Chemical Weathering Processes on the Earth’s Surface

Abstract

The exhumation of rocks from the Earth’s crust implies that they must adjust thermodynamically to the conditions existing on the surface, which are extremely different from those prevailing during their formation, with higher temperature, pressure, and often exposed to chemically aggressive fluids. The processes involved in chemical weathering, such as dissolution, hydrolysis, etc., are at the core of the adjustment mechanism, transforming solid, and usually refractory rock material, into particles-typically stripped from part of their original components and dissolved phases, both of which are amenable to be transported from the continents to the sea. Although anthropogenic actions have altered natural denudation rates, still a relatively minor portion of the material thus produced stays for a longer period on the continents, temporarily sequestered in depositional systems. The most important participants in the weathering scenario are mineral dissolution, silicate hydrolysis, and redox reactions.

4.1 Introduction

As seen in a previous chapter, physical weathering is a mechanical process which fragments rocks into smaller particles without a substantial change in chemical composition; in contrast, biological weathering exerts jointly a biophysical and biochemical effect. As an example of the synergism that dominates in rock weathering, these processes are very important in the exogenous cycle since they vastly increase the surface area of crustal material exposed to the agents of chemical weathering, i.e., air and water. A larger surface area means that more mineral matter will be accessible to react with fluids (Fig. 4.1). All weathering processes can be viewed as the adjustment of rocks and minerals formed at high temperature and pressure contrasting with the situation prevailing on the Earth’s surface, where low temperatures and pressure prevails (e.g., Anderson et al. 2007). Therefore, minerals adjust to the new set of conditions to recuperate stability; this adjustment may be relatively fast for a Ca-rich plagioclase, for example, or slow as in the weathering of some silicates, like alkali feldspar or muscovite.

Fig. 4.1

The specific surface area (SSA) is the total surface area of a material per unit of mass, for example, m² g⁻¹. The figure illustrates the increase of the surface area as the size of particles decreases

Among the many factors that control rock weathering such as climate, relief, biota, time, and the properties of the rock itself, climate plays a major role determining water availability and reaction kinetics which jointly determine the extent of chemical leaching. A clear example was presented by Benedetti et al. (2003), which studied the response of weathering in a region exposed to extremely high precipitation (1.8 to 12 m yr⁻¹). Clearly, chemical weathering is set off by water (mostly water with slightly acid pH) and gases (e.g., oxygen), which attack minerals (e.g., Drever 2005). Some compounds and ions of the original mineral are removed in solution, percolating through mineral debris to feed groundwater, rivers, and lakes. A proportion of fine-grained solids may be washed away from the weathering site, leaving a modified solid residue (i.e., regolith) which forms the basis of soils.

4.2 Mechanisms of Chemical Weathering

Chemical weathering is essentially an intricate process not only because of the complexity of the chemistry involved—some of which remains unknown—but also because minerals react differently to the aggression of weathering agents (e.g., White and Brantley 1995). Diverse mechanisms of chemical weathering are recognized and a variety of combinations occur together during the breakdown of the majority of minerals and rocks.

4.2.1 Dissolution

The most straightforward weathering reaction is the dissolution of soluble minerals. The water molecule is efficient in separating ionic bonds, such as in the dissolution of halite that results in an electrolyte solution:

$$ {\text{NaCl}}_{{({\text{s}})}} \leftrightarrow {\text{Na}}^{ + }_{{({\text{aq}})}} + {\text{ Cl}}^{ - }_{{({\text{aq}})}} $$

(4.1)

Halite’s solubility product is K _sp  = 37.6.

Clearly, halite is highly soluble; at 25 °C and a pressure of 100 kPa, its solubility is 6.15 mol kg⁻¹ (~351 g L⁻¹). Since no hydrogen ions (H⁺) are involved, the process is independent of pH. Due to a number of factors, like the remarkable abundance in seawater of the two ions involved, several authors have probed into the chemical characteristics of this simple mineral. For example, Gavrieli et al. (1989) studied the solubility of halite as a function of temperature in the highly saline brine system of the Dead Sea, and Alkattan et al. (1997) experimented on its dissolution kinetics.

Similarly, anhydrite, which is markedly less soluble than halite (i.e., its solubility is 0.67 g L⁻¹), is subjected to congruent dissolution when in contact with water;

$$ {\text{CaSO}}_{{4\left( {\text{s}} \right)}} \leftrightarrow {\text{Ca}}^{2 + }_{{\left( {\text{aq}} \right)}} + {\text{ SO}}^{2 - }_{{4\;\left( {\text{aq}} \right)}} $$

(4.2)

In contrast with halite, anhydrite’s K _sp  = 2.4 10⁻⁵.

In some cases, the formation of a hydrate occurs with a definite number of molecules of water of crystallization;

$$ {\text{CaSO}}_{{ 4({\text{s}})}} + {\text{ 2H}}_{ 2} {\text{O}} \leftrightarrow {\text{CaSO}}_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}}_{{({\text{s}})}} $$

(4.3)

4.2.2 Hydration

The hydration of anhydrite to gypsum results in “bubbles,” often discernible in playas, as a consequence of expansion and mechanical deformation. Anhydrite and gypsum, as well as halite, are typical minerals of saline lakes and playas, where they crystallize as a consequence of evaporation, following a sequence that is governed by their solubility product (Fig. 4.2). Moreover, these minerals, abundant in arid environments, are easily windblown and transported away from their source, thus contributing significantly to the chemical composition of raindrops. Likewise, ocean aerosols determine that most atmospheric precipitation is of the Na⁺-Cl⁻- type. As rainfall moves away from the sea coast, the continental chemical signature becomes more prominent (e.g., Forti Adolpho et al. 2012) due to the participation of windblown dust and soluble aerosols. It follows, then, that a significant part of the dissolved phases in atmospheric precipitation are, in fact, recycled salts that originate in the sea and return to the sea after flowing over and through the outcropping continental crust, as rivers and aquifers. Many authors that have examined chemical denudation employed a methodology that allows correcting the contribution of chemical weathering, by subtracting the atmospheric input (e.g., Stallard and Edmond 1987; Gaillardet et al. 1999).

Fig. 4.2

a Typical playa basin (Salina de Ambargasta, ~29º S ~64º W), in the NW corner of Córdoba Province (Argentina), photographed during the desiccation stage (i.e., austral winter); the structure left by microbial mats can be clearly observed. b Pseudo-hexagonal gypsum crystal from Barranca Colorada (Salina de Ambargasta) obtained at 5 cm depth. c Cubic halite crystals from the surface salt crust at the playa basin of Salina de Ambargasta (Córdoba, Argentina) (Zanor et al. (2013). Photographs by G.A. Zanor

Silica is very abundant in the Earth’s crust and, although it occurs naturally as eight distinct forms with uneven abundance, the species of real interest are ordinary quartz, opal-A, and amorphous silica (e.g., Wray 1997). Amorphous silica has a solubility of 60–80 mg L⁻¹ at 0 °C, 100–140 mg L⁻¹ at 25 °C, and about 300–380 mg L⁻¹ at 90 °C (Krauskopf 1956; Krauskopf and Bird 1995). Clearly, silica solubility increases markedly with temperature. After its congruent dissolution, Si(OH)°₄ is the most representative monomer and usually exists as the uncharged acid, H₄SiO₄ (Fig. 4.3). Wray (1997) has completed an extensive review on solutional weathering in sandstones, and Piccini and Mecchia (2009) have studied the solution weathering rate and the origin of karst landforms and caves in quartzites of Venezuela.

Fig. 4.3

Species in equilibrium with amorphous silica. In natural waters dissolved silica is present as monomeric silicic acid. The line surrounding the shaded area gives the concentration of maximum soluble silica. The mononuclear wall represents the lower concentration limit below which multinuclear silica species are not stable (Stumm and Morgan 1996). Reproduced with permission, John Wiley and Sons

4.2.3 Carbonation

Carbonation is perhaps the Earth’s commonest geochemical mechanism and is concerned with the weathering effects of CO₂ in aqueous solution and, particularly, the interaction with CaCO₃ that in some parts may give rise to peculiar karstic landscapes. Water in equilibrium with the atmosphere has a pH of ~5.6 due to presence of CO₂ but groundwater may be more acidic as the concentration of CO₂ can be 20 to 30 times higher than in the atmosphere due to the production of CO₂ by biological respiration and the resulting higher partial pressure in soil air. Microbes, moss, and fungi are significant producers of CO₂ (see Chap. 3) and plants respire about 40 % of their CO₂ through the root system. In tropical soils, the interstitial air may contain up to 11 % of CO₂ due to the exuberant biological activity.

The following equation is of significant importance in chemical weathering because it produces carbonic acid:

$$ {\text{CO}}_{{ 2({\text{g}})}} + {\text{ H}}_{ 2} {\text{O}} \leftrightarrow {\text{H}}_{ 2} {\text{CO}}_{{ 3({\text{aq}})}} $$

(4.4)

The weak carbonic acid can dissociate further:

$$ {\text{H}}_{2} {\text{CO}}_{{3\left( {\text{aq}} \right)}} \leftrightarrow {\text{H}}^{ + }_{{\left( {\text{aq}} \right)}} + {\text{HCO}}^{ - }_{{3\;\left( {\text{aq}} \right)}} $$

(4.5)

Bicarbonate ion itself dissociates also

$$ {\text{HCO}}^{ - }_{{3\;({\text{aq}})}} \leftrightarrow {\text{H}}^{ + }_{{\left( {\text{aq}} \right)}} {\text{ + CO}}^{ - }_{{3\;({\text{aq}})}} \, $$

(4.6)

Equations 4.5 and 4.6 clearly show the dependency of the system with the pH of the solution, as Bjerrum plots clearly show (e.g., Drever 1997). Moreover, the corresponding equilibrium constant for the above reactions and a detailed treatment of carbonate chemistry has been also presented by Stumm and Morgan (1996) and by Langmuir (1997).

Limestone is dissolved by water which contains dissolved CO₂ (or carbonic acid) and the reaction becomes:

$$ {\text{CaCO}}_{{3\left( {\text{s}} \right)}} + {\text{ H}}_{2} {\text{O }} + {\text{ CO}}_{{2\left( {\text{aq}} \right)}} \leftrightarrow {\text{Ca}}^{2 + }_{{\left( {\text{aq}} \right)}} + {\text{ 2HCO}}^{ - }_{{3\;\left( {\text{aq}} \right)}} $$

(4.7)

The reaction in Eq. 4.7 explains the formation of caves in calcareous terrains, like the extraordinary cenotes of the Mayan peninsula (Fig. 4.4). It is evident that if the equilibrium of Eq. 4.7 is displaced to the left-hand side of the chemical equation, due to Le Chatelier’s principle, one mole of CO₂ is returned to the gaseous pool and another may be sequestered as calcite. At any rate, carbonation is a mechanism that exports alkalinity from the continents to the sea. Harmon and Wicks (2006) have brought together many case studies that deal with the geomorphologic, hydrologic, and geochemical aspects associated with carbonation processes.

Fig. 4.4

Karstic “cenotes” in the Yucatán Peninsula (Mexico). The term derives from low-land Maya and refers to any location with accessible groundwater. It is a deep natural sinkhole that results from the collapse of limestone bedrock. In the Peninsula’s N and NW, cenotes may be up to 100 m deep, with the water flow being more likely dominated by aquifer matrix and fracture flows. Conversely, the cenotes along the Caribbean coast often provide access to extensive underwater cave systems. a The so-called “sacred cenote” at the archeological area of Chichen Itzá (Wikimedia Commons). b A typical cenote (Chan Sinicché) at Cuzamá, on the eastern side of Yucatán (Wikimedia Commons)

4.2.4 Hydrolysis

Another significant mechanism in chemical weathering is the hydrolysis of minerals. It may occur under neutral, acidic, or basic conditions, with compounds in which either cations give rise to weak bases or anions give rise to weak acids, or both the cation and anion give rise to weak bases and acids. The suffix “-olysis” implies the chemical reaction or breaking down of a chemical compound, such as a silicate.

A typical acid hydrolysis, which is closely associated with the carbonation mechanism and which is of enormous significance in continental denudation is the incongruent dissolution of sodium feldspar to clay (i.e., kaolinite) by water saturated with CO₂:

$$ 2 {\text{NaAlSi}}_{3} {\text{O}}_{{8\left( {\text{s}} \right)}} + {\text{ 9H}}_{2} {\text{O + 2H}}_{2} {\text{CO}}_{{3\left( {\text{aq}} \right)}} \leftrightarrow {\text{A1}}_{2} {\text{Si}}_{2} {\text{O}}_{5} \left( {\text{OH}} \right)_{{4\left( {\text{s}} \right)}} + 2{\text{Na}}^{ + }_{{\left( {\text{aq}} \right)}} {\text{ + 2HCO}}^{ - }_{{3\;\left( {\text{aq}} \right)}} {\text{ + 4H}}_{4} {\text{SiO}}_{{4\left( {\text{aq}} \right)}} $$

(4.8)

Gibbs free energy of formation values, which can be found in the literature (e.g., Faure 1998), allow to calculate ΔG ⁰ = 69.1 kJ for the above reaction. The positive value of ΔG⁰ means that the reaction is not energetically favored and, hence, it is not spontaneous. There is ample evidence, however, that it occurs. The corresponding equilibrium constant,

$$ { \log }_{10} {K } = \, {\Updelta {{G}^{ \circ } } \mathord{\left/ {\vphantom {{\Updelta {\text{G}}^{ \circ } } {\left( { 2. 30 3R \, T} \right)}}} \right. \kern-0pt} {\left( { 2. 30 3R \, T} \right)}} \, = \, - 1 2. 10 2 2 { } $$

(4.9)

R is a constant with the value of 8.314 JK⁻¹mol⁻¹ and T is the temperature in degrees Kelvin; 2.303 is the conversion factor to transform ln K into log₁₀ K.

Therefore,

$$ {\text{K }} = { 7}. 9 { 1}0^{ - 1 3} $$

(4.10)

The value of K shows that a relatively large quantity of carbonic acid is needed for the reaction to occur. Hence, the scenario that comes into focus is one of an aqueous solutions moving away from the mineral surface under chemical attack and a significant replenish rate of fresh carbonated water coming into contact with the mineral surface, thus sustaining the continuous hydrolysis process. This chemical mechanism plays a key role in the transfer of major dissolved ions via riverine or groundwater pathways to the world’s oceans (Meybeck 2005).

Many authors have probed into the significance of silicate hydrolysis in the overall picture of continental chemical weathering, (Fig. 4.5) and there are many examples that can be cited (e.g., Zakharova et al. 2005; Banks and Frengstad 2006; Zakharova et al. 2007; Rajamani et al. 2009) as important contributions to increase the present knowledge. At any rate, White (2005) has completed a valuable in-depth review on the natural weathering of silicate minerals .

Fig. 4.5

Dissolution rates of different minerals as a function of pH (25 °C). For data, see Stumm and Morgan (1996). Reproduced with permission, John Wiley and Sons

Chemical weathering not only supplies major chemical components to the hydrosphere, like cations and anions. They are accompanied by numerous trace elements (i.e., concentrations lower than 1 ppm), which usually also are trace elements in rocks, although there are some exceptions: Fe, Al, and Ti are major rock components but, due to their low solubility, occur as traces in natural waters (Gaillardet et al. 2005). Of special interest among trace components are the group or rare earths (REE), which are freed during weathering but are rapidly adsorbed onto colloids and thus constitute valuable tracers of the original rock (e.g., Elderfield et al. 1990; Sholkovitz 1995) (Fig. 4.6).

Fig. 4.6

Upper continental crust (UCC)-normalized REE spidergrams for Paraná River. a Paraná total suspended sediments (shaded area, N = 35, data from Depetris and Pasquini 2007); Post Archean Australian shale (PAAS, Taylor and McLennan 1985); and Average Andean Arc (AAA, http://www.geokem.com/) normalized compositions are included for comparison. Note the pronounced Eu positive anomaly in the Paraná River and in the AAA. b Dissolved REE in the middle Paraná River (data from Depetris and Pasquini 2007); world average dissolved REE composition (Gaillardet et al. 2005) is included for comparison. Note that the dissolved fraction preserves the Eu positive anomaly observed in total suspended sediments

4.2.5 Oxidation-Reduction Processes

Redox reactions are common in aqueous solution, whether as weathering reactions or in the early stages of sediment diagenesis, in the water-mud interface (e.g., Potter et al. 2005). Although oxidation and reduction occur jointly, in weathering processes one product may be more relevant than others, so the following treatment of the subject matter is divided into oxidation (i.e., when oxidized products are the significant part) and reduction (i.e., when reduced phases are the more important byproducts). One interesting aspect of this weathering mechanism (it also happens in early diagenetic processes) is that it is often mediated by microbes (as seen in Chap. 3) and organic debris.

Oxidation usually occurs with free oxygen (i.e., dissolved in water) as the oxidizing agent. For example, in the reaction:

$$ 4 {\text{FeO}}_{{({\text{s}})}} + {\text{ O}}_{{ 2({\text{g}})}} \to 2 {\text{Fe}}_{ 2} {\text{O}}_{{ 3({\text{s}})}} $$

(4.11)

The corresponding value for ΔG ⁰ = –240 kJ. The associated equilibrium constant for the reaction is K = 1.08 × 10⁴², thus indicating that the reaction will go practically to completion, as direct observation of rusting iron proves.

Sulfides are common in coal deposits, mineral veins, and mudrocks; for example, the oxidation of Fe(II) and S in pyrite generates sulfuric acid:

$$ 2 {\text{FeS}}_{{ 2({\text{s}})}} + { 7}{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}{\text{O}}_{{ 2({\text{g}})}} + {\text{ 7 H}}_{ 2} {\text{O}} \to 2 {\text{Fe}}\left( {\text{OH}} \right)_{{ 3({\text{s}})}} + {\text{ 4H}}_{ 2} {\text{SO}}_{{ 4({\text{aq}})}} $$

(4.12)

As was presented in Chap. 3, microorganisms play a very significant role and are closely involved in sulfide oxidation. The resulting acidity (e.g., pH may be as low as 1 or 2; according with Norstrom et al. (2000) it may be even negative in natural environments) enhances the solubility of aluminum and other metals which causes toxicity in aquatic ecosystem. Clearly, this mechanism, when active, promotes mineral dissolution and chemical denudation. Another example of oxidation worth addressing is the one depicted by the reduced iron-bearing fayalite, the iron-rich olivine:

$$ {\text{Fe}}_{ 2} {\text{SiO}}_{{ 4({\text{s}})}} + \, {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}{\text{O}}_{{ 2({\text{g}})}} + {\text{ 5H}}_{ 2} {\text{O}} \to 2 {\text{Fe}}\left( {\text{OH}} \right)_{{ 3({\text{s}})}} + {\text{ H}}_{ 4} {\text{SiO}}_{{ 4({\text{aq}})}} $$

(4.13)

The products of this reaction are silicic acid and colloidal hydrated iron oxide which, after dehydration, yields a variety of iron oxides whose common occurrence reflects their insolubility under the oxidizing Earth surface conditions. Both phases—dissolved and solid—are common participants in global continental denudation.

In weathering processes, reduction is less significant than oxidation reactions. It occurs frequently in anaerobic environment-types, such as waterlogged soils whose greenish-grayish color is determined by reduced iron oxides. Organic matter typically operates as a reducing agent in weathering processes. Tissue or organic debris are oxidized to form CO₂, as seen above, or to form new organic compounds. SO₄ ²⁻ may be reduced by microbes which use the oxygen in the sulfate to oxidize organic material. The resulting sulfide (S²⁻) can go on to react in various ways. One of them is the reaction that may take place:

$$ 2 {\text{H}}^{ + }_{{({\text{aq}})}} + {\text{ S}}^{ 2- }_{{({\text{aq}})}} \to {\text{H}}_{ 2} {\text{S}}_{{({\text{g}})}} $$

(4.14)

Besides entering the gas pool, sulfide can react with metals to precipitate, for example, FeS or MnS (Fig. 4.7).

Fig. 4.7

A comparison between typical anoxic and oxic aqueous environments. T represents turbulence (Potter et al. 2005). Note the location of the sulfate reduction zone in both, anoxic and oxic environments; organic matter is better preserved in saline sulfate rich environments. Reproduced with permission from Springer Science

In addition to their capability of alternately mobilizing or immobilizing metals associated with naturally occurring aquifer materials, redox processes can also affect the chemical quality of ground water contributing to the generation of byproducts such as Fe²⁺, H₂S, CH₄, and to the preservation or degradation of anthropogenic contaminants (e.g., McMahon and Chapelle 2008) (Fig. 4.8).

Fig. 4.8

pH-Eh (i.e., a measure of the redox state of a solution) diagram showing the fields for natural waters. Data from different environments have been added for comparison (Pasquini and Depetris 2012 and unpublished sources). The diagram shows the evolution of waters through the cycle as it becomes less oxic and more buffered with respect to pH

4.2.6 Exchangeable Ions

The fine-grained mineral particles that result from the weathering of minerals and rocks are particularly important in supplying a reservoir of exchangeable cations and anions, which may be involved in supplementary weathering processes. These clays (particle size <2 μm) and colloids (particle size <0.2 μm) may be mineral matter or humus and carry many negative charges on their surface, and individual positive exchangeable ions attached to the surface. Cation exchange (e.g., H⁺, Na⁺, Ca²⁺, Al³⁺, etc.) is more common than anion exchange (Fig. 4.9).

Fig. 4.9

Schematic representation of a diffuse-layer adsorption model showing the association of ions to an exchange surface such as a clay particle

Applying the mass law to exchange reactions:

$$ \left\{ {{\text{Na}}^{ + } {\text{R}}^{ - } } \right\} + {\text{ K}}^{ + } \leftrightarrow \left\{ {{\text{K}}^{ + } {\text{R}}^{ - } } \right\} + {\text{ Na}}^{ + } $$

(4.15)

$$ 2\left\{ {{\text{Na}}^{ + } {\text{R}}^{ - } } \right\} + {\text{ Ca}}^{ 2+ } \leftrightarrow \left\{ {{\text{Ca}}^{ 2+ } {\text{R}}^{ - }_{ 2} } \right\} + {\text{ 2 Na}}^{ + } $$

(4.16)

In the previous equations, R⁻symbolizes the negatively charged network of the cation exchanger; for most clays the general order of ion affinity (known as the Hofmeister series) is Cs⁺>K⁺>Na⁺>Li⁺, and Ba²⁺>Sr²⁺>Ca²⁺>Mg²⁺ (Stumm and Morgan 1996).

Different particles (or different soils) have different cation exchange capacity (or CEC), which is the concentration of cations in milliequivalents per 100 g of soil or sediment. These adsorbed cations may have an effect on the chemical composition of the soil solution, which in turn may modify weathering processes in the regolith.

Adsorption and desorption processes may be important controlling isotope fractionation in soil profiles, as was shown for Mg isotopes in an extreme weathering scenario, in the basaltic island of Hainan, China (Huang et al. 2012).

Glossary

Cation exchange capacity

: Is the maximum quantity of total cations that a soil or sediment is capable of holding, and is available for exchange with the soil solution at a given pH value.

Cenote

Is a deep natural pit, or sinkhole, characteristic of Yucatán (Mexico) that results from the collapse of limestone, exposing the groundwater underneath.

Congruent dissolution:

Weathering reaction between a mineral and water that results in its complete dissolution.

Diagenetic processes:

Are changes that occur in sediment or sedimentary rocks during and after rock formation (lithification), at temperatures and pressures less than that required for the formation of metamorphic rocks or melting.

Hydrate:

A substance that contains water of crystallization or water of hydration, in a definite ratio as an integral part of the crystal.

Incongruent dissolution:

Weathering reaction between a mineral and water that results in its partial dissolution and a solid residue.

Karst:

Is a geological formation shaped by the dissolution of soluble bedrock, usually carbonate rock but also in gypsum. Given the right conditions it can also occur in weathering-resistant rocks, such as quartzite.

Monomer:

Is a molecule that may bind chemically to other molecules to form a polymer.

Playa:

Is an ephemeral lakebed, or a remnant of an endorheic lake, consisting of fine-grained sediments infused with alkali salts. Alternative names include dry lake or alkali flat.

Regolith:

I Layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons

Solubility product:

Is a constant (K_sp) for a solid substance dissolving in an aqueous solution; represents the level at which a solute dissolves in solution.