Allevardite-type ordering

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 proportion of the illitic layers in the I/M increases with depth and the unordered phases are transformed into ordered phases of the allevardite type (Fig. 8.1). This transition is easily recognized in diffractograms of samples saturated with ethylene-glycol by the disappearance of the 17-A peak and the appearance of a peak at 13-14 A indicative of a structural ordering in short chains (Fig. 8.2). Such a phase transformation has been initiated at various levels in the Oued el-Mya Basin at depths of 2 300-2 500 m. The transformation of I/M of the allevardite type into those of the kalkbergite type with >80% illitic layers in the lattice takes place at a depth of 4.3-4.S km in the Paleozoic sediments. 

Corrensite, a mixed-layer mineral of the chlorite-montmorillonite type with an ordered structure, occurs at several levels within the Triassic Basin and in particular in the area of the Hassi R Mel deposit (Plate 15). Corrensite is a highly useful geothermal indicator in sediments (Porrenga 1967 Kiibler 1973)- In the area mentioned it starts to appear at a depth of 2.1 km and remains stable down to 2.3 km. The maximum temperatures reached were reconstructed on the basis of the appearance or disappearance of allevardite, kalkbergite and corrensite mixed-layer minerals (Fig. 8.2). Min-eralogical and crystallochemical analyses of mixed-layer clay minerals reveal the pro-... 

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