Silica sediments transformation

Thermodynamic properties and solubility of the sediments. From a comparison of the tabulated thermodynamic constants (Naumov et al., 1971) it follows that the transformation of glass into a-Si02 is accompanied by a 2.2 kcal/mol change in enthalpy. Values of the isobaric-isothermic potential of formation of amorphous silicas were calculated on the basis of a comparison... 

The rate of diagenetic transformations of iron cherts is determined not only by T and P, but also by the particulars of deposition, form of migration, dispersion of the particles, and pH of the environment. The time it takes to transform amorphous sediments into crystalline minerals is very great. For iron hydroxides it is several million years, and for silica it may be still more. 

Figure 4 X-ray diffraction patterns contrasting various crystallinities of silica (a) radiolarian silica, Porcelanite (opal-CT) and a-Cristobalite (made by heating silica gel at 1,350 °C for 4 h) from Calvert (1983) (b) diatom assemblage from Antarctic plankton tow, deep-sea siliceous ooze (Holocene in age) from beneath the Antarctic Polar Front, and two chert deposits from state of New York. The sharpness of the silica peak(s) between 20° and 26° two theta increases as silica undergoes diagenetic transformation from a fresh-diatom assemblage to buried sediment for...

D iogenically precipitated silica is a metastable silica polymorph which must eventually alter to quartz under the earth s surface conditions. Present observations of deep-sea sediments suggest that this transformation may occur directly or through an intermediate, alpha cristobalite. Several models have been proposed to ascertain the rate at which these processes occur. This series of papers tests these models and offers simple but powerful methods for detecting changes in crystal form as a function of geologic age. 

Illite clays may result from the weathering of micas and feldspars. Their formation in soils and sediments is favored by high K" " and moderate silica concentrations. When smectites or mixed-layer smectite/illite clays are buried in deep sedimentary basins, they are gradually transformed into more stable illites by a combination of time and temperature (diagenesis) (cf. Velde and Vasseur 1992 Huang et al. 1993 Cuadros and Linares 1996). The reaction involved might be... 

The purpose of the present study is to measure and understand the isotopic composition of boron in purely marine cherts, thus expanding the database of 5"B in sediments and sedimentary rocks (see Fig. 1). For this we use a sequence of cherts from the Deep Sea Drilling Project (DSDP). This suite of samples offers the possibility to determine the post-deposition changes in 6"B that accompany silica transformations during diagenesis. These transformations pose strong limits on the use of B isotopes in marine cherts as a potential paleo-tracer. 

Pb(CH3)4 is totally adsorbed from aqueous solutions onto silica gel, the adsorbed species being transformed relatively rapidly Into [Pb(CH3)3]+ [37]. It is also adsorbed from aqueous samples onto glass walls of bottles [37, 42]. Therefore, extractions from aqueous samples should be carried out inside the glass sampling bottles [42]. Solutions of inorganic lead and mercury salts release tetraalkyllead compounds from silica and sediment surfaces [37]. 

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