Transport in Silicates

Solid state reactions and creep deformation are important processes in the earth s mantle. They occur only if at least two ionic species migrate simultaneously in order 

We remember that minority point defect concentrations in compounds depend on the activity of their components. This may be illustrated by the solubility of hydrogen in olivine since it depends on the oxygen potential in a way explained by the association of the dissolved protons with O and O- as minority defects [Q. Bai, D. L. Kohlstedt (1993)]. Similarly, tracer diffusion coefficients and mobilities of Si and O are expected to depend on the activity of Si02. The value (0 lnDf/0 In aSio2) = Si and O, should give information on the disorder type as discussed in Section 2.3. 

Olivine and other orthosilicates have been exposed to oxygen potential gradients in order to investigate the demixing of solid solutions and internal reactions (oxidation, decomposition). The corresponding formalism was outlined in Chapters 8 and 

Whereas we now begin to understand solid state kinetics in orthosilicates, this understanding is still unsatisfactory for other silicates with interlinked tetrahedra. Let us turn to the discussion of chemical kinetics in layered silicates since they play a prominent role in soil chemistry. For illustration we will concentrate on transport 

T = tetrahedral coordination sheet O = octahedral coordination sheet 1 = interlayer. 

---

Zhang Y., Stolper E. M., and Wasserburg G. J. (1991) Diffusion of a multi-species component and its role in oxygen and water transport in silicates. Earth Planet Set Lett 103, 228-240. 

Hastie, J. W., Horton, W. S., Plante, E. R., and Bonnell, D. W. Thermodynamic Models of Alkali Vapor Transport in Silicate Systems, lUPAC Conf., Chemistry of Materials at High Temperatures, Harwell, U.K., August 1981 High Temp. High Press, in press. 

Initial °Th and Pa are generally considered to be associated with a detrital component that becomes cemented, or occluded, within the speleothem. This component may be composed of clays, alumino-silicates or Fe-oxyhydroxides (Fig. 3) with strongly adsorbed and Pa. Th and Pa incorporated in speleothems and similar deposits may also have been transported in colloidal phases (Short et al. 1998 Dearlove et al. 1991), attached to organic molecules (Langmuir and Herman 1980 Gaffney et al. 1992) or as carbonate complexes in solution (Dervin and Faucherre 1973a, b Joao et al. 1987). 

Bockris J. O M., Kitchener J. A., and Davies A. E. (1952a). Electric transport in liquid silicates. Trans. Faraday Soc, 48 536-548. 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

Kubicki, J.D. and Lasaga, A.C., Molecular dynamics and diffusion in silicate melts, in Diffusion, Atomic Ordering, and Mass Transport Selected Problems in Geochemistry, Ganguly, J., Ed., Springer-Verlag, New York, 1991, P- L... 

Future studies should be pursued under controlled doping conditions and in atmospheres containing CO2 and O2. The known synergistic effect of CO2 on 02-solubility in silicate melts at very high gas pressures has, in fact, been interpreted in terms of Na2C03 formation in solution. Effects of this type could significantly enhance alkali vapor transport in practical combustion systems. 

We believe that a similar water vapor solubility enhancement of alkali vapor transport is possible in soda-lime-silica glass systems, and work is in progress to verify this. Some of the disparities between various glass vaporization studies may well result from variations in water content and, hence, alkali activities. The common explanation for water vapor enhanced alkali vapor transport over silicates has revolved around formation of volatile NaOH (77) and KOH (53) species. However, no direct test for the presence of these species has been made, and the possibility of water vapor enhancement of atomic Na and K transport exists in these systems. 

Laboratory studies have suggested that there are three modes of transport for silicic acid (reviewed by Martin-Jezequel et al., 2000) first, silicic acid may be rapidly transported across the cell membrane, following surge uptake kinetics. This occurs primarily in Si-starved cells with cell quotas (Droop, 1968, 1973) near minimal values. Second, sdicic acid uptake can be controlled internally, presumably due to regulation ofsihcaprecipitation and deposition (e.g., Hildebrand et al., 1997). Third, silicic acid uptake may be controlled externally due to substrate hmitation. 

Silicon is mobilized by the weathering of silicate minerals, mainly feldspars (see eqn. 4.14), and is transported in natural waters at near-neutral pH as undissociated silicic acid (H4Si04), an oxyanion (Fig. 5.2). Silicate minerals weather slowly, such that rates of input—and concentrations—of silicon in most freshwaters are quite low. Despite this, where silicates are the main component of bedrock or soil, H4Si04 can be a significant dissolved component of freshwater. 

The analyte is accumulated on the filter as a solid compound which is further dissolved and directed towards the detector. Matrix interferences are circumvented and sensitivity is improved, as demonstrated in the determination of copper in silicate rocks by flame atomic absorption spectrometry [295]. The analyte was precipitated by rubeanic acid, the precipitate was separated by continuous filtration and the depleted sample zone was directed towards waste. After precipitate collection, a potassium dichromate stream was allowed to pass through the filtering unit, thus dissolving the retained precipitate and transporting it towards the detector. The dynamic concentration range was expanded (0.3—200 -igL 1 Cu) by passing different sample volumes through the... 

---