Orthoclase feldspar acid water clay mineral potassium ion soluble silica. The ions released from silicate minerals in the weathering process are sodium, potassium, calcium, iron, and magnesium ions. They are carried away by rain and river waters or become important soil nutrients. The chemical stability of minerals : This is related to the tendency of a mineral to remain in a given chemical form rather than to react spontaneously to become a different chemical substance.
The stability of a specific mineral is determined by the specific set of conditions e. These minerals crystallize earlier and at higher temperatures and pressure, are therefore least stable and most subject to chemical weathering. Climate is determined by the temperature of a region plus the amount of precipitation it receives. Climate is weather averaged over a long period of time. Chemical weathering increases as:. So how do different climates influence weathering? A cold, dry climate will produce the lowest rate of weathering.
A warm, wet climate will produce the highest rate of weathering. The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering. Under these conditions, metals such as copper, zinc, and lead are quite soluble, which can lead to toxicity for aquatic and other organisms.
For many years, the river downstream from the Mt. Washington Mine had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the situation has improved. The hydrolysis of feldspar and other silicate minerals and the oxidation of iron in ferromagnesian silicates all serve to create rocks that are softer and weaker than they were to begin with, and thus more susceptible to mechanical weathering.
Some weathering processes involve the complete dissolution of a mineral. Calcite, for example, will dissolve in weak acid, to produce calcium and bicarbonate ions. The equation is as follows:. Limestone also dissolves at relatively shallow depths underground, forming limestone caves. This is discussed in more detail in Chapter 14, where we look at groundwater. The main processes of chemical weathering are hydrolysis , oxidation , and dissolution.
Complete the following table by indicating which process is primarily responsible for each of the described chemical weathering changes:. Frost Wedging - Upon freezing, there is an increase in the volume of the water that's why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze.
As the water freezes it expands and exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles. Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed. Because of these differing conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface.
These are igneous minerals that crystallize from a liquid. Note the minerals that occur low on this list are the minerals that crystallize at high temperature from magma. The higher the temperature of crystallization, the less stable are these minerals at the low temperature found near the Earth's surface. As you can see from the above, clay minerals and oxide minerals including quartz are the most common byproducts of chemical weathering.
Thus clay minerals and quartz are the most abundant contributors to clastic sediment and soil. We here discuss the structure, properties, occurrence, and identification of clay minerals, but first we need to discuss the phyllosilicates in general. Clay Minerals Clay minerals are an important group of minerals because they are among the most common products of chemical weathering, and thus are the main constituents of the fine-grained sedimentary rocks called mudrocks including mudstones, claystones, and shales.
In addition, clay minerals are the main constituent of soils. Understanding of clay minerals is also important from an engineering point of view, as some minerals expand significantly when exposed to water. Clay minerals are used extensively in the ceramics industry and are thus important economic minerals. Based on their structures and chemical compositions, the clay minerals can be divided in to three main classes:. Each of these are formed under different environmental and chemical conditions.
Kandites The kandites are clays that show a T-O structure , with the octahedral layer similar to the gibbsite structure.
Since the layers are electronically neutral, the bonding between layers is by weak van de Waals bonds. Other kandites with similar structures are Anauxite, Dickite, and Nacrite. Kaolinite is formed by weathering or hydrothermal alteration of aluminosilicate minerals.
Thus, rocks rich in feldspar commonly weather to kaolinite. In order to form, ions like Na, K, Ca, Mg, and Fe must first be leached away by the weathering or alteration process.
This leaching is favored by acidic conditions low pH. Granitic rocks, because they are rich in feldspar, are a common source for kaolinite.
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