Illite sedimentary

Illite Sedimentary rocks a) From muscovite or biotite by leaching of potassium ions b) From montmorillonite by adsorption of potassium ions c) As neoformation from weathering solutions To muscovite/ biotite, to chlorite in marine environments (by addition of magnesium ions) or brines Either residual or neoformation... 

Figure 2. Illustration of the possible displacement of a sedimentary bulk composition upon reduction of Fe + to Fe + during diagenesis (after Velde, 1968). I = illite Chi = chlorite Kaol = Kaolinite M03 = expanding trioctahedral phases 1 = initial bulk composition 2 = reduced bulk composition. Upper diagram shows initial assemblage and the lower diagram those after reduction of Fe +.

Pyrophyllite is probably not stable below some 300°C at 1 Kb pressure. This temperature will be reduced at lower total water pressure but probably will remain at a substantial value (Velde and Kornprobst, 1969). Its existence in sedimentary rocks should be indicative of relatively high temperatures if it is stable. It is typically found with illite-chlorite or occasionally with allevardite (Dunoyer de Segonzac, 1969 Ehlmann and Sand, 1959). The reaction Kaolinite + quartz = pyrophyllite is an important marker in phyllosilicates parageneses when it can be observed. 

Probably the most passionately debated mineral (if one might use this adverb in a discussion of clays) found in argillaceous sediments, rivalling perhaps the enigmatic dolomite and dolomitization in the realm of sedimentary rocks, is the mineral or group name illite. Defined and redefined by its originator, R. E. Crim debated and further redefined, denied a proper existence and reprieved, this species has attracted the attention of clay mineralogists for the past two decades. It represents, in fact, the dominantly potassic, dioctahedral, aluminous, mica-like fraction of clay-size materials. Known as sericite or hydro-mica in studies of hydrothermal alterations, soil mica or illite in soils and illite... 

Si, Fe and Fe is variable. Illite also appears to be the early product of weathering in cycles of intense alteration or one of the stable products under intermediate conditions (Jackson, 1959). It is apparently stable, or unaffected by transport in rivers for relatively short periods of time (Hurley, et al., 1961) but does change somewhat in the laboratory when in contact with sea water (Carroll and Starkey, 1960) it has been reported to be converted to chlorite or expandable minerals upon marine sedimentation (Powers, 1959). However, Weaver (1959) claims that much sedimentary illite is "reconstituted" mica which was degraded to montmorillonite by weathering processes. It is evident that a certain and usually minor portion of illite found in sedimentary rocks is of detrital origin (Velde and Hower, 1963) whether reconstituted or not. 

Thus detrital sediments can contain illite of at least four origins material crystallized during weathering reconstituted degraded mica, detrital mica formed at high temperatures and of course unaffected detrital illite from sedimentary rocks. 

During the process of lithification and deep burial illite appears to remain stable or at least is slow to react with other minerals. It is by far the most dominant species of clay mineral in argillaceous sedimentary rocks (Grim, 1968 Millot, 1964). In early burial, the overall illite content of a specimen may decrease during adjustment to bulk chemical and physical restraints (Perry and Hower, 1970) however most studies of deeply buried sediments show a definite increase in illite as pressures and temperatures increase (Millot, et al., 1965 Dunoyer de Segonzac, 1969 ... 

In sediments and sedimentary rocks lMd is the predominant polymorph of illite which indicates a diagenetic origin for more of the material. 

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. 

Figure 11 indicates the necessary change in composition which a muscovite would need to become stable under conditions in a sedimentary rock where chlorite is present (x to y). The solid solution for mica-illites is delimited by the shaded area which represents a much larger variation than is possible under metamorphic or igneous conditions. The detrital muscovite (composition x) is in itself stable if the bulk composition of the sediment as projected into the coordinates is found at x. 

The AG between the assemblage of muscovite + chlorite at composition y and illite of this is likely to be relatively small and the tendency to recrystallize the muscovite from x to y compositions will be small at sedimentary conditions. However, as more thermal energy is added to the rock system, under conditions of deeper burial, the recrystallization will proceed more rapidly as temperature is increased. Evidence for such an effect can be found in Millot (1964) where sedimentary rocks coming from deeply buried or slightly metamorphosed series show the "chloritization" or kaolinitization" of detrital mica grains in splendid photographs. 

The effect is most marked in more mature sedimentary rocks where temperatures have been relatively high. The phenomenon represents not only the chloritization or kaolinitization of muscovite but also its illitization. 

It is obvious then that A1 is not synonymous with Fe in sedimentary mica-like minerals. The increasing influence of trivalent iron in a sedimentary system will obviously provoke the crystallization of a specific mineral series which is not contiguous with illite and which would not be present otherwise. The development of glauconite in sediments should be due to specific local conditions which permit the chemical evolution of an initial montmorillonite material to celadonite mica-like phase. In fact previous observations have consistently led to this conclusion as to the origin of glauconite in sediments and sedimentary rocks. 

Tomita, et al., 1970 Meilhac and Tardy, 1970.) The prevalence of montmorillonites, in river sediments and those studied as deep-sea cores in the numerous JOIDES reports leads one to believe that montmorillonite is a very common weathering product. Certainly a portion of it is derived from degraded micas but if one considers that the next most common sedimentary mineral is illite, one is forced to conclude that either continental rocks are for the major part made of micas or that many other minerals are transformed into montmorillonite during the weathering process. 

However, both 7 and 14 8 chlorites are considered to be iron-rich when found in low temperature environments. Why are the diagenetic or authi-genic chlorites found in sedimentary rocks ferrous The answer can be found in the phase relations of the minerals common in sedimentary rocks. Basically, 14 8 chlorite is formed either through the destabilization of the montmorillonite-illite mixed layered mineral or kaolinite in the majority of argillaceous sedimentary rocks (Dunoyer de Segonzac, 1969 van Moort, 1971 Perry and Hower, 1970 Muffler and White, 1969). The increase in chlorite content is frequently observed in the presence of illite or a mixed layered mineral with a high non-expandable layer content. 

In each of the different parageneses outlined here, the instability of a mineral can be denoted by its replacement with one or usually several minerals. The rocks in these facies are typified by multi-phase assemblages which can be placed in the K-Na-Al-Si system. This is typical of systems where the major chemical components are inert and where their masses determine the phases formed. The assumptions made in the analysis up to this point have been that all phases are stable under the variation of intensive variables of the system. This means that at constant P-T the minerals are stable over the range of pH s encountered in the various environments. This is probably true for most sedimentary basins, deep-sea deposits and buried sedimentary sequences. The assemblage albite-potassium feldspar-mixed layered-illite montmorillonite and albite-mixed layered illite montmorillonite-kaolinite represent the end of zeolite facies as found in carbonates and sedimentary rocks (Bates and Strahl,... 

The absence of sepiolite and palygorskite from sediments and sedimentary rocks in other parts of the world is most likely due to a lack of attention on the part of researchers who have looked at clay mineral suites in the past. This can be explained in part by the similarity of the respective major low-angle peaks which can be confused with montmorillonite (12 8 sepiolite-one water layer montmorillonite) and illite (10.5 8 palygorskite-slightly "expanded" illite). A priori there is no reason why these minerals should be particular to French sedimentary rocks except that workers from this country have been particularly alert to their presence. This opinion is reinforced by the now-frequent reports of sepiolite and, to a lesser extent, palygorskite in sea sediments of the Atlantic shelf and ridge, Mediterranean, Red Sea and Pacific deep sea (see JOIDES reports—National Science Foundation Publications). 

The initial increase in hydrostatic pressure in a sedimentary basin appears not to change mineral stabilities from those of the weathering environment. The formation of potassic, iron-rich micas such as ferric illite and glauconite both in lacustrine and shallow ocean basins demonstrates their stability at low pressures and temperatures. The same is true of the 7 8 chlorite chamosite or berthierine. Most likely the chemical variables of pH, Eh and the activity of the various ions in solution are predominant in silicate phase equilibria in sedimentary environments. 

K-Ar dating of sedimentary illite polytypes. Bull. Geol. Soc. Arne., 73, 1167-70. 

Clay minerals occur in all types of sediments and sedimentary rocks and are a common constituent of hydrothermal deposits. They are the most abundant minerals in sedimentary rocks perhaps comprising as much as 40% of the minerals in these rocks. Half or more of the clay minerals in the earth s crust are illites, followed, in order of relative abundance, by montmorillonite and mixed-layer illite-montmorillonite, chlorite and mixed-layer chlorite-montmorillonite, kaolinite and septachlorite, attapulgite and sepiolite. The clay minerals are fine-grained. They are built up of tetrahedrally (Si, Al, Fe3+) and octahedrally (Al, Fe3+, Fe2, Mg) coordinated cations organized to form either sheets or chains. All are hydrous. 

Much of the derived expanded clay, even that which resembles montmorillonite (holds two layers of ethylene glycol), will contract to 10 A when exposed to a potassium solution. Weaver (1958) has shown that these clays can obtain sufficient potassium from sea water and readily contract to 10 A. Vermiculite and mixed-layer biotite-vermiculites are rare in marine sedimentary rocks. Weaver (1958) was unable to find any expandable clays in marine sediments that would contract to 10 A when treated with potassium. A few continental shales contained expanded clays that would contract to 10A when saturated with potassium. Most vermiculites derived from micas and illites have high enough charge so that when deposited in sea water they extract potassium and eventually revert to micas and illites. Some layers may be weathered to such an extent that they do not have sufficient charge to afford contraction and mixed-layer illite-montmorillonites form. 

So-called mixed-layer chlorite-vermiculites are common in marine sedimentary rocks, but it appears that in most, if not all, instances the vermiculite layers will not contract when saturated with potassium and the expanded layers are probably some form of smectite. These clays probably formed from volcanic material, montmorillonite or chlorite, rather than from the degradation of micas and illites. 

Evernden, J.F., Curtis, G.H., Obradovich, J. and Kistler, R., 1961. On the evaluation of glauconite and illite for dating sedimentary rocks by the potassium-argon method. Geochim. Cosmochim. Acta, 23 78-99. 

Velde, B. and Hower, J., 1963. Petrologic significance of illite polymorphism in Paleozoic sedimentary rocks. Am. Mineralogist, 48 1239. 

FIGURE 3.3 X-ray diffractograms of the B-I.b. bentonite sample (sedimentary) (upper) and B-II.b. upper bentonite sample (bentonitized tuff) (lower), mm = montmorillonite, ab = albite (plagioclase), q = quartz, i = illite, s = sanidine (feldspar). (Reprinted from Nagy and Konya 2005, with permission from Elsevier.)... 

The transformation of smectite to mixed layer smectite-illite, and ultimately to illite, with increasing temperature is an extremely important reaction in many sedimentary basins, including the northern Gulf of Mexico Basin (Hower et al., 1976 Boles and Franks, 1979 Kharaka and Thordsen, 1992). The water and solutes released and consumed by this transformation are major factors in the hydrogeochemistry of these basins, because of the enormous quantities of clays involved. Several reactions conserving aluminum or maintaining a constant volume have been proposed for this transformation (Hower et al., 1976 Boles and Franks, 1979). The reaction proposed below (Equation (4)) conserves aluminum and magnesium, and is probably a closer approximation based on the composition of formation waters in these systems ... 

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