Historical Geology/Chemical weathering

Chemical weathering is the breakdown of rocks into sediment by chemical processes.

The reader who does not appreciate chemistry may skip the chemical formulas in this article and simply note the results of the reactions, as described in the summary section below. The results themselves cannot be skipped over: understanding chemical weathering is essential to answering such basic questions as: "Why is sand mostly made of quartz?" and "Where does clay come from?"

Agents of chemical weathering

The main agents of chemical weathering are:

Water. Some minerals, such as rock salt, will dissolve readily in water; others such as pyroxene will also do so, though at a much slower rate.

Carbonic acid. Rainwater and groundwater are not pure water; some of the molecules of water react with the carbon dioxide in the atmosphere (in the case of rainwater) or produced by bacteria and plant roots (in the case of groundwater) producing carbonic acid, as follows:

H₂O (water) + C O₂ (carbon dioxide) → H₂C O₃ (carbonic acid)

Oxygen. This is a highly reactive chemical, and the only reason that there's so much of it about in the atmosphere is its constant production by biological processes and a shortage of things that it hasn't reacted with already.

Chemical weathering of common minerals

In this section we shall look at how some common minerals are affected by chemical weathering. We have arranged the list more or less in order from the minerals most susceptible to chemical weathering to the most resistant.

Halite. Salt, of course, dissolves in water. This is why you are unlikely to see rock salt on the surface except in desert environments.

Gypsum. This, like halite, is soluble in water; similar remarks apply to it.

Calcite. This, you should recall from previous articles, is the mineral forming limestone and its metamorphic counterpart, marble. It can just dissolve in water; it also reacts with carbonic acid as follows:

Ca C O₃ (calcite) + H₂C O₃ (carbonic acid) → Ca²⁺ + 2(H C O₃)^- (dissolved ions of calcium and bicarbonate).

The ease with which limestone dissolves (relative, at any rate, to other minerals) produces the distinct topography of a region built on limestone rocks, with underground caves full of stalactites; sinkholes where the land has subsided; streams disappearing into the ground or rising out of it as springs. This is known as karst topography.

This is also the reason why a marble tombstone, though handsome in appearance, is not a good long-term investment.

Silicate minerals. Of silicate minerals in general we may observe that the more mafic minerals with higher melting points (those to be found to the right of our diagram in the article on igneous rocks) are more susceptible to chemical weathering than felsic low-temperature minerals. This will be reflected in the order in which we list them below.

Olivine. This mafic mineral has the formula (Mg,Fe)₂Si O₄. Recall that the (Mg,Fe) in the formula means that it is a solid solution in which varying quantities of magnesium or iron can play the same chemical role. It reacts with carbonic acid as follows:

(Mg,Fe)₂SiO₄ (olivine) + 4H₂CO₃ (carbonic acid) → 2(Mg,Fe)²⁺ + 4H C O₃^- (dissolved ions of magnesium/iron and bicarbonate) + H₄Si O₄ (silicic acid).

As with limestone, the constituent parts of the mineral are now entirely dissolved in water, leaving no residual mineral.

On the other hand, iron olivine can react with water and atmospheric oxygen like this:

2Fe₂Si O4 + 4H₂O + O₂ → 2Fe₂O₃ + 2H₄Si O₄

And the hematite can further react with water as follows:

Fe₂O₃ + H₂O → 2Fe O(O H) (goethite)

Hematite and goethite are both very insoluble in water: they remain as residual minerals. It is these iron oxides that give many soils their reddish or yellowish color.

Pyroxene. The typical rock-forming pyroxines have the formula (Mg,Fe)Si O₃. This can react with carbonic acid as follows:

(Mg,Fe)Si O₃ + H₂O + 2H₂C O₃ → (Mg,Fe)⁺² + 2H C O₃^- + H₄Si O₄

Again, as with olivine, the constituent parts of the mineral are dissolved in water. However, as with olivine, iron pyroxene can react with oxygen and water to produce the residual mineral hematite:

2Fe S O₃ + 4H₂O + 2O₂ → 2Fe₂O₃ + 2H₄Si O₄

Again, the hematite can turn to goethite.

Mica and amphibole, minor constituents of felsic and intermediate rocks, undergo rather more complicated reactions. (Details may be found here for biotite, a mica, and here for amphibole.)

In summary, the residual minerals produced are clay minerals; iron oxides; and, in the case of biotite, the mineral gibbsite (Al(O H)₃), which is usually found in association with clay.

Potassium and sodium feldspars produce residual clay minerals. Here, for example, is the reaction by which potassium feldspar produces kaolinite (plus various dissolved substances):

4K Al Si₃O₈ (K feldspar) + 4H₂C O₃ (carbonic acid) + 2H₂O (water) → Al₄Si₄O₁₀(O H)₈ (kaolinite) + 4K⁺ (potassium ions) + 4H C O₃^- (bicarbonate ions) + 8Si O₂ (dissolved quartz)

As sodium is chemically almost indistinguishable from potassium (see the article on Chemistry for geologists for further details) potassium feldspar reacts in a similar way.

Quartz is the most stable of the silicate minerals. This is why quartz sand is so common as a sediment; when all the other constituents of igneous rocks have either dissolved or been converted to clay, grains of quartz will remain. This is why there is no such thing as a large grain of sand: the maximum size of such grains is limited by the size of the quartz crystals that form in granite and similar rocks.

Note, however, that quartz sandstone is vulnerable to chemical weathering, because although the grains of quartz themselves are resistant, the minerals cementing them together may not be.

Clay minerals are very resistant to chemical weathering, because they are, as we have seen, a product of chemical weathering, and, like all minerals, they are stable under the conditions under which they were formed.

Iron oxides. These, as we have seen, are a product of chemical weathering of iron-bearing forms of such mafic minerals as olivine and pyroxene. We have noted that hematite can be converted to goethite by oxidation:

Fe₂O₃ + H₂O → 2Fe O(O H)

Once iron oxide has formed, there is very little that can happen to it except conversion to another sort of iron oxide; these are regarded as the most stable of all common classes of minerals.

Summary

The residual minerals left after chemical weathering has done its work are quartz, clay minerals, a scattering of iron oxides, and sometimes a little gibbsite. The other constituents of minerals are dissolved; their usual fate is to be carried by rivers to the sea, where they contribute to the dissolved mineral content of seawater.

We may note that most land-formed sediments are in fact quartz sand, clay, or a mixture of the two. This demonstrates the predominance of chemical weathering over mechanical weathering and erosion. If sand or mud were produced simply by mechanical crushing of granite, then they would be 60% feldspar; but they are not. When we find any appreciable amount of feldspar in sand (as in arkose sandstone), we may infer that there has been a higher than usual ratio of mechanical to chemical processes.

How do we know?

The processes of chemical weathering are sufficiently slow that it is reasonable to wonder how we know that they take place at all.

In the first place, we know that they ought to take place. According to the theory of chemistry, in the chemical equations given above, under the conditions in which chemical weathering takes place, the situations described by the right-hand side of the equation are more stable than the situations described on the left hand side; so the reactions described should happen.

We can speed up reactions which, in nature, involve weak, highly-diluted carbonic acid, and instead use something stronger, such as hydrochloric acid (H Cl). In principle the only difference this should make (besides, obviously, the substitution of chloride ions for bicarbonate ions) is that as hydrochloric acid gives up its hydrogen ions more readily, the reaction will go faster. So, for example, we can use hydrochloric acid to convert potassium feldspar to clay as follows:

4K Al Si₃O₈ (K feldspar) + 4H Cl (hydrochloric acid) + 2H₂O (water) → Al₄Si₄O₁₀(O H)₈ (kaolinite) + 4K⁺ (potassium ions) + 4Cl^- (chloride ions) + 8Si O₂ (quartz)

And this happens fast enough for us to observe it happening. Alternatively, to experiment under more natural conditions, you can bury samples of minerals in a nice moist acidic soil, where chemical weathering occurs fastest, leave them for a few years, and see what's happened to them; they will not weather completely, but if the process involves conversion of one mineral to another, rather than just dissolution, then the chemical changes are observable on the surface of the mineral.

Without any of these artifices, we can find naturally occurring rocks which appear to be in the process of weathering: for example, the exterior of such mafic rocks as basalt can often be seen to be rusty, as a result of the iron pyroxene being converted to iron oxides. In the same way, granite boulders can be found with a weathering rind, where on the outside of the boulder the feldspar on the outside has been partly converted to clay, while the feldspar on the inside is still relatively intact. In tropical soils, it is possible to find granite boulders in which the feldspar has become so "rotten" with clay that it is literally possible to kick the boulder to pieces. Under a microscope, the feldspar crystals will appear corroded and pitted. Or if a road cut or railroad cut goes through a hill (an event which always delights geologists) then since the rocks near the top are more weathered, we can take a series of samples going up through the vertical section from unaltered rock to completely weathered rock (saprolite) as in this study of the weathering of biotite.

We can observe the effect of weathering on old tombstones or on dressed stones used for building; as you would expect, this is most noticeable in the case of limestone or marble. In such cases we can usually be quite certain that the stones in question have suffered little from merely physical processes such as abrasion by sandstorms, tidal action, transport in rivers, and so forth.

Finally, we may note that the processes we have described explain the nature of sediment: they explain why so much of it is quartz sand and clay; they explain, as we have seen, the size of sand grains; they explain the origin of the minerals that cement together the grains in sandstone; and they explain the origins of the minerals found in sea water as dissolved ions.

In summary: these processes ought to happen; we can simulate them happening; and what we see in nature is just what we should see if they did happen.

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