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 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
N

1.5

Ti group

C

I
Br

)

1

Alkali metals and
alkaline earths

Zr

/crust0.5
Zr

S
Zr

0

Ti
Li

Fe

Si
Al P Cl

log (soil

-0.5

Rb
Sr

Tm

Dg Yb
Nd
Lu
Eu
Tb
Cs Pr Sm
Gd
Hf
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(octa)

smectite
group
vermiculite
group
kaolin
group
silica group
iron oxides

(montmorillinite)
Mx(Al3.2Fe0.2Mg0.6)(Si8)O20(OH)4 (1)
(trioctahedral vermiculite)
Mx(MgFe)6(Si8-xAlx)O20(OH)4(2)
(kaolinite)
(Al4)(Si4)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) CaCO3
NA

NA
NA

2 (tetra):1(octa)
1 (tetra):1(octa)
Tectosilicates
Oxides

NA
NA

aluminum
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 York (1989).
(2) from L.A. Douglas, 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. 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
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 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