Glauconite mixed layering

The maximum potassium content is less than 10 wt.% when the samples show no smectite component by XRD. According to Thompson and Hower (1975) this maximum potassium content represents less than one atom of potassium per unit cell of mica (in fact 0.65 atoms). XRD determinations show that the evolution of glauconite mixed-layer minerals and iUite/smectite are similar (Velde and Odin, 1975). However, the geological conditions under which these transformations occur are in general not the same. The glauconite micas show a significant substitution of low-charge ions in both the tetrahedral and octahedral sites, as do Ulites. 

Some glauconites have been identified in hydrothermally altered basalts together with celadonites (Alt et al., 1992 Clayton and Pearce, 2000). This material appears to form a mixed-layer mica-ferric smectite series. The formation of glauconite mixed-layer minerals is therefore not restricted to peloids under shallow-ocean-bottom conditions. However, the identification of glauconite as distinct from ferric Ulite is difficult. Perhaps these mineral occurrences should be given another name. 

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 ... 

Lee SY, Jackson ML, Brown JL (1975a) Micaceous occlusions in kaolinite observed by ultrami-cotomy and high resolution electron microscopy. Clays Clay Miner 23 125-129 Lee SY, Jackson ML, Brown JL (1975b) Micaceous vermicuUte, glauconite and mixed-layered kaolinite-montmorillonite by ultramicotomy and high resolution electron microscopy Proc Soil Sci Soc Amer 39 793-800... 

Table 5.53 lists the general classification of micas with their main compositional terms. Stoichiometry obeys the general formula XF2 3Z40io(OH,F)2, where X = interlayer cations, Y = octahedrally coordinated cations of the 2 1 mixed layer, and Z = tetrahedrally coordinated cations of the 2 1 mixed layer. It must be noted that several compositional terms are indeed solid mixtures of more elementary components. In particular, glauconite has a complex chemistry and an Al Si diadochy of 0.33 3.67. (R and R terms in table 5.53 identify generic divalent and trivalent cations, respectively.)... 

The approach used is to compare the composition of mixed layered mineral series—the illite-montmorillonite and the glauconite-montmorillonites. 

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... 

The most important observation which can be made after the analysis of the chemical and phase equilibria information is that illite and glauconite mineral series, do not overlap. Solid solution is not continuous, neither in the mica-like phase alone nor in mixed layering between mica and expanding layers. Glauconite is not a subspecies of illite. 

Those chlorites associated with mixed layered clay minerals are most silica-rich and have the greatest compositional variations for grains in a single thin section they tend to be iron-rich and aluminous. One chlorite vein was found to transect a glauconite pellet. This chlorite was quite iron-poor indicating attainment of a local chemical equilibrium between chlorite and iron mica upon its crystallization. 

Mixed-layer illite-montmorillonite is by far the most abundant (in the vicinity 90%) mixed-layer clay. The two layers occur in all possible proportions from 9 1 to 1 9. Many of those with a 9 1 or even 8 2 ratio are called illites or glauconites (according to Hower, 1961, all glauconites have some interlayered montmorillonite) and those which have ratios of 1 9 and 2 8 are usually called montmorillonite. This practice is not desirable and js definitely misleading. Other random mixed-layer clays are chlorite-montmorillonite, biotite-vermiculite, chlorite-vermiculite, illite-chlorite-montmorillonite, talc-saponite, and serpentine-chlorite. Most commonly one of the layers is the expanded type and the other is non-expanded. 

Celadonite, then, is a low-temperature mineral, but it is formed above the temperatures where glauconite is assumed to form (near-ocean bottom-water temperature). As far as one can tell from the rather scarce XRD information, no interlayered smectite/celadonite mixed-layer series is usually present. Data from Desprairies et al. (1989) and Holmes (1988) suggest that the glauconite is a 10 A phase, and that the associated nontronite is a potassic smectite (12.8 A, one hydration layer in the natural, air-dried state). However, information is extremely sparse and this generalization is very tenuous. 

Normally weathered (subaerial, surface alteration) basalts can contain nontronite (a ferric smectite) and celadonite which form at the same time but by pseudomorphism of different basalt mineral grains during intermediate stages of alteration (i.e., between rock and soil) as summarized by Righi and Meunier (1995). These observations would lead one to beheve that celadonite can be formed in terrestrial environments. If so, celadonite is not entirely restricted to relatively low-temperature hydrothermal formations in marine environments. It does not, apparently, form a significant mixed layer mineral series with smectite minerals as do glauconite and ilhte. 

Figure 6 Representation of chemical compositions of potassic, low-temperature micas in space. The poles represent feldspar, dioctahedral clays, and trioctahedral clays, respectively. M = Na, Ca, and especially K ions, R = Al, Fe R = Fe Mg. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite), smectite, and mixed layer mica/smec-tites are indicated. Initial materials are kaolinite (kaol) and iron oxides. A second step is the production of an iron-aluminous smectite and then the formation of either illite via an iUite/smectite mixed layer mineral or glauconite via a glauconite mica/iron-smectite mixed layer phase.

Formation of nontronite from pelletal freshwater sedimentary material recalls the formation of berthierine or glauconite, but in a freshwater context (Pedro et al., 1978). Again there seems to be little tendency to form a mixed layer mineral. 

Shallow sediments under conditions of low sedimentation rate can produce either glauconites or berthierine-verdine minerals. Given the miner-alogical information presented above, one can think of the glauconite/ferric-aluminous smectite mixed layered mineral series culminating in the formation of the micaeous (nonexpanding) mineral glauconite and the berthierine/smectite series... 

Figure 11 Representation of the evolution of clay pellets in shallow shelf sediment areas according to the oxido-reduction conditions locally present. Lower arrow shows berthierine formation through reduction of iron, shifting the pellet composition from the ferric (R = Fe ) pole to the ferrous pole (R = Fe ). This reaction passes through a chemical evolution by the formation of a berthierine/smectite mixed layer mineral (chi in the figure). The arrow towards glauconite indicates the change in composition with increase in potassium and some reduction of ferric iron. The diagram represents feldspar, dioctahedral clays, and trioctahedral clays, respectively. R + = Fe +, R = Al, Fe. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite) and end-member celadonite (Ce) are indicated.

Fig. 8.3. a Correlation of the temperature-dependant clay mineral assemblages in shales and sandstones, Saharan basins b Distribution of mont-morillonites, illites and mixed layers (I/M) within the compositional triangle pyrophyllite -muscovite - celadonite (glauconite)... 

Figure 5.8 (lower figure) shows an example (Paleozoic carbonates, mixed carbonates and siliciclastics of the Gipsdalen Group/Barents Sea). Zone 1 shows mixed layer clay and/or illite Zone 5 shows a glauconitic or feld-spathic sandstone with mica and illite. 

Toler, L. G., and J. Hower, 1959. Determination of mixed-layering in glauconites by index of refraction. Am. Mineralogist 44 1314. 

Mehra and Jackson [1959] showed, for a wide range of soil clays, that the sum of planar sorption surface of expansible 2 1 layer siUcates and the mica unit-cell interlayer surface (measured by K2O content), when corrected to exclude quartz, chlorite, and kaolinite in mixed clays, is constant within the experimental error of about 2 %. Manghnani and Hower [1964] obtained somewhat similar results for glauconites, but the decrease in K2O corresponded to less expansible layer material than indicated by the study of Mehra and Jackson. 

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