Silicate diagenesis

Weathered fragments of continental crust comprise the bulk of marine sediments. These particles are primarily detrital silicates, with clay minerals being the most abmidant mineral type. Clay minerals are transported into the ocean by river runoff, winds, and ice rafting. Some are authigenic, being produced on and in the seafloor as a consequence of volcanic activity, diagenesis and metagenesis. 

Opal (or opaline silica) An amorphous silicate formed through the polymerization of silicic acid molecules. Though most is biogenic in origin, some forms as a result of diagenesis. 

Most likely, the chemical system remains closed, as far as the other components in the silicate phases are concerned, as diagenesis or low grade metamorphism becomes more evident. Although there may be transfer of calcium, it seems, from bulk chemical analysis, that there is no systematic increase in potassium nor decrease in sodium content of argillaceous sediments. The transfer of Na and K is between the two size fractions—clay and coarse fraction—or between phyllosilicates and tectosilicates. Albitization of argillaceous rocks should be a common phenomenon where mixed layered phases are predominant in clay assemblages and especially evident in the illite-chlorite zone. 

MURAtA (K.J.) and LARSON (R.R.), 1975. Diagenesis of Miocene siliceous shales, Tremblor Range, California. Journ. Res. U.S. Geol. Survey,... 

SIEVER (R.), 1962. Silica solubility, 0°-200°C, and the diagenesis of siliceous sediments. Journ. Geol. TO, 127-150. 

Tectonic events can play a major role in the later history of chalk diagenesis. Subduction and very deep burial can lead to the conversion of calcite to a calc-silicate with release of CO2. As an example, Figure 8.22 illustrates the P-T conditions necessary for the reaction... 

Recent sediments of water basins. In recent basins iron sediments consist mainly of the iron hydroxides Fe(OH)3 or Fe203-nH20, but in very rare cases silicates and carbonates of Fe ", pyrite, and hydrotroilite enter into the composition of the sediment all together they constitute reactive (mobile) iron, which actively takes part in the diagenetic processes. A mixture of clastic minerals, which decompose negligibly and take practically no part in the processes of diagenesis, constitute another group. 

In the analysis of the deposition of iron sediments it has already been mentioned that quite likely both iron silicates and carbonates and amorphous iron hydroxide were formed, which could convert to other forms both during the formation of the sediment and in subsequent diagenesis. Reduction of hydroxide could have been controlled by external (atmospheric) or internal (organic matter, free carbon in the sediment) oxidation-reduction buffer systems. All these variants need additional consideration in the thermodynamic analysis of diagenetic processes. 

On the basis of the data obtained Chukhrov et al. (1977) assume that in all epochs of the geologic history of the Earth, including the Proterozoic, iron was supplied to the sedimentary basins chiefly in the form of ferrihydrite, which either was converted to hematite (in the absence of organic matter) or was reduced by organic matter during diagenesis to siderite and iron silicates, as was assumed in the works by Strakhov (1960) and Plaksenko (1966). 

On the basis of thermodynamic constants obtained for hydroxide compounds of iron with different aging time and also of experimental data, the physicochemical character of the diagenetic transformations of iron sediments of various compositions (oxide, silicate, carbonate, sulfide) can be traced. The results obtained are represented graphically in the form of stability diagrams of iron compounds as a function of variations in the main parameters governing the physicochemical character of the environment of diagenesis—pH, Eh, activity of iron and dissolved forms of sulfur and carbon dioxide. 

Diagenesis of iron silicate sediments in the absence of active forms of sulfur and carbon dioxide leads to some decrease in the silicate field due to expansion of the goethite field towards the region of reducing conditions (see Fig. 58). As in amorphous silicate sediments, the magnetite field in crystalline rocks is completely replaced by the greenalite field. 

Diagenesis of oxide-silicate-carbonate-sulfide sediments. If reactive sulfur, carbonic acid, and silicic acid are present at the same time in original iron sediments of any composition, the mineral associations formed are determined by the concentrations of these active forms. Figure 59a gives a diagram of the relationships between the crystalline iron compounds in the system Fe-Si02-C02-S-H,0 for a — 10 and = 10 g- ion/1. 

Fig. 59. Composite diagrams of mineral equilibria after the end of diagenesis in oxide-silicate-carbonate-sulfide rocks in an open system (up = 10 g-ipn/1) a—in coordinates of A-pH at Og = 10 b—in coordinates of Eh-a at pH = 6 c—in coordinates of Eh-a at pH = 8.

The formation of silicates in the process of diagenesis is also controlled mainly by sulfur compounds in the waters. Thus, in environments with pH = 8 greenalite can be formed only when Og < 10 g-ion/1 (see Fig. 59). In more alkahne environments the minimum values of at which greenalite is formed are somewhat higher at pH = 9 and 10, the Us(min) values are 10 and 10 g-ion/1, respectively. 

It is presumed that at the end of deposition and diagenesis there is no siderite in rocks of this type, the stable Fe " silicate is a mineral of greenalite type, there is excess silica, ferric oxides possibly are present, and the fluid phase in the intergranular space consists mainly of water. 

---