Carbonate minerals surface area

Keil R. G. and Cowie G. L. (1999) Organic matter preservation through the oxygen-deficient zone of the NE Arabian Sea as discerned by organic carbon mineral surface area ratios. Mar. Geol. 161, 13-22. 

Figure 15.8 Weight percent organic carbon (%OC) versus mineral surface area (SA). Capital letters represent bulk samples from Obidos (O), Vaigen Grande (V), and Manacapuru (M) lowercase letters represent data from SPUTT-fractionated samples from Vargem Grande (v) and Manacapuru (m).

Figure 3 Loss of terrestrial OC in deltaic systems, (a) Organic carbon to mineral surface area ratio (OC SA) plotted against bulk stable carbon isotopic compositions for riverine suspended sediments (closed symbols) and deltaic surface sediments (open symbols). A shift to lower OC SA values indicates net loss of organic matter, and a shift to heavier (i.e., C-enriched) isotopic compositions indicates increasing contributions from marine organic matter, (b) The average ( 1 SD) total amount of terrestrial OC persisting in deltaic sediments, based on the changes in OC SA and composition between river suspended sediments and deltaic sediments for four river systems...

Figure 8 Weight percentages of organic carbon (%OC) plotted versus mineral surface area for surficial sediments from a range of depositional regimes. M and P represent data for samples from the Mexican and Peruvian margins, respectively (source Hedges and Keil, 1995).

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

Since sorption is primarily a surface phenomenon, its activity is a direct function of the surface area of the solid as well as the electrical forces active on that surface. Most organic chemicals are nonionic and therefore associate more readily with organic rather than with mineral particles in soils. Dispersed organic carbon found in soils has a very high surface-to-volume ratio. A small percentage of organic carbon can have a larger adsorptive capacity than the total of the mineral components. 

The density of the corrosive current of jamesonite in NaOH solution is basically the same as that in Ca(OH)2 solution, but it is minimal in Na2C03 solution, about a fraction of the fourth of the former. There are obvious appearances of passivation and its breaking-down in strong polarization area in NaCOa solution Because COj ion is easier to form insoluble alkaline carbonate than OH ion, the carbonate salts are passive on the mineral surface to inhibit oxidation reaction. 

Approximately 40 to 50% of the total amount of phenolics sorbed was retained by the organic matter fraction (27). In surface soil layers, organic matter is frequently intimately associated with the mineral components present, providing a large surface area and reactive sites for surface interaction. Soil acidity has a major influence on phenolic adsorption by the organic carbon fraction, since the degree of dissociation of the phenolic acids is pH-dependent. Whitehead and coworkers (28) observed that the extractability of several phenolic acids was highly dependent upon the extractant pH between pH 6 and 14. The amount extractable continually increased with extractant pH thus the extracted acids could not be readily classified into distinct fractions. 

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

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