Calcium carbonate chemical weathering

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

Rainwater is essentially free of mineral solutes. It is usually slightly acidic due to the presence of dissolved carbon dioxide, or more highly acidic because of acid rain-forming constituents. As a result of its slight acidity and lack of alkalinity and dissolved calcium salts, rainwater is chemically aggressive toward some kinds of mineral matter, which it breaks down by a process called chemical weathering. 

A typical chemical reaction involved in weathering is the dissolution of calcium carbonate (limestone) by water containing dissolved carbon dioxide ... 

Acid Hydrolysis. The water that enters soil as rain or snow is in equilibrium with CO2 in the atmosphere, which dissolves to form carbonic acid. Unpolluted rainwater has a pH of approximately 5.7, whereas water in soil pores may be exposed to air containing a higher partial pressure of CO2 than the free atmosphere, and hence soil water may be more acidic (see Section 5.4). It is the attack on soil minerals by this weak carbonic acid that is the major chemical weathering process in most soils. For example, acid hydrolysis of calcium carbonate yields calcium and bicarbonate ions ... 

As the amount of CO2 stored increases, it becomes progressively more difficult to guarantee a physical barrier that prevents CO2 from returning to the atmosphere. Chemical conversion to a thermodynamically lower state would thus be desirable and is indeed possible. CO2 is the anhydrous form of carbonic acid and therefore can be used to displace weaker acids such as silicic acid. The formation of carbonates from silicates is well known as geological weathering. Thermodynamically, CO2 can be bound as a carbonate. In many instances, these carbonates dissolve in water, but some, such as magnesium and calcium carbonates, are remarkably stable as solids. Thus, mineral sequestration would provide a means of storing CO2. 

In this paper, we describe an onsite weathering experiment designed to identify acid-rain increased dissolution of carbonate rock. This experiment is based on the measurement of the change in rainfall-runoff composition from the interaction of a rock surface with incident acid rain 2. The experiment involves conducting long-term exposures of two commercially and culturally important calcium carbonate dimension stones (i.e., Indiana Limestone (commercial name for Salem Limestone) and Vermont Marble (commercial name for Shelburne Marble)) (3-5). This technique appears to give a direct measurement of the chemical dissolution of carbonate rock from the combined reactions of wet and dry deposition. Preliminary results from the initial months of onsite operation are presented to illustrate the technique. 

The chemical components of calcium carbonate — dissolved calcium ions and carbon dioxide — are widely distributed. Calcium is the fifth most common element in the earth s crust (after oxygen, silicon, aluminium and iron). It was extracted from early igneous rocks by the combined effects of erosion by the weather and corrosion by acidic gases (oxides of sulfur, oxides of nitrogen and carbon dioxide dissolved in rain water). Carbon dioxide makes up about 0.03 % by volume of the earth s atmosphere and is dissolved in both fresh and sea water. Combination of dissolved calcium ions and carbon dioxide resulted in the sedimentary deposition of calcium carbonate, which was subsequently converted into limestone rock. Early limestones (Precambrian — Table 2.1) are believed to have been deposited as precipitates of CaCOa, and/or as a result of the biochemical activity of very simple organisms, such as bacteria. 

All the physical and mechanical properties are altered at the same time when a filler is added, and the extent of the change depends on particle size and geometry. Chemical resistance may be affected (calcium carbonate is attacked by mineral acids) and weathering behaviour can alter. 

Sedimentary rocks are derived from weathered rock masses and deposited by the action of water or other means, or by the sedimentation of bioliths (mineral skeletons, shells, etc., of plants and animals) into layers, or, rarely, by chemical precipitation. Massive deposits of china clay or kaolin are found as a result of weathering, followed by movement and deposition. Limestone, the general term for natural calcium carbonate rocks, is the most abundant of the sedimentary rocks and is formed by the deposition of countless skeletons and shells. Gypsum and diatomaceous earths occur widely in sedimentary rocks. Sedimentary rocks are almost universally found in layered beds, which may have folded or otherwise been altered by subsequent geological events. Layers will differ from each other in texture, mineralogy and particle size. 

Photo-chemical stability in PVC mixtures depends strongly on surface activity, particle size distribution, and the filling load of the fillers used. Aging behavior improves with small amounts (0 to 1 vol.%) of silicon dioxide due to improved stabilizer distribution. Larger SiOj loads, however, result in intensified photo-oxidative degradation. Ground natural chalks (calcium carbonate) up to a filler load of 10 vol.% do not influence natural weathering [683]. 

This is a very sketchy depiction of the deep carbon cycle because it illustrates only the behaviors of calcium and silica. In reality, a wide variety of other cations are present in the silicate minerals, such as in the plagioclase feldspars (Table 13.2). Furthermore, not all of the limestone is converted into siUcate minerals some remains as limestone. Uplift of the limestone onto land, followed by chemical and biological weathering, is another sink for atmospheric CO2, via... 

Sulfuric acid is found in nature in the vicinity of volcanoes. It is also used in industry for manufacturing numerous consumer products. Therefore, the chemical may be released to the environment as a waste product or from unintentional, accidental releases. If released to soil, it will dissolve in soil moisture and migrate with either soil moisture or groundwater flow. If released to water, it will dissolve or create sulfate salts. Dissolved sulfuric acid will react with calcium and magnesium to produce sulfate salts. Sulfuric acid can contribute to the weathering of soil and rocks by reacting with calcium and carbonates contained in soil and rocks. 

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