Oxide geochemistry

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

What are the relative contributions of these two sources Two approaches have been taken. One is to establish the geology and hydrology of a basin in great detail. This has been carried out for the Amazon (Stallard and Edmond, 1981) with the result that evaporites contribute about twice as much sulfate as sulfide oxidation. The other approach is to apply sulfur isotope geochemistry. As mentioned earlier, there are two relatively abundant stable isotopes of S, and The mean 34/32 ratio is 0.0442. However, different source rocks have different ratios, which arise from slight differences in the reactivities of the isotopes. These deviations are expressed as a difference from a standard, in the case of sulfur the standard being a meteorite found at Canyon Diablo, Arizona. 

Reed, M.H. and Spycher, N.F. (1985) Boiling, cooling and oxidation to epithermal systems. A numerical modeling approach. In Berger, B.R. and Bethke, P.M. (eds.). Geology and Geochemistry of Epithermal System. Reviews in Economic Geology, 2, 249-272. 

In aqueous geochemistry, the important distinguishing property of metals is that, in general, they have a positive oxidation state (donate electrons to form cations in solution), but nonmetals have a negative oxidation state (receive electrons to form anions in solution). In reality, there is no clear dividing line between metals and nonmetals. For example, arsenic, which is classified as a nonmetal, behaves like a metal in its commonest valence states and is commonly listed as such. Other nonmetals, such as selenium, behave more like nonmetals. 

Most of the chemical processes discussed before (acid-base equilibria, precipitation-dissolution, neutralization, complexation, and oxidation-reduction) are interrelated that is, reactions of one type may influence other types of reactions, and consequently must be integrated into aqueous- and solution-geochemistry computer codes. 

Williams, P. A. Oxide Zone Geochemistry , Ellis Horwood Chichester, 1990 p. 115. 

An autocatalytic reaction is one promoted by its own reaction products. A good example in geochemistry is the oxidation and precipitation of dissolved Mn11 by C>2(aq). The reaction is slow in solution, but is catalyzed by the precipitated surface and so proceeds increasingly rapidly as the oxidation product accumulates. Morgan (1967) studied in the laboratory the kinetics of this reaction at 25 °C and pH > 9. 

Leaching and desorption of As from its associated mineral surfaces such as iron, aluminum and manganese oxides under the influence of the aquifer complex geochemistry, largely take part in its transport from sediment to aquifer pore-water. Adsorption has widely been considered as the retardation of As transport (Smedley 2003). 

Armstrong, J.E. 1976. Quaternary geology, stratigraphic studies and revaluation of terrain inventory maps, Fraser Lowland, British Columbia. Geological Survey of Canada, Paper 75-1, Part A, 377-380. Bowell, R.J. 1994. Sorption of arsenic by iron oxides and oxyhydroxides in soils. Applied Geochemistry, 9, 279-286. 

KEYWORDS Cu-Zn tailings, acidification, oxidation, dewatering, geochemistry... 

Ritchie, A.I.K. 1994. Sulphide oxidation mechanisms-controls and rates of oxygen transport. In Alpers, C.N. Blowes, D.W. (eds.), Environmental Geochemistry of Sulphide Oxidation, ACS Symposium Series 550. Washington DC. 

Brown Jr., G. E. (1990), "Spectroscopic Studies of Chemisorption Reaction Mechanisms at Oxide-Water Interfaces", in M. F. Hochelia Jr. and A. F. White, Eds., Minerai-Water Interface Geochemistry, pp. 309-363. 

This paper is based primarily on the understanding of reactions at the oxide-electrolyte interface gained through the study of collodial suspensions of oxides. From the point of view of the geochemist, these suspensions of pure oxides are pristine systems, interesting, but perhaps of only marginal relevance to geochemistry. To the pure chemist, these systems are almost too ill-defined to warrant serious scientific consideration. 

As an introduction to the discussion of electrical and chemical models for reactions at oxide-electrolyte interfaces, some reflections on the importance of these interfaces in geochemistry are presented. 

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