CHEMICAL WEATHERINGRock is durable over a single human lifetime. Over longer expanses of geologic time, however, rocks decompose chemically at the Earth’s surface. The most important processes of chemical weathering are dissolution, hydrolysis, and oxidation. Water, carbon dioxide, acids and bases, and oxygen are common substances that cause these processes to decompose rocks.
DISSOLUTION
If you put a crystal of halite (rock salt) in water, in dissolves and the ions disperse to forma solution. Halite dissolves so rapidly and completely that this mineral is rare in moist environment. A small proportion of water molecules spontaneously dissociate (break apart) to form an equal number of hydrogen ions and hydroxyl ions. Many common chemicals dissociate in water to increase either the hydrogen or the hydroxyl ion concentration. Thus, a solution can become either an acid or a base. Hydrogen and hydroxyl ions are chemically reactive and therefore acids and bases are much more corrosive than pure water. Water found in nature is never pure. Atmospheric carbon dioxide dissolves in raindrops and reacts to form a weak acid called carbonic acid. As a result, even the purest rainwater, which falls in the Arctic or on remote mountains, is slightly acidic. This acidic rainwater dissolves limestone. Industrial pollution can make rain even more acidic. Limestone outcrops commonly show signs of intense chemical weathering as a result of natural and polluted rain. In addition, when water flows through the ground, it dissolves ions from soil and bedrock. In some instances, these ions render the water acidic; in other cases the water becomes basic. Flowing water carries the dissolved ions away from the site of weathering. Weathering by solution produces spectacular caverns in limestone. Most solution reactions are reversible. A reversible reaction can proceed in either direction if conditions change. For example, calcite dissolves readily in acid to form a solution. If a base is added to the solution, solid calcite will precipitate again.
HYDROLYSIS
During dissolution, a mineral dissolves but does not otherwise react chemically with the solution. However, during hydrolysis, water reacts with a mineral to form a new mineral, with the water incorporated into its crystal structure. Many common minerals weather by hydrolysis. For example, feldspar, the most abundant mineral in the Earth’s crust, weathers by hydrolysis to form clay. As feldspar converts to clay, flowing water carries off soluble cations such as potassium. Tha water combines with the less soluble ions to form clay minerals. Quartz is the only rock-forming silicate mineral that does not weather to form clay. Quartz resists weathering because it is pure silica, and does not contain any of the more soluble cations. Because quartz is so tough and resistant to weathering, it is the primary component of sand. Much of it is transported to the sea coast, where it concentrates on beaches and eventually forms sandstone.
OXIDATION
Many elements react with atmospheric oxygen. Iron rusts when it reacts with water and oxygen. Rusting is one example of a more general process called oxidation. Oxidation reactions are so common in nature that pure metals are rare in the Earth’s crust, and most metallic elements exist in nature as compounds. Only a few metals, such as gold, silver, copper, and platinum commonly occur in their pure states. Iron is abundant in many minerals, including olivine, pyroxene, and amphibole. If the iron in such a mineral oxidizes, the mineral decomposes. Many metallic elements, such as iron, copper, lead, and zinc, occur as sulfide minerals in ore deposits. When metallic sulfides oxidize, the sulfur reacts to form sulfuric acid, a strong acid. For example, pyrite oxidizes to form sulfuric acid and iron oxide. The sulfuric acid washes into streams and ground water, where it may harm aquatic organisms. This, many natural ore deposits generate sulfuric acid when they weather. The same reaction may be accelerated when ore is dug up and exposed at a mine site.
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