Glauconitization, process

Over longer time scales, clay minerals can undergo more extensive reactions. For example, fossilization of fecal pellets in contact with a mixture of clay minerals and iron oxides produces an iron- and potassium-rich, mixed-layer clay called glauconite. This mineral is a common component of continental shelf sediments. Another example of an authigenic reaction is called reverse weathering. In this process, clay minerals react with seawater or porewater via the following general scheme ... 

It is interesting to note that the 1M polymorph represents an ordered form while lMd structures are disordered (Guven and Burnham, 1967) and that the typical sequence in the process of glauconitization is lMd to 1M (Burst, 1958). Illite remains, for the most part, disordered even in Paleozoic sedimentary rocks (Velde and Hower, 1963). This would suggest that the glauconite structure, being more symmetric, might be more stable than illite, a point which will be discussed when experimental studies are considered. 

Eh and pH at which the process takes place. In both cases the role of organic material is evident and similar, acting as a motor in changing the aspect of the silicate in the pellet both in chemistry and mineralogy from that of the enclosing sediment. The reasons for the development of one mineral species or the other are at the moment somewhat obscure but, considering the similarity of the occurrence, the mechanism is probably the same. With this in mind the analysis which follows for sedimentary glauconites can be considered applicable for sedimentary 7 X chlorite pellets. 

The organic remains in the initial pellets are generally accorded great importance in the process of glauconitization. Indeed it appears that the incipient pellet is mineralogically and chemically similar to the surrounding sediments but notably different in organic matter. 

The material within the pellet would be then more reducing than the sediment in which it lies, and would have a higher local Eh potential. Thus, the initial impetus to the process is a A Eh between sediment and pellet. The concentrations of the elements which must be present in the sea water solution to promote glauconite formation are largely governed by the type of sediment in contact with the water. Equilibrium is thus established punctually between pellet and sea water effecting a transfer of material between the two media. On a larger but still somewhat local scale the dissolution of detrital silicates in sea water provides the basic "reservoir" of material in solution and hence determines the activities of various elements in the solution. 

One should notice the possibility of producing single-phase illite materials by the same type of process. If, for reasons unknown at the moment, the path of chemical change leads to aluminous illite instead of iron glauconite, i.e., parallel to the K axis with low initial iron content, one could produce single phase illite or mixed layered mineral assemblage. These are apparently rare, but such an explanation could be used to explain the illite and mixed layered mono-mineral layers of "metabentonite" deposits which cannot be explained as recrystallization of an eruptive rock. Mono-mineral layers in carbonate rock the so called... 

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

It might be noted that, upon burial in an argillaceous matrix, the process of both glauconite and 7 8 chlorate formation is largely stopped. 

It is only implied that the process of formation of sedimentary glauconites and 7 X chlorites in pelletal form is restricted to areas near the sediment-water interface. Given the proper chemical conditions, these minerals will form at depth. These conditions, however, are probably unusual. 

Since this review was originally completed, Foster (1969) published a review in which similar conclusions are drawn about the glauconites and celadonites. The lack of correlation between iron and potassium content in glauconite is substantiated in her paper. Foster considered the process of glauconitization to be of two separate, unrelated processes, incorporation of iron into the crystal structure and fixation of potassium in interlayer positions, with incorporation of iron and development of negative layer charge preceding complete fixation of potassium . 

Courbe C., Velde B., and Meunier A. (1981) Weathering of glauconites areversaloftheglauconitization process inasoil profile in western France. Clay Min. 16, 231-243. 

Although these materials are still too costly to be used in commercial processing they do possess potential in this respect and may be employed as reagents in redox titrations. An interesting exception is afforded by the mineral glauconite which when impregnated with manganese(IV) oxide acts as a redox couple, thus ... 

Stille P, Clauer N (1994) The Process of Glauconitization. Chemical and Isotopic Evidence. Contrib Mineral Petrol ii7(N3) 253-262... 

The final step in the interpretation process is to derive mineralogy from the specific lithology. A list containing some of the common sedimentary minerals used for formation evaluation is provided in Table 5.18. Potentially identifiable minerals are quartz, potassium-feldspar, albite, calcite, dolomite, siderite, anhydrite, iUite/smectite, kaoUnite, glauconite, chlorite, pyrite and others. 

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