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)