Granite, a common igneous rock, is composed of several key minerals: quartz, feldspar (both orthoclase and plagioclase), mica (biotite and muscovite), and occasionally some hornblende. Here's what happens to each mineral during complete chemical weathering in a humid environment:
Quartz:
* Highly resistant: Quartz, composed of silicon dioxide (SiO2), is highly resistant to chemical weathering.
* Slight alteration: While it generally remains intact, prolonged exposure to acidic solutions can cause some dissolution, leading to a slight rounding of its sharp edges.
* End result: In the long run, quartz grains will be the most resistant to weathering and often end up as sand grains in the soil.
Feldspar:
* Susceptible to hydrolysis: Feldspar minerals, like orthoclase (KAlSi3O8) and plagioclase (NaAlSi3O8 or CaAl2Si2O8), are susceptible to hydrolysis. This reaction involves the interaction of feldspar with water and hydrogen ions (H+) from acidic rainwater or organic acids.
* Formation of clay minerals: Hydrolysis breaks down feldspar into clay minerals like kaolinite (Al2Si2O5(OH)4) and other soluble ions.
* Liberation of potassium and calcium: The process also releases potassium (K+) and calcium (Ca2+) ions, which can contribute to soil fertility.
Mica:
* Susceptible to hydrolysis and oxidation: Both biotite (a dark mica with iron) and muscovite (a light mica without iron) undergo hydrolysis, but biotite is also vulnerable to oxidation due to the presence of iron.
* Clay mineral formation: Hydrolysis breaks them down into clay minerals and releases potassium (K+) and magnesium (Mg2+) ions.
* Iron oxidation: Biotite's iron content leads to the formation of iron oxides like hematite (Fe2O3), giving weathered granite a reddish-brown color.
Hornblende:
* Vulnerable to hydrolysis and oxidation: Hornblende, a dark silicate mineral containing calcium, magnesium, iron, and aluminum, undergoes both hydrolysis and oxidation.
* Formation of clay minerals and iron oxides: Similar to biotite, it breaks down into clay minerals and releases soluble ions like calcium (Ca2+) and magnesium (Mg2+). The iron content leads to the formation of iron oxides, contributing to the red coloration.
Overall, the chemical weathering of granite in a humid environment leads to:
* Formation of clay minerals: These are important components of soil, influencing its fertility and structure.
* Release of soluble ions: These ions contribute to the nutrient content of the soil.
* Formation of iron oxides: These oxides color the weathered granite red or brown.
* Presence of quartz: This mineral remains relatively unaltered and contributes to the sandy texture of weathered granite.
This complete weathering process takes a long time, but eventually, the granite will be transformed into a mixture of clay minerals, iron oxides, and quartz grains, forming the basis for new soil development.
