Allevardite

Allevardite is one specific mineral name and/or mineral group which should be more closely defined. Essentially this is an ordered, mixed layered mineral, that is one with regularly alternating non-expanding and expandable layers. The major character of these minerals is the... 

Although the type mineral allevardite is sodi-potassic the term is... 

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. 

Figure 9. Phases found between the compositions muscovite (Mu)— pyrophyllite (Py) at 2Kb pressure (after Velde, 1969). M = mica (tending to an illite-like phase) ML = random mixed layered phase All = allevardite-like phase Mo = fully expandable phase Kaol = kaolinite ...

If we now consider the bulk compositions of the mixed-layered minerals which contain both expandable and non-expandable layers, two series are apparent, one between theoretical beidellite and illite and one between theoretical montmorillonite and illite (Figure 25). The intersection of the lines joining muscovite-montmorillonite and beidellite-celadonite (i.e., expandable mineral to mica), is a point which delimits, roughly, the apparent compositional fields of the two montmorillonite-illite compositional trends for the natural mixed layered minerals (Figure 26). That is, the natural minerals appear to show a compositional distribution due to solid solutions between each one of the two montmorillonite types and the two mica types—muscovite and celadonite. There is no apparent solid solution between the two highly expandable (80% montmorillonite) beidellitic and montmorillonitic end members. The point of intersection of the theoretical substitutional series beidellite = celadonite and muscovite-montmorillonite is located at about 30-40% expandable layers— 70-60% illite. This interlayering is similar to the "mineral" allevardite as defined previously. It appears that as the expandability of the mixed... 

Ca and Mg is inversed for vermiculite and montmorillonite (Levy and Shainberg, 1972). Further, the natural mica-beidellite interlayered minerals (rectorite) are sodi-calcic while the mica-montmorillonite minerals (allevardite) are sodi-potassic. Quite possibly, the site of charge imbalance changes the selectivity coefficients for exchangeable ions. The montmorillonite series of interlayering will produce illite and the beidel-litic series could lead to a paragonitic or possibly calcic mica. 

P-T space of the reaction mixed layered --- allevardite + phyllosilicate... 

If we look back to the experimental studies on natural expandable minerals at high pressures, it can be recalled that the production of a chlorite-phase occurred when interlayering in the natural dioctahedral mineral had reached about 30% interlayering. It is possible that below this transition only expandable phases are present for most magnesium-iron compositions one is dioctahedral, the other would be trioctahedral. Thus, at temperatures below the transition to an ordered allevardite-type phase, dioctahedral mixed layered minerals will coexist with expandable chlorites or vermiculites as well as kaolinite. The distinction between these two phases is very difficult because both respond in about the same manner when glycollated. There can also be interlayering in both di- and... 

The data of van Moort (1971), Perry and Hower (1970), and Hower, t al., (1976) suggest such a conclusion in that chlorite content appears to decrease until the allevardite zone is reached where chlorite content begins to increase. 

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Rocks in the Assejmi core indicate that the allevardite transition occurs at 3Km depth and 150°C. However, these rocks contain the assemblage allevardite + pyrophyllite + chloritoid. The latter phase is typical of metamorphic rocks. Obviously the rocks had a more complex history than is apparent. Other deep cores, up to 5Km in depth reported by Dunoyer de Segonzac (1970) Perry and Hower (1970, 1972) Iijima (1970) and Hower, et al , (1976) show no signs of destabilization of allevardite at temperatures as high as 180°C. 

Martin Vivaldi and MacEwan, 1960) is a rough equivalent of rectorite or allevardite-like minerals. However, corrensite is less frequent in sedimentary rocks and sediments than the dioctahedral mixed layered minerals. It is possible that corrensite has been overlooked and is more common in argillaceous rocks than so far reported. It is nevertheless evident that corrensite occurs in a variety of geologic environments. 

Two phase assemblages of any of these minerals are known. It should be noted that aluminous phases, such as kaolinite, have never been reported with corrensite neither are sedimentary phyllosilicates such as 7 8 chlorite or glauconite. Non-phyllosilicates in association with corrensite frequently include diagenetic quartz, albite and dolomite. Pelitic rocks, specially associated with those containing corrensite, contain allevardite and fully expanding montmorillonite (dioctahedral). 

The available data indicate that, in shaley layers, petroleum or petroleum-like substances will be formed near the temperature of montmorillonite instability. At great depths, allevardite or the regular... 

ML = mixed layered mica-montmorillonite All = allevardite-like phase ... 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

In zone II, that of normal mixed layered dioctahedral minerals, there are few characteristic mineral reactions. However, the change of the interstratified material as it becomes "allevardite-type" mineral, i.e., showing a discrete super-lattice reflection, is undoubtedly complex. 

Zone 111 is defined by the presence of an ordered mixed layered dioctahedral mineral which has an obvious superlattice reflection. Mixed layered proportions vary from 50% to 25% expandable material. The mixed layered phase is called here "allevardite-Iike". Indications from studies on deeply buried and shallow rocks suggest that as pressure increases, the mixed layer superlattice reflection appears at lower temperature. 

DUNOYER DE SEGONZAC (G.) and HEDDEBAUT (C.), 1971. Paleozoique anchi-metamorphism a illite, chlorite, pyrophyllite, allevardite et paragonite dans les Pyrenees basques. Bull. Carte ser. Geol. Als. Lorr. 24, 277-90. 

Brindley, G.W., 1956. Allevardite, a swelling double layer mica mineral. Am. Mineralogist, 41 91 — 103. 

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