People sometimes are surprised to loam that many rocks exposed at the earth’s surface are really not in chemical equilibrium with their environment but rather are unstable and are slowly but continuously reacting with atmospheric components to dissolve or change to new substances that are more nearly stable at the earth’s surface.

Geologically, weathering proceeds in a accordance with a generalization called the “rule of stability,” which states that a mineral approaches stability most closely in an environment similar to that in which it formed.

Intrusive igneous rocks, of course, are formed under conditions of high temperature, high pressure, and a deficiency of free oxygen and fresh water. When such rocks are laid bare on the continents, they find themselves, in an environment of low temperature low pressure, and abundant oxygen and water. Chemical change is inevitable, and the minerals most susceptible to change are those that formed under physical and chemical conditions most removed from conditions at the earth’s surface.

For example, among the common rock-forming silicate minerals, olivine forms at high temperatures and pressures early in the crystallization of magma. Consequently, it rapidly weathers in the environments that exist at the earth’s surface. Quartz forms much later under less extreme conditions of temperature and pressure and is less susceptible to weathering. In reference to the Browen Reaction Series it is apparent that minerals nearer the top of the series generally weather more rapidly than those near the base.

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We noted how silicon oxygen tetrahedra are joined together by sharing some oxygen ions and that the number of metallic ions needed to neutralize the crystal is reduced by this sharing.

This increased oxygen sharing by silicon results in a greater number of strong covalent like bonds between silicon and oxygen, and this in turn greatly increases the ability of the mineral to resist weathering. For example, the ratio of oxygen to silicon in olivine is 4, in pryoxene 3, in hornblende 2.7, in biotite 2.5 and in quartz 2. The diminishing ratios correlate nicely with greater resistance to weathering. All of the oxygen atoms are shared by silicon atoms in quartz, and this partially accounts for its great resistance of weathering.

The chemical weathering of rocks involves reactions that are interrelated that may occur simultaneously, and that utilize water, oxygen, carbon dioxide, and organic acids. These processes include hydrolysis, oxidation, carbonation, solution, hydration, and chemical changes induced by the growth of plants.

Hydrolysis

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Hydrolysis is a chemical reaction between a mineral and water. It involves a reaction between the H+ or OH” ions in the water and relatively active metallic ions such as sodium, calcium, potassium and magnesium. Hydrolysis is particularly important in causing the decomposition of silicate minerals. Although it may occur in the presence of pure water, in nature hydrolysis nearly always involves carbon dioxide. To illustrate, small quantities of carbon dioxide from the atmosphere or soil are dissolved in water to form carbonic acid.

The potassium ions released from the feldspar may be carried away in solution, utilized by plants, or become incorporated into clay minerals. A small part of the silica is removed in solution, although the greater part remains in the clay-rich weathering residue.

Carbonation

As implied by the term, carbonation involves the chemical addition of carbon dioxide to earth materials. Carbon dioxide in the atmosphere (and in the air trapped within soils) is readily absorbed to water to form carbonic acid. Although relatively weak, carbonic acid nevertheless has a pervasive cumulative effect in the chemical weathering of a variety of different kinds of rocks. It is involved in the dissolution of common silicate minerals and is particularly effective in dissolving limestones and dolostones. The reaction for limestone is indicated below

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H20 + C02 H2C03

water carbon dioxide carbonic acid

H2C03 + CaC03 -> Ca(HCCy2

carbonic acid calcite soluble calcium

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(in limestone) bicarbonate

In order for carbonation to occur, water must be readily available. For this reason carbonation is most vigorous in most climates.

In the weathering of silicate minerals, carbonation and hydrolysis work together as the earth’s most important processes for achieving decomposition of rocks. The hydrolysis component provides clay minerals and takes silica into solution, while simultaneously carbonation removes metallic elements as ions in solution.

Oxidation

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Oxidation, the addition of oxygen to a compound, is one of the main kinds of changes produced in rocks by chemical weathering. Oxygen has a strong affinity for iron, which may be present in such silicate minerals as hornblende, and olivine, as well as sulphides such as pyrite (FeS).

The oxidation of the iron (essentially what we call rusting) takes place chiefly in the presence of atmospheric moisture and results in the range of red and brown colourations we see in soils, and weathered rocks. In the oxidation process, oxygen gas dissolved in water reacts with iron to from hematite (Fe203) or limonite (Fe203 H2O). The process is illustrated by the illustrated by the following formula:

4Fe + 302 + nH2O -> 2(Fe203) nH20

iron oxygen water “limonite” iron

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hydroxide or “rust”

(n means a variable amount)

The oxidation of a common nonsilicate such as pyrite (fool’s gold”) involves combining oxygen with both and sulphur as follows;

4FeS2 + nH20 + 1502 2Fe203 nH20 + 8H2S04

pyrite water oxygen “limonite” sulphuric acid

Here, the relatively insoluble iron compounds may remain as a coating on the rock, whereas the sulphuric acid is leached away and becomes available for chemical reactions with other minerals.

Hydration

Hydration is a process whereby water is absorbed by a mineral and incorporated into the weathering product. For example the mineral anhydrite (CaS04) may take in water to become alabaster gypsum (CaS04 nHzO), or hematite (Fe203) may be converted to limonite (Fe203 nH20). Hydration is an important process in the development of clay and accounts for the presence of water within many clay minerals. Another aspect of hydration is that the hydrated mineral, because of the water it has taken up, larger than the parent mineral. The increase in volume causes growing hydrated crystals to exert pressure on the walls of the spaces they occupy, and such pressure may contribute to rock disintegration.

Solution

We have been how the dissolving power of water for certain rocks and minerals is increased when carbon dioxide is present. Even without the addition of carbon dioxide, however, water has the ability to dissolve rocks and minerals.

The dissolving away of thick beds of salts and gypsum is an example of simple solution weathering. A quartz sandstone also may weather by solution, although the process is exceedingly slow because of the low solubility of quartz.

The ability of water to dissolve substance is related to the configuration and electrical properties of the water molecule itself. A water molecule consists of a large strongly negative oxygen ion and two hydrogen ions.

The hydrogen ions have contributed their electrons to the molecule and thus exist as two small positively charged protons. Both of the protons are located on the same side of the molecule, so that it has an electrically positive side and an electrically negative side. It is termed a dipolar molecule.

The electrical charges at either end of the water molecule not only attract their opposites in other water molecules but also attract compounds in rocks and minerals that likewise have separate centers of positive and negative electrical charge. These compounds are taken into solution as weathering proceeds.