interaction soil

8/10/2019 interaction soil

1/27

1

Degradation of Soil

Minerals

y Organic

cids

KimH. Tan

2

The decomposition of soil minerals by humic acids

HAs)

has attracted

considerable attention since the early history of soil science. Long before

Dokuchaiev formulated his pedological concept, soil organic acids, in-

cluding HAs, were expected to play an important role in the dissolution of

rocks and minerals (Sprengel, 1826). Since then, conflicting arguments

were reported

as

to the effectiveness of these acids in rock and mineral

weathering. Mainly due to lack of supporting experimental evidence, a

large number of scientists questioned the importance of HAs as a dissolu-

tion agent (Clarke,

1911;

Fetzer,

1946;

Krauskopf,

1967;

Loughnan,

1969).

However,

an

equally large number of authors can also be found in

the literature defending the role of humic acids

as

a weathering agent

(Graham,

1941;

Van der Marel,

1949;

Kononova et aI.,

1964).

With the increased knowledge in HA chemistry, evidence

is

accumu-

lating suggesting

that

humic compounds

playa

significant role in mineral

dissolution. Today s data indicate

that

the acidity and chelating capacity

of these organic acids bring about the degradation of many rocks and

minerals (Singer Navrot,

1976;

Schalscha et aI.,

1967;

Baker,

1973;

Schnitzer Kodama,

1976; Tan 1980).

The subsequent release of metal

cations in the form of complexes or chelates has an important bearing in

soil formation and nutrient supply to

plant

roots. Not only will the

mobilization

and

precipitation of the metal chelates result in horizon dif-

ferentiation giving rise to different kinds of soils (De Coninck, 1980;

Birkeland, 1974),

but

depending on stability such chelates are thought to

provide the carrier mechanism by which depleted nutrients at the root

surface can be replenished (Lindsay,

1974).

1-1 SOIL MINERALS

The inorganic fraction of soils, subject to degradation processes, is

composed of rock fragments

and

minerals of varying size and composi-

tion. On the basis of size, the following major fractions are generally

I Contribution of the Dep. of Agronomy, Univ. of Georgia, Athens, GA.

2

Professor of agronomy, Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602.

Copyright 1986 Soil Science Society of America,

677

S. Segoe Rd., Madison,

WI

53711,

USA. Interactions o Soil Minerals with Natural Organics and Microbes SSSA Spec. Pub.

no. 17.

1

Published 1986


2/27

2

T N

Table 1-1. The six categories of soil silicates on the basis of the arrangement

of the SiO. tetrahedra in their structure.

Soil silicate

Nesosilicates

Sorosilicates

Cyclosilicates

Inosilicates

Phyllosilicates

Tectosilicates

Structural

arrangement of SiO.

Separate Si04

tetrahedra

Two or more linked

tetrahedra

Closed or double

rings of SiO.

tetrahedra

Single or double

chains of SiO.

tetrahedra

Sheets of SiO.

tetrahedra

Framework of SiO.

tetrahedra

Mineral species examples

Phenacite, olivine, garnet, zircon, andalusite,

sillimanite, kyanite, topaz, chloritoid, nd

sphene

Epidote group

Beryl, cordierite, tourmaline, and axinite

Pyroxenes, pyroxenoids, and amphiboles

Serpentine, mica, kaolinite, smectite, illite,

vermiculite,

nd

chlorite

Quartz, chalcedony, tridymite, crystobalite,

opal, alkali and plagioclase feldspars,

feldspathoids, scapolite, and zeolites

recognized: gravel > 2.0 mm); sand 2.0 to 0.050 mm); silt 0.050 to

0.002 mm); nd clay 0.002 mm). Despite the variability in composi

tion, these fractions are mostly silicates and oxides. The soil silicates are

classified into

six

categories on the basis of the silica Si0

4

)

tetrahedra

linkages in their structure Table 1-1). The sand and silt fractions are

mostly neso-, soro-, cyclo-, ino-, or tectosilicates, whereas the silicate

clays belong mainly to the phyllosilicates. Phyllosilicates also occur in

sand

nd

silt fractions, whereas feldspars belonging to the tectosilicates

are frequently found in the clay fraction. Frequently, the terms

second ry

nd prim ry miner ls

are used to distinguish the clays from the other

minerals. Although a number of pedologists may raise some objections to

the use of these terms, for practical purpose and convenience, this article

will apply the term primary minerals to minerals which persist in the soil

chemically unchanged from the p rent rocks, nd the term secondary

minerals to minerals which have been formed by the weathering of

primary minerals.

1-1.1 Weathering Sequences of Soil Minerals

The breakdown nd stability of soil minerals are quite complex, nd

can be studied in several ways. One method to study these minerals

is

to use

weathering sequences and indexes, which appear to

e

popular in the past. A

weathering sequence

is

defined as a ranking of minerals in increasing or

decreasing) order of resistance to weathering. Weathering indexes are

ex-

pressed in terms of molar ratios of elements released from the minerals, or

in terms of weathering stages, or weathering means Jackson Sherman,

1953). A large number of weathering sequences are present in the litera

ture. Since it

is

not within the scope of this chapter to discuss weathering


3/27

DEGRADATION OF SOIL MINERALS

3

Table 1-2. Relative stability of primary minerals on the basis of hardness

(Hunt, 1972; Tan, 1981 .

Mineral

Talc

Gypsum

Calcite

Fluorite

Apatite

Orthoclase

Quartz

Topaz

Corundum

Diamond

Scale of hardness (ease of scratching, Mohs scale

Easy to scratch with fingernail (very soft, 1

Just scratches with fingernail (soft, 2

Scratched by copper coin, not by fingernail (slightly hard, 3

Easy to scratch by glass, not by Cu coin (moderately hard 4

Just scratched by glass (hard, 5

Mineral scratches glass easily (hard 6

Difficult to scratch by glass, mineral scratches glass very

easily (very hard 7

Difficult to scratch by glass (very hard, 8

Difficult to scratch by glass; mineral cuts glass

(very hard 9

Very difficult

to

scratch by glass; mineral cuts glass

(extremely hard, 10

Table 1-3. Weathering sequence of primary minerals according to sequence

of crystallization (Goldich, 1938,.

Dark

Olivine

Augite

Hornblende

Biotite

Minerals

Light

Anorthite

Labradorite

Andesine,

Oligoclase

Albite

Orthoclase,

Microcline

Muscovite

Quartz

Sequence of

crystallization

Early

Late

Resistance

to

weathering

Least resistant

Most resistant

processes in general, only a few examples, which may have some bearing

on the topic of degradation of minerals by humic acids, will be discussed

below for illustrations. One example of a weathering sequence is the

listing of minerals in order of increasing hardness from 1 to 10, known

as

the Mohs scale (Table

1-2 .

The degree of hardness of most soil minerals

ranges only from 1 to 7, since minerals with a

hardness>

7 (e.g., topaz,

corundum, and diamond) are relatively uncommon in soils. Quartz

(hardness

= 7

is generally considered the hardest mineral in soils, and

because of this, it is the soil mineral most resistant to weathering. The

question arises as

to how far this concept can be applied to weathering.

Micas have a hardness of about 2 to 3, yet they are relatively resistant to

weathering.

Another example is the listing of minerals according to the sequence

of crystallization (Table 1-3). The

data

in Table

1 3

indicate

that

the

least stable minerals to weathering, represented by olivine and anorthite,

were formed first, and have less silica than the more resistant ones.

Ac-

cording to Goldich (1938), the Si/Al ratio of both olivine and anorthite is

1:1. The molar SiiAl ratio ofanorthite is > 2.0. In biotite and orthoclase,

the more resistant minerals on the list, this ratio increases to 3:1. This


4/27

4

Weathering Sequence

Ferromagnesians

\

Olivine

\

Pyroxene

\

Amphibole

\

Biotite

\

Muscovite

Quartz

Feldspars

/

Anorthite

Ca- feldspar

Albite

Na- f e l d s p ~

/

Orthoclase

K- feldspar)

/

TAN

Fig. 1-1.

Weathering

sequence of primary minerals adapted from Goldich (1938). The direc-

tion of

the arrow

points to increasing stability.

weathering sequence

is

perhaps the most known and quoted in many

books in a slightly different version

as

follows (Fig. 1-1).

As

indicated

earlier, many other weathering sequences have been formulated. For the

reader interested in this topic, reference

is

made to Jackson and Sherman

(1953) and Jenny (1941).

1 1.2

Weathering Indexes of Soil Minerals

Weathering indexes are defined by Jackson

and

Sherman (1953) in

terms of numbers expressing the degree or rate of weathering.

As

stated

before, several types of weathering indexes have been devised, e. g., molar

ratios, weathering stages,

and

weathering means. They are

less

relevant

than weathering sequences in the study of mineraI'degradation by

HAs

and will be mentioned briefly for completeness only.

The most used weathering index

is

perhaps the molar ratio, which is

the ratio of the molar concentrations of elements in the mineral or the

ratio of molar concentrations of elements released during mineral

weathering. Examples of such ratios are

Si0

2

/sesquioxide ratios,

Si0

2/

Al

2

0

3

ratios,

Si0

2

/ferric oxide ratios, bases/

Al

2

0

3

ratios, alkali

Al

2

0

3

ratios, alkaline earth/Al

2

0

3

ratios, leaching ratios, etc. For more details,

reference

is

made to Jenny (1941).

The weathering stage,

as

defined by Jackson

and

Sherman (1953),

is

the concentration of specific minerals associated with a given degree of

weathering. These authors indicated that one or two minerals would

dominate in any soil horizon.

The weathering mean

is

calculated using the formula

as

follows:

m

=

(ps)/ p

where m = weathering mean, p = percentage of a mineral in soil, and s

= weathering stage. The summation ( )

is

the addition of the various

p

x

s

values of all the minerals found in a given soil.


5/27

DEGRADATION OF SOIL MINERALS

5

Table 1-4. Selected physical and structural properties of minerals as related

to

bonding type (Evans, 1939; Tan, 1982).

Mineral

property

Mechanical

Thermal

Electrical

Optical

Structural

Ionic

Strong, hard

High melting

point

Low thermal

expansion

Nonconducting

Variable

High

coordination

Moderately

high density

Type of bonds

Homopolar Metallic van der Waals

Strong, hard Variable Weak, soft

High melting Variable Low melting

point melting point point

Low thermal High thermal

expansion expansion

Nonconducting Conducting Nonconducting

High refractive Opaque Transparent

index

Low

Very high Very high

coordination coordination coordination

Low density High density

1 1.3

Crystal Chemistry

and

Stability of Soil Minerals

Although a large number of factors account for the stability of soil

minerals, perhaps the factors related to the structure of the mineral are of

greater importance in mineral degradation by humic acids

than

any other

factors discussed earlier. Mineral stability depends to a large degree on

the strength of the atoms or ions binding their neighboring ions in the

crystal lattice. Four major types of bonding forces between atoms in

crystals have been reported (Table 1-4). As indicated in Table 1-4, many

of the mineral properties vary according to bond types. The ionic and

homopolar bonds between atoms yield, in general,

hard

crystals with

high melting points. On the other hand, van der Waals forces give rise to

weak bonds and relatively soft crystals with low melting points.

Most of the bonds in the structure of soil minerals are ionic in nature.

In

the case of soil silicates, single or several units of

Si0

4

tetrahedra can be

linked together by mutually sharing the oxygen atoms, or by linkages

through cations, such as

Ca and

Mg For example, in inosilicates, double

chains of silica tetrahedra can be linked together by

Ca

and Mg (Fig. 1-2)

as

is the case in amphiboles. In tectosilicates, the

Si0

4

and

Al0

4

tetrahedra

are linked together by alkali

and

alkaline earth metals located in the lat

tice interstices.

An

example of the lat ter

is

feldspar. The cations acting

as

the connecting linkage are considered nonframework ions, and form the

weakest spots in the crystal. Whatever the structural linkage is, it is noted

that

a progressive increase in sharing of framework oxygen atoms between

adjacent silica tetrahedra generally yields the minerals more resistant to

weathering.

In

terms of energy relations, the Si-O-Si linkage, called the silox ne

ond

(Sticher Bach, 1966), requires the greatest energy to form, com

paresl to other cation-oxygen bonds (Table 1-5). The

data

in Table 1-5

show

that

Si-O bonds are the strongest bonds, requiring

13

164.9 to


6/27

6

TAN

Table 1-5. Energies of formation of cation-O bonds Paton,1978; Keller, 1954).

Cation

Si

4

+ nesosilicates)

Si

4

+ inosilicates, single chain)

Si

4

+ inosilicates, double chain)

Si

4

+ phyllosilicates)

Si

4

+ tektosilicates)

AP

framework)

AP

nonframework)

Fe

J

Mg2

Ca2

H+ inOH)

Na

K

t

5 tetrahedron t

weak

sp t

t f

strong bonds

Energy of formation

kJ mol-

1

13164.9

13118.9

13102.1

13085.4

13030.9

7868.8

7512.7

3850.6

3821.3

3515.4

2 157.8

1349.2

1252.8

kcalmol-

1

3142

3131

3127

3123

3110

1878

1793

919

912

839

515

322

299

Fig. 1-2. Schematic linkages of silica tetrahedra. Top: Linkage of two silica tetrahedra by Ca

ion. Bottom: Linkage of several tetrahedra by mutually sharing oxygen atoms. The Si-O-Si

bond, called the siloxane bond is a very strong bond.

13030.9 kJ mol-I for their formation. Aluminum-oxygen bonds are the

next strongest 7868.8 to 7512.7

kJ

mol-I needed for formation), whereas

the bonds between nonframework cations and O

2

are the weakest 1252.8

to 3850.6

kJ

mol-lor 299 to 919 kcal mol-I). f the following hypothesis

is

valid,

that

bonds requiring the greatest energy to form will also be the

most resistant to weathering attack, then the data in Table

1-5

indicate

that

nonframework cation-O bonds, such

as Na-O

and K-O will be first

to rupture. Next in line will be the

H-O Ca-O

Mg-O, and Fe-O bonds,

while the most difficult bond to break

is

the siloxane Si-O-Si bond.

On

the basis of a progressive increase of oxygen sharing between adjacent

silica tetrahedra, Keller 1954) ranked the stability of the silicate groups

as follows: nesosilicates < sorosilicates < inosilicates < phyllosilicates

or

Documents

interaction soil