Weathering produces a number of changes in rocks and minerals.
Weathering is a natural process in which factors, including rain, wind and microbes, break down or change the structure of geological formations such as rocks, minerals and soils. Some weathering processes, such as grinding between rocks, are purely physical, but many are due to a chemical reaction of some sort. There are several different types of chemical weathering, often involving the presence of water.
When atmospheric carbon dioxide gas is dissolved in water, it forms the chemical carbonic acid (H2CO3). Carbonic acid is a weak acid but is potent enough to react with the mineral calcite (CaCO3). This reaction breaks the calcite down into positively charged calcium ions and negative bicarbonate ions, which are water soluble. Since calcite is the major component of limestone formations, this process results in the limestone slowly dissolving and the formation of underground caves and tunnels.
Chelation occurs when complex organic molecules released by plants combine with positive metal ions found in rocks and minerals. Metals such as iron and aluminum can be drawn out of rocks in this manner and rendered water soluble. This process is sometimes associated with lichen species which cling to geological formations. The end result is a gradual breakdown of the rock or soil exposed to the lichen. Some fungi work to transfer nutrients from rocks to tree roots through this process.
Hydrolysis is the most common chemical weathering process. It occurs when water is rendered slightly acidic by the presence of dissolved carbon dioxide gas. During hydrolysis, positively charged hydrogen (H+) ions from the marginally acidic water replace positive metal ions (such as potassium or sodium) found in various minerals. The result is a change in the chemical composition of the mineral. This process is responsible for the formation of clay from rock species such as potassium feldspar.
Oxidation is a chemical process in which an element loses electrons, either gaining or increasing a positive charge in the process. It occurs when oxygen dissolved in water reacts with metallic elements. The iron-based chemical species ferrous oxide [Fe(II)O], for example, readily combines with oxygen to form the mineral hematite [Fe(III)2O3], which lends a characteristic rust-red coloration to soils and rocks. Other metals susceptible to oxidation include magnesium, aluminum and chromium.