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)

Secondary Minerals in California Soils: Sierra Nevada

Soil Mineralogy vs. Elevation