Mineral phases, formation

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

Rey, C., Renugopalakrishnan, V., Shimizu, M., Collins, B. and Glimcher, M.J. 1991 A resolution-enhanced Fourier transform spectroscopic study of the environment of the COj ion in the mineral phase of enamel during its formation and maturation. Calcified Tissue International 49 259-268. 

The study of separate mineral phases or of granulometric fractions is another approach which can be used to recover temporal information from radioactive disequilibria in weathering profiles. Such approaches rely on the assumption that the fractions only contain or concentrate minerals phases specific of a single or of few stages of formation and evolution of weathering profiles, and hence can help to characterise the time constants of the corresponding stages. 

The initial stages of iron incorporation requires the ferroxidase sites of the protein. Thereafter the inner surface of the protein shell provides a surface which supplies ligands that can partially coordinate iron but which leave some coordination spheres available for mineral phase anions, thereby enabling the biomineralization process to proceed, with formation of one or more polynuclear ferrihydrite crystallites. Iron is transferred from the ferroxidase sites to the core nucleation sites by the net reaction (Yang et ah, 1998) ... 

Let us consider a rock at temperature T whose chemical composition q (recipe) is expressed as the vector of all the molar fractions x0 of s elements or oxides. It is assumed that it can be made by an arbitrarily large number p s of mineral phases exclusive of solid solution. B is the component matrix of these minerals for the selected set of elements or oxides. Let nj be the number of moles of mineral j and gj its Gibbs free energy of formation AGf T estimated when formed from either the elements or the oxides. The function to be minimized is the Gibbs free energy G given by... 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

Estimation of diffusion distance or diffusion time is one of the most common applications of diffusion. For example, if the diffusion distance of a species (such as °Ar in hornblende or Pb in monazite) is negligible compared to the size of a crystal, it would mean that diffusive loss or gain of the species is negligible and the isotopic age of the crystal reflects the formation age. Otherwise, the calculated age from parent and daughter nuclide concentrations would be an apparent age, which is not the formation age, but is defined as the closure age. This has important implications in geochronology. Another example is to evaluate whether equilibrium between two mineral phases (or mineral and melt) is reached if the diffusion distances in the two phases are larger than the size of the respective phases, then equilibrium is likely reached. 

It is now known that teeth undergo a continuous process of demineralisation and remineralisation (see Table 2), which is driven by changes in the plaque composition [29]. In the presence of fermentable carbohydrates plaque microorganisms generate characteristic organic acids, that is, lactic and acetic [17], and these diffuse through the pellicle to the tooth surface and cause demineralisation [30]. Ions are then liberated from the mineral phase into this low pH liquid [31], and they diffuse outwards and re-precipitate at the surface layer of the demineralised lesion [32,33]. If this process is sufficiently rapid, there is a net loss of tooth mineral and irreversible cavity formation. 

Fluoride also brings about a change in composition in natural hydroxypatite, since it not only undergoes a simple exchange with hydroxyl ions but also promotes the formation of a phase containing less carbonate than the initial hydroxyapatite [65]. Fluoride is taken up more readily by demineralised enamel than by sound enamel [66], which means its availability causes a self-healing effect in the mineral phase of the hard tissue. 

Abrajano, T. A., Bates, J. K., Woodland, A. B., Bradley, J. P. Bourcier, W. L. 1990. Secondary phase formation during nuclear waste-glass dissolution. Clays and Clay Minerals, 38,537-548. 

A final example concerns the formation of heteropolynuclear hydroxide complexes.116 The complexes [(OH)Fe(OH)2Cr]3+, [(OH)Fe(OH)2Cr(OH)]2+ and [(OH)2Fe(OH)2Cr(OH)]+, or polymers such as Fe(OH)2M "+ (M = V, Cr, Mn, Co, Ni, Cu n = 2-4) have been studied with a view to an understanding of the inclusion of transition metals in iron ores as mixed oxides rather than their occurrence as discrete mineral phases. Many other examples might have been chosen in this section. Reference should be made to the general reviews given above. However it should be clear that simple inorganic coordination complexes play a major role in the chemistry of natural aqueous systems at low temperatures. 

Acidic amino acids seem to play a key role in (1) the fixation of calcium, (2) the nucleation of CaC03 crystals, and (3) the oriented growth of the mineral phase. There appears to be no essential difference between biochemical or geochemical template-induced mineral formation. The only requirement is the presence of a calcium-specific template and an environment suitable for the deposition of calcite or aragonite. 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

In 1962 it was said that the nature of the local mechanism of calcification is one of the most important unsolved problems in biochemistry 484. This statement still holds true in spite of all the significant work that has been done in the years in between. There is no easy way out for a reviewer who tries to coordinate the various findings, hypotheses, and ideas that have been made in this direction but no universally acceptable model on the mechanisms of biomineralization has yet emerged. There is no need to duplicate the efforts of other reviewers. Instead, the present work will only concentrate on one key aspect of biomineralization where some new insight has recently been gained binding and transfer of calcium ions, and subsequent formation of mineral phases. 

Chiarizia, R., Thiyagarajan, P., Jensen, M.P. et al. 2003. Third phase formation in TBP solvent extraction systems as a result of interaction between reverse micelles. In Leaching and Solution Purification, Vol. 1. Proc. Hydrometallurgy 2003 5th Int. Conf. in Honor of Prof. I. Ritchie. Young, C. A. et al. Eds. The Minerals, Metals and Materials Society, Warrendale, PA, pp. 917-928. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

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