Silicate depletion

Malone, T., Garside, C., and Neale, P. (1980). Effects of silicate depletion on photosynthesis hy diatoms in the plume of the Hudson River. Mar. Biol. 58, 197—204. 

It was at one time thought that even the terrestrial planets themselves formed directly by condensation from a hot solar nebula. This led to a class of models called heterogeneous accretion models, in which the composition of the material accreting to form the Earth changed with time as the nebula cooled. Eucken (1944) proposed such a heterogeneous accretion model in which early condensed metal formed a core to the Earth around which silicate accreted after condensation at lower temperatures. In this context the silicate-depleted, iron-enriched nature of Mercury makes sense as a body that accreted in an area of the solar nebula that was kept too hot to condense the same proportion of silicate as is found in the Earth (Lewis, 1972 Grossman and Larimer, 1974). Conversely, the lower density of Mars could partly reflect collection of an excess of silicate in cooler reaches of the inner solar nebula. So the... 

The argument goes back to Officer and Ryther (1980), who point out that it is also necessary to take account of recyling rates, which are faster for P than N and for N than Si. Silicate depleted by uptake during the spring bloom, for example, is often replaced more slowly than DAIN and phosphate, and this may force seasonal succession from diatoms to dinoflagellates or small phytoflagel-lates. 

If hardness breakthrough occurs and goes undetected for any length of time, the treatment reserve is swamped and quickly becomes depleted. This loss of treatment is serious because calcium carbonate scaling can result. The reduction in alkalinity also can permit silicate scaling to occur and prevent adequate maintenance of the magnetite film, which protects the waterside metal surfaces from corrosion. 

The acidity of soils also strongly affects the availability of metal cations such as K, Mg, and Ca. Most soils contain significant amounts of clay, whose chemical composition is dominated by aluminosilicates. Silicates are anionic, and the anions can be neutralized either by accepting protons or by associating with metal cations. When soil is too acidic, protons replace these metal cations, and the soil becomes depleted in these essential nutrients. 

In a study of dental silicate cements, Kent, Fletcher Wilson (1970) used electron probe analysis to study the fully set material. Their method of sample preparation varied slightly from the general one described above, in that they embedded their set cement in epoxy resin, polished the surface to flatness, and then coated it with a 2-nm carbon layer to provide electrical conductivity. They analysed the various areas of the cement for calcium, silicon, aluminium and phosphorus, and found that the cement comprised a matrix containing phosphorus, aluminium and calcium, but not silicon. The aluminosilicate glass was assumed to develop into a gel which was relatively depleted in calcium. 

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

Presolar stardust discovered to date in meteorites constitute no more than 0.5%o of the total mass of the samples, and one common chemical property is that they are acid resistant. Isotopic heterogeneity could also be present in less refractory phases like silicates, provided parent body metamorphism did not erase the differences. Noble gases are not discussed here because they are depleted by many orders of magnitude relative to the Sun and can be dominated by trace exotic minerals. Nitrogen is not discussed for the same reason. The... 

Copper in meteorites is depleted in the heavier 65 isotope with respeet to the Earth (Luek et al. 2003 Russell et al. 2003). Luck et al. s (2003) study of the four main groups of carbonaceous chondrites CI-CM-CO-CV showed that Cu depletion is maximum (-1.5%o) for the C V chondrites (e.g., Allende) for which the depletion of volatile elements is strongest, which indicates that volatilization does not accormt for the observed isotopic heterogeneity (Fig. 4). Luck et al. (2003) found that 8 Cu in CI-CM-CO classes correlates with O excess, but this does not seems to be the case for CV (Luck et al. 2003) nor for the CR, CB, and the particularly Cu-depleted CH-like classes (Russell et al. 2003). In contrast, chondritic Zn is relatively heavy with 8 Zn up to 1 %o (Luck et al. 2001). The rather high 5 Zn values of iron meteorites (up to 4%o)is reminiscent of a similar fractionation of Fe isotopes between metal and silicates (Zhu et al. 2002). 

Alteration at Michelin is dominated by intense soda metasomatism and potash depletion, coupled with locally intense hematization much of the mineralization consists of fine-grained uraninite within sodium-rich silicates such as aegirine and arfvedsonite. The recently discovered Jacques Lake deposit, which is an important resource in its own right (about 17 million pounds UsOs), has many similarities to Michelin, although its metavolcanic host rocks are compositionally distinct. The age of the uranium mineralization in these deposits is... 

Although the role of crystalline phases in the leachability of HT materials is unclear and must be examined from case to case, the identified silicates and oxides are overall more resistant to corrosion than silicate glass and residues of incineration (Scholze 1991). Thus, a clear assessment of the durability of HT materials as a function of crystalline components must take into account the combined effects of their enrichment or depletion in trace metals, their individual leachability, the increase (but sometimes decrease) in overall reactivity due to local heterogeneities and increased Sspec (Jacquet-Francillon et al. 1982 Bickford Jantzen 1984 Jantzen Plodinec 1984 Scholze 1991 Adams 1992 Sproull et al. 1994 Sterpenich 1998). 

The geochemistry of angrites is characterized by strong silica undersaturation, by which we mean that there is not enough SiC>2 to combine with various cations to form common silicate minerals. The result is the formation of silica-poor minerals like kirschsteinite and nepheline. These meteorites also show strong depletions in moderately volatile elements. They are thought to have formed as partial melts of a chondritic source under oxidizing conditions. 

A gradient of Cs/Mn ratios with distance from the crater beyond 1500 feet is present owing to the fractionation of those radionuclides during the detonation, and subsequent deposition of ejecta and fallout. Figure 14 shows Cs/Mn ratios for 6-inch depth ejecta samples collected at Sedan crater from the lip to 3000 feet from ground zero. Any increase in Cs/Mn ratios observed at sites distant from the crater lip must be evaluated in view of the fractionation that is evident in the ejecta with respect to these two radionuclides. The Cs/Mn ratio at crater lip sites and those out to 1500 feet are all close to 0.01-0.02. A sample of fused silicate material, which represents a class of ejecta extremely depleted in volatile radionuclides, had a low Cs-Mn ratio of 0.006. 

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