Silica concentration pattern

The silica-concentration pattern observed in the Mattole explains both the lack of high correlation between silica and discharge or specific conductance and the relatively small changes in silica concentration with discharge noted by investigators for other streams. 

Figure 14. Concentration patterns of adsorption tests with different silica modifications. The exposed surface area is different for each sample initial silicic acid concentration is 78 ngrams Si02/ml.

Figure 11 Silica concentration in pore fluids versus depth for Cenozoic units of the Gulf of Mexico Basin. Most fluids are oversaturated with respect to quartz (1 ft = 0.3048 m) (reproduced by permission of SEPM (Society from Sedimentary Geology) from Basin-wide Diagenetic Patterns, 1997, p. 36).

The decrease in silica concentration during rising discharge, rapid rise in silica near the discharge peak, and leveling off in silica concentration 18-24 hours after peak discharge are easily seen in the case of an isolated runoff event. However, when one storm follows another in rapid succession, the pattern of variation in silica and the ratio Si02 Spec. Cond. with time becomes complex. Nevertheless, if one keeps in mind the basic pattern outlined above, the reasons for the observed silica concentrations become evident. 

Silica concentration was plotted vs. the square root of time to test for possible diffusion control (17, 18) in the release of silica. An allowance was made for filtering time. Soil composite A displayed a release pattern which suggested diffusion control at one rate for the first 8 hours (Figure 15), and then the slope changed to a rate similar to that seen in soil composite B. A break in slope in the line for soil B apparently occurred at the third aliquot collected. 

Silica concentration in the Mattole River displays a consistent pattern with respect to stream flow, specific conductance, and sediment concentration. However, the silica variation is out of phase with respect to both discharge and dissolved electrolytes, and silica varies inversely with sediment concentration. Any explanation of the silica variation must account for these facts. 

Some tentative conclusions can now be drawn regarding the way in which silica is released from Mattole soils. The linear relation between silica concentration and the square root of time in soil and sediment suspensions, where dissolved silica is less than about 1 mg/liter, suggests that a diffusion mechanism controls the release of silica from mineral particles. Such a mechanism would be in agreement with studies by others (14, 17, 18). Those studies suggest that in the initial release of silica from feldspar only a diffusion mechanism would be apparent, but as the silica concentration increased a subsequent sorption (precipitation ) reaction on the altered solid surface would slow the net release of silica until a relatively steady condition existed. This appears to be a pattern that would explain the silica released from both low and high concentrations of prewashed Mattole soil and sediment in water. 

The chemical analyses of the Wyatt Formation in Appendix 7.8.1 demonstrate that the rocks have high silica concentrations ranging from 67.46% to 70.3%. Although the Wyatt Formation appears to be a deposit of volcanic ash that was extruded on the surface of the Earth, the pattern of decreasing concentrations of CaO + MgO, total Fe, and increasing Na O + K O with increasing silica concentration in Fig. 7.5 resembles the effects of fractional crystallization of magma. In addition, the pattern of variation of the chemical compositions of the Wyatt Formation in Fig. 7.5... 

Fig. 16.17 The total alkali and silica concentrations of the notephrite, to tephriphonolite. This stratigraphic pattern also DVDP-1 core define two stratigraphic sequences consisting of occurs on the Hawaiian islands, where early-formed tholeiite...

The inductively coupled plasma source (Fig. 20.11) comprises three concentric silica quartz tubes, each of which is open at the top. The argon stream that carries the sample, in the form of an aerosol, passes through the central tube. The excitation is provided by two or three turns of a metal induction tube through which flows a radio-frequency current (frequency 27 MHz). The second gas flow of argon of rate between 10 and 15 L min-1 maintains the plasma. It is this gas stream that is excited by the radio-frequency power. The plasma gas flows in a helical pattern which provides stability and helps to isolate thermally the outside quartz tube. 

FIGURE 10.4 Silica gel 60 TLC pattern of commercially available Bj2 reagents, hydroxo-cobalamin and dicyanocobinamide. The concentrated solution (4 pi) was spotted on the silica gel TLC sheet and developed with 2-propanol/NH40H(28%)/water (7 1 2, v/v) at room temperature in the dark. 1 — hydroxocobalamin, 2 — dicyanocobinamide. 

FIGURE 10.5 Elution profile on OH-B12 treated by microwave heating for 6 min during silica gel 60 column chromatography. Fifty milliliters of the treated OH-B12 solution (5 mmol/1) was evaporated to dryness and dissolved in a small amount of w-butanol/2-pro-panol/water (10 7 10, v/v) as a solvent. The concentrated solution was put on a column (1.4 X 15.0 cm) of silica gel 60 equilibrated with the same solvent and eluted with the same solvent in the dark. The eluate was collected at 4.0 ml with a fraction collector. Fractions I to V were pooled, evaporated to dryness, dissolved with a small amount of distilled water, and analyzed with silica gel TLC. Inset represents the mobile pattern of the OH-B12 degradation products of fractions I to V on the TLC plate. Data are typical, taken from one of five experiments. (Reprinted with permission from Watanabe, F. et al., J. Agric. Food Chem., 46, 5177-5180, 1998. Copyright (1998) American Chemical Society.)... 

Preliminary tests with samples of vitreous silica, quartz, cristobalite, and tridymite not treated with NaOH showed an effect which for quartz was previously described (7, 13) repeated tests on the same sample did not reproduce the dissolution pattern but gave reduced values. The example in Figure 8 shows three of four repeated runs on the same cristobalite sample. Although the surface area remained constant, the concentration increase with time was always slower than in the preceding... 

The alkaline mafic volcanics are less enriched in silica and more enriched in incompatible trace elements than oversaturated tholeiitic rocks with similar MgO contents. REE show smooth and fractionated patterns (Fig. 9.7). Mantle normalised incompatible element patterns of mafic rocks generally display a small upward convexity with moderate enrichments in Ta and Nb. The large majority of the mafic volcanic rocks exhibit positive spikes of Ba and Pb, which are small or absent in the southern outcrops of Capo Ferrato, Guspini and Rio Girone (Fig. 9.7). Silicic volcanics exhibit a large range of incompatible element concentrations. Sr- and Nd-isotopic... 

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