lecture 6 mineral

Mineral Weathering and Secondary Mineral Formation weathering: chemical alteration of minerals in soils, involves water, gases, acids, etc.. Parent material soil Desilication via weather

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

or group of columns) AND the element’s ionic potential

Z/R = ionic potential z=charge, r=radius

Classes:

oxides/hydroxides form (Al, Fe)

Ionic potential of important elements

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

Li

Na

K

Rb

Cs

Be

Mg

Ca

Sr

Ba

F

Cl

Br

I

Ce Pr

Nd

SmEuGdTb Dg

HoEm

Tm

YbLu Ti

Zr

Hf

C N

S

Si P Al

Fe

Th U

log (soil

Zr

/crust

Zr

)

atomic number

A lkali metals and alkali ne 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.

Classification

Tetrahedral Sheet Arrangement

Example Chemical Formula of Specific

Minerals

Si/Al+Fe CEC

(meq(+)/100g mineral

Phyllosilicates 2 (tetra):1(octa) smectite

group

(montmorillinite)

Mx(Al3.2Fe0.2Mg0.6)(Si8)O20(OH)4 (1)

47-162) (5)

group

(trioctahedral vermiculite)

Mx(MgFe)6(Si8-xAlx)O20(OH)4(2)

144-207) (2)

group

(kaolinite) (Al4)(Si4)O10(OH)8

(6)

Tectosilicates NA silica group (opal) SiO2 •nH2O) (3) infinity 0

Oxides NA iron oxides (geothite) FeOOH 0 ~0 (pH

dependent) (4)

dependent)

dependent) aluminum

oxides

dependent)

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, 2 nd Ed., Soil Science Society 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

opal

calcium

calcite

calcium

- K

PHYLLOSILICATES biotite trioctahedral trioctahedral

-K

muscovite dioctaheral dioctahedral

TECTOSILICATES plagioclase

feldspars

quartz

Si(OH) 4

INCREASING DEGREE OF DESILICATION

1:1 phyllosilicates: kaolinite

•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)

(and property of cation exchange

capacity)

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)

weathering (desilication)

Non-silicate secondary minerals: oxides

Fe oxides

2 Hematite

Non-silicate secondary minerals: carbonates

Calcite

weathering source

environments when soil

solution becomes saturated

to MAP

related to MAP

Geographical distribution related to climate

•Greater than

100cm/yr removes

carbonate

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