Ordered mixed layered minerals

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... 

Figure 18. Schematic representation of several possible types of solid solution. Shaded and blank layers represent expanding and mica-like units (2 1 structures). Solid and unfilled circles represent two species of interlayer ions, a totally random in all aspects b = interlayer ion ordering, single phase montmorillonite c = ordered interlayer ions which result in a two-phase mica structure, two phases present d = randomly interstratified mineral, one phase e = regular interstratification of the 2 1 layers giving an ordered mixed layered mineral, one phase present f = ordered mixed layered mineral in both the interlayer ion sites and the 2 1 interlayering. This would probably be called a single phase mineral.

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

The apparent discrepancy could reside in the fact that if potassium ions are available at all, they will form a mica at temperatures near 100°C. Montmorillonite structures below these conditions (pressure and temperature) need not contain potassium at all. However, at the correct physical conditions the 2 1 portion of the montmorillonite must change greatly (increase of total charge on the 2 1 unit) in order to form a mica unit in a mixed layered mineral phase. Since neither Na nor Ca ions will form mica at this temperature, potassium will be selectively taken from solution. Obviously this does not occur below 100°C since cation exchange on montmorillonites shows the reverse effect, i.e., concentration of calcium ions in the interlayer sites. If potassium is not available either In coexisting solids or in solutions, the sodi-calcic montmorillonite will undoubtedly persist well above 100°C. 

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... 

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. 

Mixed-Layer Minerals. In addition to polymorphism resulting front the disordering and proxying of one element for another, clay minerals exhibit ordered and random intercalation sandwiches with one another. 

Complete unmixing of the mixed-layered minerals requires temperatures on the order of 400°C or a relatively high degree of metamorphism (phengite -> muscovite + chlorite). Velde (1964), Weaver (1965), Raman and Jackson (1966), and Weaver and Beck (1971a) have presented data to indicate chloritic layers are present in most, if not all, 10A illites. Based on chemical data, Raman and Jackson concluded that the illites they had examined contained 20—29% chlorite layers. Weaver and Beck believe that most of the chloritic layers are dioctahedral. 

The clay mineral spectrum is notably less differentiated than in the other facies, the dominant minerals being trioctahedral chlorites and dioctahedral illites. In the chlorite structure, non-swelling layers predominate whereas the alternation of layers of different types exhibits a trend towards ordering. The proportion of mixed-layer minerals of the iUite-montmorillonite type decreases especially as one approaches the massive layers of rock salt. 

Reconstruction of Temperatures from Degree of Structural Ordering in Mixed-Layer Minerals... 

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-... 

Mixed layered clays, most often ordered, are present up to temperatures near 200°C at depths of 500 to 1500 meters. The minerals form in two distinct zones. At shallow depths (between 100 and 200°C) mixed layering is between 90 and 0% montmorillonite. Above 200°C or so no expandable minerals are present. In the second zone (1.5Km. depth) one finds ordered interlayering showing the superstructure reflection... 

Figure 28. Depth-temperature plot of natural mineral assemblages for the fully expandable phases (Mo), random and ordered 30-80% mixed layered (ML) and superstructured, ordered 30-20% mixed layered (All) minerals. Data from Steiner (1968), S Muffler and White (1969), M Perry and Hower (1970, 1972), P Iijima (1970), I Browne and Ellis (1970), B Dunoyer de Segon-zac (1969), D and Weaver and Beck (1971), W. I-C illite, chlorite paragenesis. Tertiary or younger sediments are represented in these studies.

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. 

Hower, J., 1967. Order of mixed-layering in illite/montmorillonites. Clays Clay Miner., Proc., 15 63-74. 

For samples taken up-dip of the Setif and Medjounes areas the values range from +2.9 to -5.8%o (Tables 7.2,7.3 Fig. 7.2). The light carbon makes itself felt in the carbonates where we find traces of recrystallization or of other mineralogical neoformations and in particular the appearance of mixed-layer clay minerals with perfectly ordered crystal structures. In the same samples microfissures are filled by secondary calcite or dolomite with detrital carbonate cement frequently being present between the crystals. In other cases we observe entire fields of neoformed minerals (dolomite... 

Fig. 8.2. Paleotemperature gradients as determined by clay mineral geothermometers and present-day thermal gradient from uncorrected borehole temperatures. Clay mineral temperatures were determined from changes in ordering and composition of mixed-layer illite-montmorillonite (I/M) clay and the appearance of corrensite. (TTiassic province, except pyro-phillite which was largely found in deeply buried Lower Paleozoic shales of Timimoune, Reggane and Tindouf basins). Depths are only of indicative signification...

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