Nitrate deposition

G. E. Ericksen, Geology and Origin of the Chilean Nitrate Deposits, U.S. Government Printing Office, Washington, D.C., 1981. 

The Chilean nitrate deposits are located in the north of Chile, in a plateau between the coastal range and the Andes mountains, in the Atacama desert. These deposits are scattered across an area extending some 700 km in length, and ranging in width from a few kilometers to about 50 km. Most deposits are in areas of low rehef, about 1200 m above sea level. The nitrate ore, caUche, is a conglomerate of insoluble and barren material such as breccia, sands, and clays (qv), firmly cemented by soluble oxidized salts that are predominandy sulfates, nitrates, and chlorides of sodium, potassium, and magnesium. Cahche also contains significant quantities of borates, chromates, chlorates, perchlorates, and iodates. 

The nitrate deposits are made up of several layers (Fig. 1). The ore bodies are very heterogeneous and variable in size, thickness, composition, and hardness. The overburden may include chuca a layer of unconsoHdated sand, silt, and clay, andpanqueque a layer of semiconsoHdated and porous material poody cemented by salts over poody cemented gravel. The ore composition has degraded considerably since the eady days of the industry, when it was reported that ores of up to 50% sodium nitrate were mined. There are stiU reserves that can be commercially mined well into the twenty-first century (1). 

The first iodine-containing mineral (Agl) was discovered in Mexico in 1825 but the discovery of iodate as an impurity in Chilean saltpetre in 1840 proved to be more significant industrially. The Chilean nitrate deposits provided the largest proportion of the world s iodine until overtaken in the late 1960s by Japanese production from natural brines (pp. 796, 799). 

Prior to World War I the principal sources of nitrogen compounds were some nitrate deposits in Chile. Fritz Haber, a German chemist, successfully developed the process we have just described, thus allowing chemists to use the almost unlimited supply of nitrogen in the atmosphere as a source of nitrogen compounds. 

Corrosion permitted leak of nitric acid into the ammonia feed passing through the cooling tubes of a nitric acid plant. An ammonium nitrate deposit built up and exploded when the tubes were cut out with a welding torch for replacement. 

Iodine is widely distributed in nature, found in rocks, soils and underground brines. An important mineral is lautarite, which is anhydrous calcium iodate found in nitrate deposits in Chde. The element also occurs in brown seaweeds, in seawater, and in many natural gas wells. Its concentration in the earth s crust is an estimated 0.5 mg/kg and in seawater 0.06 mg/L. 

CALICHE (Nitrate). The gravel, rock. soil, or alluvium cemented with soluble salts of sodium in the nitrate deposits of the Atacama Desert of northern Chile and Peru. The material contains from 14 to 25% sodium nitrate, 2 to 3% potassium nitrate, and up to I i sodium iodaic. plus some sodium chloride, sulfate, and borate. At one time, this was an important natural fertilizer. 

Since practically all nitrates are soluble in water, one would not expect to find extensive deposits of these salts in nature excepting in arid or semiarid regions. In addition to the nitrate deposits in Chile, limited quantities of potassium nitrate occur in East Asia. Most of the potassium nitrate used at the present time is made by the metatheti-cal reaction between sodium nitrate and potassium chloride,... 

Another consequence of these films of grease is the prevention of oriented overgrowths .4 It is often found that if a crystal with a clean, freshly cleaved surface, is moistened with a solution of an isomorphous, soluble salt, which is then allowed to evaporate, the crystals deposited from the soluble salt are oriented with their edges parallel to those of the original crystal. Such overgrowths are well shown by sodium nitrate deposited on calcite. They are only obtained in perfection, however, if the surface of the first crystal is free from contamination the power of orienting deposited isomorphous crystals is much diminished if the freshly cleaved surface is left in the air. 

Specifications of "benefit" from reduction of deposition has required a major investment in studies of airborne acid-forming substances and their consequences to aquatic and terrestrial ecosystems. A major part of these studies has resulted in considerably improved knowledge of the sulfate and nitrate deposition conditions in North America, as well as knowledge of the atmospheric processes affecting these distributions. In this paper, aspects of the current state of knowledge in deposition patterns are summarized, with notes about unresolved issues. 

The intersite correlations illustrate well the regional character of sulfate and nitrate deposition in the EUS. Circumstantial evidence for a similar spatial scale of influence also has been reported for the West (6). 

Bohlke, J. K., Ericksen, G. E., and Revesz, K. (1997). Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, USA. Chem. Geol. 136,135—152. 

Fisher, H., and Oppenheimer, M. (1991). Atmospheric nitrate deposition and the Chesapeake Bay estuary. Ambio 20, 102. 

Ericksen G. E. (1981) Geology and origin of the Chilean nitrate deposits. Geological Survey Professional Paper 1188, US Geological Survey, 37pp. 

Nr creation because rice cultivation creates an anaerobic environment that enhances nitrogen fixation). Although estimates are not available for 1890, Smil (1999) estimates that in 1900 cultivation-induced Nr creation was — 15 Tg N yr. Additional Nr was mined from guano (—0.02 TgN yr ) and nitrate deposits (-0.13 TgN yr- ) (Smil, 2000). 

In sediments and soils, the chemistry of selenium differs from that of sulfur in that the stability of selenite is similar to that of sulfate. The reduction of selenite to elemental selenium, which tends to immobilise selenium in soils and water, is an important process in the natural environment. Selenates are only stable under alkaline oxidising conditions and have been found, for example, in the Chilean nitrate deposits. 

Combustion produces oxides of nitrogen, which can be oxidized to nitrate (NO3) in the atmosphere the nitrate subsequently may be deposited by both wet and dry processes. Cite three specific regional or global-scale problems, or potential problems, that may be exacerbated by increased levels of nitrate deposition in precipitation. 

Sodium nitrate deposits are less widespread than the other major types of chemical crust or sediment that are found in different parts of the world. Indeed, the only deposits of any great spatial extent and thickness are those of portions of the hyper-arid Atacama Desert in South America. These materials are, in the words of Ericksen (1983, p. 366), so extraordinary that, were it not for their existence, geologists could easily conclude that such deposits could not form in nature . Ericksen notes a series of features of the deposits that defy rational explanation. These include their restricted distribution in a very salty area, their occurrence in a wide variety of topographic settings, the abundance of nitrate minerals, and the presence of a series of other minerals, such as perchlorate, that do not occur in any other saline complexes and the origin of which is obscure. 

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