Soils: chemical transformations during weathering and
soil formation
Stephen U. Aja
Weathering
Weathering is the process of breakdown of rocks at the surface of the earth and reflects
an adjustment of rocks to conditions of temperature, humidity and fluid composition on
the earth’s surface. Weathering processes may be classified into two main types, namely
physical (or mechanical) and chemical weathering. During physical weathering, rocks
break in response to stresses that have been established within the rock. The breaking
may occur along fracture planes in the rock or along mineral grain boundaries. Physical
weathering does not produce any change in the chemical composition of the rock rather
the rock becomes broken up into fragments smaller than the initial volume of rock.
Chemical weathering, on the other hand, are chemical changes undergone by rocks
exposed near the earth’s surface. Rocks typically form in pressure and temperature
conditions far removed from those at or near the earth’s surface. Hence, when such rocks
become exposed to the water compositions, temperature and pressure conditions on the
earth’s surface, the rocks will adjust themselves to the prevailing surficial geochemical
conditions. This adjustment produces significant chemical changes in the composition of
the rock such that the major element chemistry of the weathered material is distinct from
that of the parent rock. These chemical changes are manifested by depletion of the
original minerals in the rock, formation of clay minerals, changes in the chemistry of
water draining the rock and also changes in the color of iron-bearing minerals.
The agents of chemical weathering include water, carbon dioxide, oxygen and organic
acids; organic acids are derived from the alteration of plant litter. Water provides the
medium for dissolution of minerals and the breakdown of complex minerals such as
feldspars by reaction with water is known as hydrolysis reactions. The effect of
hydrolysis reactions is to make an aqueous solution more basic. Carbon dioxide makes
rainwater moderately acidic; this increased acidity leads to increased dissolution of
minerals during weathering. Oxygen is an important agent for the weathering of minerals
containing iron. In iron-bearing rock-forming minerals(e.g., olivine, pyroxene), iron
occurs in a reduced state whereas in weathering or near-surface environments, iron
occurs in the oxidized state as in minerals such as hematite (Fe2O3) and limonite (Lab 1).
Organic acids, released by decomposing plants, increase the rate of rock weathering by
increasing the rate at which metals such as Fe or Al are removed during weathering.
1
Chemical weathering reactions of iron minerals
As noted previously, pyroxene is an iron-bearing mineral found in igneous rocks such as
basalt. Equation 1 represents a weathering reaction of the pyroxene to form hematite.
4 iron pyroxene + oxygen + 8 water
→ 2 hematite + 4 dissolved silica
(1)
4 FeSiO3 ( s )  O2 ( g )  8 H 2O( l )  2 Fe2O3 ( s )  4 H 4 SiO4 ( aq)
In the various equations employed in this exercise, the physical states of the different
species in the reactions are given by the following subscripts s, l, aq and g; these symbols
indicate that the species are in solid, liquid, aqueous and gaseous states, respectively.
Also in equation 1, stoichiometric coefficients are the numbers that appear before each
reacting species such as the 4 before iron pyroxene. The stoichiometric coefficient is a
measure of the reacting units and these reacting units represent the number of moles of
each species involved in the reaction.
Problem 1. In the chart below, identify the reactants and products in equation 1, their
physical states, and number of moles (or stoichiometric coefficient) of each species.
Identification of species in equation 1
Reactant
Physical Stoichiometric Product
Species
State
coefficient
Species
Physical
State
Stoichiometric
coefficient
Problem 2. A homogenous reaction is one in which all the reacting species are in the
same state (gas or liquid or solid) whereas a heterogeneous reaction has reacting species
in mixed states. Is reaction 1 a heterogeneous or homogenous reaction?
During weathering, a mineral may be dissolved completely or may leave a residue. If the
dissolution is complete, as when a mineral such as halite (table salt) is added to water, the
dissolution is known as congruent. But if the dissolution is partial and thus leaves a
residue (or precipitate), the dissolution is known as an incongruent dissolution.
Problem 3. Does reaction 1 represent a congruent or an incongruent dissolution? Refer to
the definition of congruent and incongruent dissolution in the discussion above.
2
Chemical Budget
Problem 4. According to reaction 1, four (4) moles of pyroxene weathered to two (2)
moles of hematite. How many moles of hematite will the weathering of half a mole (0.5
moles) of pyroxene produce?
Oxidation states of Fe
Problem 5. During the weathering of iron minerals, the Fe undergoes a change in
oxidation state (or the charge carried by the iron). In equation 1, the weathering of iron
pyroxene (FeSiO3) produces hematite (Fe2O3). [The net charge on minerals is zero; that
is the number of positive charges must equal the number of negative charges. In minerals,
the charge of Si is usually +4 and that of O is usually -2].
Calculation of the charge of Fe in pyroxene and hematite
Hints
Pyroxene
(a)
Inspect mineral formula
No of oxygen atoms in
given above
mineral
(b)
Multiply number of
Total negative charge
oxygen atoms by –2
from oxygen
(c)
Inspect mineral formula
No of silicon atoms in
given above
mineral
(d)
Multiply number of
Total positive charge
silicon atoms by +4
from silicon
(e)
Find the sum of the
Unsatisfied negative
positive and negative
charge
charges
(f)
Must be equal to the
Positive charges needed
unsatisfied negative
to balance negative
charges
charges
(g)
Inspect the formula of the
Number of Fe atoms in
mineral
mineral
Divide “f” by “g”
Oxidation state of Fe
Hematite
From your calculation in the chart above, does the charge of Fe (oxidation state of Fe)
increase or decrease during the weathering reaction shown in equation 1?
______________________________________________________________
Circle the correct answer: the charge of Fe in primary minerals increases or decreases
with weathering.
3
Weathering reactions of feldspar
Plagioclase feldspar is one of the feldspars examined in Lab 1 (on minerals); the other
being orthoclase feldspars. The weathering of feldspars may be described as a hydrolysis
reaction where hydrolysis is the reaction of silicate minerals with water. Some aspects of
these hydrolysis reactions are shown in reactions 2, 3 and 4 (below).
Plagioclase feldspar + 4 water + 4 hydrogen ion
→ sodium ion + aluminum ion + 3 silicic acid
NaAlSi3O8( s )  4 H 2O( l )  4 H (aq)
(2)
3
 Na(aq)  Alaq
 3H 4 SiO4 ( aq)
Problem 6. Does equation 2 represent a congruent or incongruent dissolution?

Problem 7. Based on equation 2, do you expect the hydrogen ion concentration ( H aq
) in
water reacted with plagioclase to increase or decrease? Explain your answer.
Meaning of pH
pH is a measure of the acidity/basicity of a solution and acidity is determined by the
concentration of hydrogen ions in the solution. The acidity of a solution is referenced to
the pH scale which varies from 0 to 14; acidic and basic (alkaline) solutions have pH
ranges of 0 to 7 and 7 to 14, respectively. pH is defined as the negative logarithm of the
hydrogen ion concentration or pH   log 10 ( H  ) ; therefore, (H+) = 10-pH.
Problem 8. The typical range of pH for minerals soils in humid and arid regions are 5 to
7.5 and 6.5 to 9, respectively. Given two soil solutions whose pH values are 7.5 and 8.5,
by how much does the hydrogen ion concentrations of the two soils differ differ?
4
Problem 9. Do you expect the pH of the solution reacted with feldspars (such as equation
2) to increase or decrease with time (or reaction progress)? Why?
Plagioclase + 4.5 water + hydrogen ion
→ 0.5 kaolinite + 2 silicic acid + sodium ion
(3)
NaAlSi3O8 ( s )  4.5 H 2O( l )  H (aq)  0.5 Al2 Si2O5 (OH ) 4( s )  2 H 4 SiO4  Na(aq)
Problem 10. Which of the above reactions, 2 or 3, would you expect to occur in a more
acidic environment such as a region affected by acid rain? Explain your answer.
Plagioclase feldspar weathering in the presence of CO2
Normally, rainwater dissolves some carbon dioxide as it passes through the atmospheric
column. If the water reacting with the plagioclase feldspar is saturated with carbon
dioxide from the atmosphere, the reaction may then be written as:
Plagioclase + 4.5 water + carbonic acid
→ 0.5 kaolinite + sodium ion + bicarbonate ion + 2 silicic acid (4)

NaAlSi3O8( s )  4.5H 2O( l )  H 2CO3( aq)  0.5 Al2 Si2O5 (OH ) 4( s )  Naaq
 HCO3( aq)  2H 4 SiO4( aq)
Problem 11. Are there any differences in either the type or amounts of the solid products
formed in reactions 3 and 4? What are the differences, if any?
Problem 12. Which of the two weathering models, reaction 3 or 4, will occur at a slower
rate? Why? [Hint: assume that the active ingredient driving the reaction is the
concentration of hydrogen ions.]
5
Problem 13. What determines the amount of solid products formed at the end of the
weathering event, the rate of reaction (how quickly the reaction proceeds) or the amount
of fresh rocks weathered?
Problem 14. Considering your answers to questions 10 and 11, under which weathering
condition, reaction 3 or 4, is the formation of a well-developed soil from the parent rock
likely to require the least amount of time? Explain your answer.
Soil Formation processes and factors
Soil, the thin veneer on the earth’s land surface, consists of mineral matter (45%), organic
matter (5%), pore spaces filled with air (25%) and water (25%). Soils are the residual
products of weathering; the intensity of weathering processes decreases from the exposed
surface of the bedrock downwards. This means that the part of the bedrock exposed to
weather (i.e., rain,
snow, ice, heat) will
undergo the most
alteration.
Soil thicknesses may
vary from 0.3 to 2m
or more. Because
alteration proceeds
from the exposed
surface downwards,
all soils are vertically
zoned beginning with
a humus-rich layer
(or layer rich in
altered plant matter)
at the surface to the
least altered mineral
layer just above the
unweathered parent
rock.
Figure 1: Idealized soil profile
A well-developed soil is one whose constituent horizons (or soil profile) are clearly
resolved (see Figure 1). The major processes responsible for the development of soil
profiles include additions (atmospheric precipitations and organic matter as plant tissue),
6
transformations (of minerals and organic matter), translocations (of clays, humus,
aqueous ions from one level in the soil to the other) and leaching or removals (of soluble
ions from weathered layers). The factors that influence these processes of soil formation
include the type of parent rock (bedrock), the local topography (steep or gentle slope), the
type of climate (humid, dry, warm, cold), and the amount of time that has elapsed.
Climate parameters such as humidity and temperature are the overwhelming factors in
soil formation. Temperature increases the rates weathering reactions and water (from
rainfall) provides the medium in which chemical weathering reactions can occur.
Chemical weathering reactions will not occur in hot, dry environments.
Different rocks contain different minerals and these minerals weather at different rates
under the surface condition of the earth. The less stable minerals (easily altered) at earth’s
surface conditions will undergo a greater extent of weathering. Secondly, the type of soil
produced by weathering also depends on the weathered bedrock. For instance, a rock that
is rich in both feldspars and quartz is likely to produce a sandy clayey soil whereas a
parent rock that lacks free quartz (e.g., basalt) is unlikely to produce any type of sandy
soil. The topography of the weathered area also influences soil formation; a rugged and
steep slope will work against deep chemical weathering and therefore poor soil
development.
Problem 15. In which horizon of the soil profile (Figure 1), would you expect to find the
greatest concentration of clay minerals (such as kaolinite)? Explain your answer.
Problem 16. Organic acids released by the decay of plant matter enhance weathering
under forested areas. Usually, the presence of organic acids enhances the leaching
reaction of iron and aluminum. In the soil profile, the A horizon is the only layer that may
contain both organic and mineral matter.
If organic acids played important roles in the weathering processes, where in the soil
profile would you expect its leaching effect to be most pronounced?
Between O and A horizon
Between A and B horizon
Between B and C horizon
___________________
___________________
___________________
Explain your answer
7
Weathering of Pliocene andesites: a quantitative treatment
The mineral composition of two Pliocene andesites from the Cascade Range of NE
California (Hendricks and Whittig, 1968; Journal of Soil Science, volume 19, pp 135146, 147-153) is depicted in Figure 2.
Problem 17. From fig. 2, list the minerals in the andesites in the order of decreasing
abundance?
Hypersthene andesite:
Olivine andesite:
How do the mineral compositions of the two rocks differ?
Hypersthene Andesite
plagioclase phenocrysts
plagioclase microlites
hypersthene
olivine
mafic groundmass
opaques
glass
Olivine Andesite
plagioclase phenocrysts
plagioclase microlites
hypersthene
olivine
mafic groundmass
opaques
glass
Figure 2: Mineralogy of the unaltered andesites: A) hypersthene andesite and B)
olivine andesite. Microlites are primarily very fine-grained crystals of plagioclase
(that may also contain quartz and potash feldspar because of their fine grain sizes)
whereas the phenocrysts are very large crystals of plagioclase. The composition of
the hypersthene andesite is phenocrysts (21%), microlites (47%), hypersthene
(21%), opaques (3%), glass (8%) whereas the olivine andesite consists of
phenocrysts (13%), microlites (47%), hypersthene (6%), olivine (8%), mafic (Mg
and Fe –rich) groundmass (20%), opaques (4%) and glass (2%). Exploded slice
shows glass composition.
8
Soils from andesites
The soils formed from the olivine and hypersthene andesites are formally described as
Cumulic Ustic Umbrihumult (soil order, Ultisol) and Andic Haplumbrept (soil order,
Inceptisol), respectively (Hendricks and Whittig, 1968).
The soil formed from the olivine andesite is thus a thick, dark colored soil having a high
humus content, low base (Ca2+, Mg2+, K+) saturation and strong structure. Its soil
moisture content (ustic) is intermediate between soils having year-round plant-available
moisture and those with water supply for about half of the plant-growing season; its
subsurface horizons (cumulic) contain clay accumulations. The olivine andesite
weathered to a fine, loamy soil with a profile thickness of 120 cm.
The soil formed from the hypersthene andesite has dark-colored surface horizons with
medium-to-low basic cation supply. It is characterized by high content of volcanic glass,
amorphous materials or poorly crystalline iron and aluminum oxides and oxyhydroxides.
This soil has a minimum horizon development (“Hapla”) and is characterized by the
presence of amorphous (short-range order) materials formed from the weathering of
volcanic glass (“andic”). The hypersthene andesite weathered to a coarse loamy soil with
a profile thickness of 118 cm. A loam is a soil having moderate amounts of sand, silt and
clay and a loamy soil has properties intermediate between a fine-textured (“ashy”) and
coarse-textured (“silty”) soil.
Problem 18. The hypersthene and olivine andesites have a combined iron oxide
compositions of 6.7 % and 8.1%, respectively. Which of these two andesites is likely to
produce a more intense reddish-colored soil?
Problem 19. How would you explain the fact that the hypersthene andesite formed a
coarse loamy soil whereas the olivine andesite produced a fine loamy soil? [Hint:
consider the mineral composition of the andesites shown in Fig. 2].
Mineralogy of Saprolites
The mantle of unconsolidated material that lies on solid unweathered rock is known as
regolith (and includes soil) whereas the term saprolite refers to chemically weathered
rock that that retains the structure of the original rock and lies below the soil; usually
saprolite has not experienced any transportation.
9
There are 4 stages of saprolite alteration of the andesites in figure 2 but only the initial
and final stages of alteration are shown in Table 1. The partial mineral compositions of
the stage 4 saprolites are shown below.
Table 1. Approximate mineral compositions of stage 4 saprolites
Percent Composition (%)
Stage 4 saprolite
olivine andesite
Quartz
Feldspar (orthoclase and plagioclase)
Mica-like clay
Amorphous phases (non-crystalline solids)
Kaolinite
Iron oxide
1
<1
2
27
30
10
hypersthene andesite
2
<1
2
28
38
8
Problem 20. Using the information in table 1 (stage 4 saprolites) and figure 2, list the
minerals or other solids present in greatest abundances in the chart below.
Hypersthene
andesite (figure 2)
Stage 4 saprolite
(from table 1)
Olivine andesite
(figure 2)
Stage 4 saprolite
(from table 1)
From your analyses above, are the minerals present in greatest abundance in the
saprolites the same as in the andesites?
________________________________________________________________
If not, list the minerals present in greatest abundances in the andesites and the ones
present in greatest abundances in the saprolites.
Andesites:
Saprolites:
Problem 21. Can the mineral abundances in the saprolites and in the andesites be
explained using feldspar hydrolysis reactions (i.e., equations 2 to 4 above)? Explain.
10
Abrasion pH
Abrasion pH is the pH measured in aqueous solutions produced by grinding rocks or
minerals in distilled water; abrasion pH for the hypersthene andesite, olivine andesite,
and the corresponding values for stage and 1 and 4 saprolites are shown in the chart
below.
Solid material
Hypersthene andesite
Abrasion pH
8.9
Solid material
Olivine andesite
Abrasion pH
8.6
Stage 1 saprolite
5.5
Stage 1 saprolite
6.3
Stage 4 saprolite
4.9
Stage 4 saprolite
5.4
Problem 22. Compare the abrasion pH of the saprolites and unweathered andesites. How
does the abrasion pH vary with weathering: increase, decrease or stay the same?
Problem 23. What is the concentration of hydrogen ions in the stage 4 saprolite? By how
much does the hydrogen ion concentration in the fresh rock and saprolite waters differ?
Problem 24. Since aqueous solutions generally increase in pH as the solids undergo
hydrolysis, could the trend of abrasion pH be attributed to hydrolysis?
Problem 25. If the abrasion pH were not due to hydrolysis, what would be an alternative
explanation?
[Hint: consider whether hydrogen ions are being consumed or release back into the
solution and also the types of minerals present in the saprolite and in the fresh rock.]
11
Extent of alteration of fresh rocks
Problem 26. For the olivine andesite, approximately 22, 43, 44 and 45 % of original rock
is removed at each of the 4 stages of saprolite alteration. During which phase of alteration
was the most rock matter removed and why?
Problem 27. During the saprolite formation, 47.4 % and 45.4 % of the original
hypersthene and olivine andesites, respectively, were lost. If there were 1 kg of original
rock present, how much rock would be left over at the end of each stage of weathering?
Hypersthene andesite:
Olivine andesite:
Problem 28. Which of the two rocks (hypersthene andesite or olivine andesite)
experienced a faster rate of weathering?
For the andesitic rock that weathered at a faster rate, was its rate of weathering
moderately or considerably greater? On what observations are your inferences based?
12
13