Why does feldspar weather easily

Table 6. Silicates fall within the middle range. The most common silicates in clastic sedimentary rocks are quartz, K-, Na-feldspars and micas. Amphiboles, pyroxene, olivine and Ca-feldspars are almost never found in sedimentary rocks.

Rocks sometime expand when exhumed. Repeated expansion and contraction of the rock during heating and cooling. Sometimes these curved layers fall away like skin on an onion. Soils may also form from transported material derived from elsewhere and deposited in a lowland or basin. Residual soils develop on plains and lowlands with moderate to gentle slopes and consist of loose, heterogeneous material left behind from weathering.

This material may include particles of parent rock, clay minerals, metal oxides and organic matter. This loose material is collectively called regolith , whereas the term soil is reserved for the topmost layer which contains organic matter. The A-horizon is the topmost layer and is usually a meter or two thick.

The upper portion of the A-horizon is often rich in organic matter, called humus , and may also contain inorganic material like insoluble clays and quartz. The A-horizon may take thousands of years to develop depending on the climate and acitivity of plants and animals. This is the layer that supports crops and other types of vegetation.

These precipitated minerals often accumulate in small pods, lenses and coatings. Organic matter is sparse in the B-horizon. The lowest layer constitutes the C-horizon and is comprised of cracked and variably weathered bedrock mixed with clays. This makes it useful as a drilling mud to keep drill holes open , and to plug leaks in soil, rocks, and dams. Montmorillinite, however, is a dangerous type of clay to encounter if it is found in tunnels or road cuts. Because of its expandable nature, it can lead to serious slope or wall failures.

Other, less common, members of the smectite group include Beidellite, Hectorite, Nontronite, Sauconite, and Saponite. The Illite clays have a structure similar to that of Muscovite, but is typically deficient in alkalies, with less Al substitution for Si. Thus, the general formula for the illites is:. Thus, the illite clays are non-expanding clays. Illite type clays are formed from weathering of K and Al-rich rocks under high pH conditions.

Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of ancient mudrocks and shales. Mixed Layer Clay. Mixed layer clays are common, and consist of clays that change from one type to another through a stacking sequence.

The sequences can be ordered and regular, or high unordered and irregular. For example montmorillinite layers can alternate with illite layers in an ordered way, or there can be several layers of montmorillinite with random layers of illite. Distinguishing Clay Minerals Generally, the clay minerals occur as such small mineral grains that they cannot be easily distinguished in either hand specimen or thin section. However, the smectites can be distinguished from the other clays in the field by the "eating test" - place some clay in your mouth.

If you can feel it expand as it becomes moistened, then it is one of the smectite clays, and is not a kandite or illite clay. X-ray techniques, are thus usually required to identify the clay minerals. First, however, the clays have to be separated from other constituents.

To do this, we first disaggregate the sample and place it in a settling tube filled with water. Particles will settle in the water according to Stokes Law:. Usually a desegregating agent Calgon is added to the water to keep the individual particles from adhering to one another.

The particles are placed in a large glass cylinder filled with water and the desegregating agent, and the mixture is stirred. One then must use Stokes law to figure out how far particles of clay size will settle in a given time. That distance is measured on the cylinder, and that amount of water is then poured off and collected.

Basically, it is the granular disintegration or grusification of granite due to the hydration of feldspar and biotite minerals which break down into clays. The swelling of these clay minerals as they abosorb water further weathers the granite and biotite minerals in particular also undergo oxidiation, leading to the reddish color of grus deposits.

Because hydration is the main chemical process breaking granite down, it follows that water plays a big role. It turns out that granite weathers best in the subsurface where soil moisture is in constant contact with bedrock granite. This why you often find in desert regions piles of rounded boulders sitting in random piles on the landscape. Below surface, the granites joints and edges more surface area are chemically attacked and the result following exhumation is a pile of random boulders.

This is known as spheroidal weathering and we will see a lot of these on the next module. In wetter regions you develop really weathered granite outcrops that border on soils and known as sapprolites.

Here's a shot of spheroidal weathered granite producing grus. In the case of the Mosul Dam, hydration of anhydrite has important consequences. The increase in volume applied force to an overlying limestone layer, breaking it into pieces. While unbroken limestone is a strong enough material upon which to build a foundation, broken limestone is too weak to provide a safe foundation. Oxidation happens when free oxygen i. This reflects a transition from an oxygen-free atmosphere to an oxygenated one.

In iron-rich minerals such as olivine, the oxidation reaction begins with taking iron out of the mineral and putting it into solution as an ion. Olivine reacts with carbonic acid, leaving dissolved iron, bicarbonate, and silicic acid:. Iron and oxygen dissolved in water react in the presence of bicarbonate to produce hematite and carbonic acid:. When the olivine in basalt is oxidized, the basalt takes on a reddish colour that is distinct from the dark grey or black of unweathered basalt Figure 8.

The oxidation reaction would be similar for other iron-containing silicate minerals such as pyroxene, amphibole, and biotite. Iron in sulphide minerals such as pyrite FeS 2 can also be oxidized in this way. Hematite is not the only mineral that can result from oxidation.

In fact, a wide range of iron oxide minerals that can form in this way, In granite, for example, biotite and amphibole can be altered to form the iron oxide and iron hydroxyoxide minerals that are referred to in combination as limonite orange material in Figure 8.

Oxidation reactions can pose an environmental problem in areas where rocks have elevated levels of sulphide minerals such as pyrite. This is because when oxygen and water react with pyrite, sulphuric acid is produced:. Some of the worst examples of ARD are at metal mine sites, especially where pyrite-bearing rock and waste material have been mined from deep underground, and then piled up and left exposed to water and oxygen.

In these cases the problem is referred to as acid mine drainage. One example is the Mt. At many ARD sites, the pH of the runoff water is less than 4 very acidic.