Montmorillonite Mixed layered minerals

Figure 31b indicates the compositional spread of chlorites from six rocks in the illite-montmorillonite mixed layered mineral facies and from the illite-chlorite zone in the French Alps (Velde, unpublished). The grains analyzed with the microprobe are chlorites replacing isolated grains of detrital mica or were newly formed grains. They are usually 15 microns in the smallest dimension. 

VELDE (B.) in press. A proposed diagram for illite, corrensite and illite-montmorillonite mixed layered minerals. 

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

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.

Figure 26. Compositional fields of natural mixed layered minerals compared to theoretical end-members (shaded area). Mu = muscovite B = beidellite Mo = montmorillonite Ce = celadonite.

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... 

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. 

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. 

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. 

Figure 32. Results of experiments on natrual minerals are schematically shown in Mr3-2R -3R coordinates. Kaol = kaolinite ML j = mixed layered beidellitlc mineral MLj, = mixed layered montmorillonitic mineral I = illite compositional field chi = chlorite Exp3 trioctahedral expandable-chlorite mixed layered mineral (expanding chlorite and corrensite).

Kaol = kaolinite Mo = beidelitic montmorillonite M 38 muscovite (or illite) ML = mixed layered minerals Anal = analcite Nat = natrolite Ze = alkali zeolites. Alkali zeolite tie-lines for specific species are not given, j) less than 100°C. 

The second division of the zeolite facies is based upon the appearance of albite as a diagenetic mineral, usually coexisting with analcite in the initial stages of its development, and also with the widespread development of montmorillonite-illite mixed layered mineral (30 to 90% expandable layers) coexisting with illite. The phase relations of this facies are indicated by Figure 35b. Assemblages can contain natrolite as above. They are ... 

Figure 46a. Development of a portion of the system K-Al-Si as an increasing number of the chemical components become intensive variables of a given system. F = feldspar Mi = mica G = gibbsite Kaol = kaolinite Q -quartz Mo = montmorillonite ML = mixed layered mineral. All chemical components are extensive variables.

If we consider three components, the phases will be arranged as in Figure 48a at conditions of initial burial. The solid solution series are somewhat abbreviated for simplicity. The phase relations are dominated by fully expanding and mixed layered minerals which cover a large portion of the compositional surface. Notably two dioctahedral expandable minerals exist as does a large undefined series of trioctahedral phases designated as expanding chlorite, vermiculite and trioctahedral montmorillonite. 

Sudo, T. and Kodama, H., 1957. An aluminian mixed-layer mineral of montmorillonite-chlorite. Z. Krist., 109 380-387. 

In addition to swelling chlorite (vermiculite) there is also in association with mixed-layer minerals of the ilUte-montmorillonite type (sometimes Fe-bearing) a chlorite that is unstable on thermal treatment. The diffractogram of oriented samples is char-... 

Iron-Bearing lllite. This hydromica mineral of the type green mica or Fe-illite is present frequently in variable quantities in the clayey fraction of the different rock types of the saline complex where it is part of the paragenetic association Mg-chlo-rite -I- mixed-layer chlorite mixed-layer mineral of illite-montmorillonite type. The Fe-illites were noted in dolomitized sandstones and dolomites as well as in the argillaceous intercalations and argillaceous siltstones. 

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. 

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-... 

Argillaceous Rocks. As a general rule, solid mineralogical criteria for the increasing degree of mineral maturity include (1) the transformation of montmorillonite to illite via a sequence of mixed-layer minerals of the montmorillonite-illite type, (2) the appearance of chlorite, and (3) the disappearance of potassic feldspars by decomposition. These mineral transformations may be described by the following reactions ... 

Hendricks, S. B, and R. S Dyal. 1952. Formation of mixed layer minerals by potassium fixation in montmorillonite. Soil Science Society of America Journal 16, no. 1 45-48. doi 10.2136/sssajl952.03615995001600010014x. 

Heystek, H., 1956. Vermiculite as a member in mixed-layer minerals. Clays Clay Min., 4 429. Hofmann, U., A. Weiss, G. Koch, A. Mehler, and A. Scholz, 1956. Intracrystalline swelling, cation exchange, and anion exchange of minerals of the montmorillonite group and of kaolinite. Clays Clay Min. Pub. 456 Nat. Acad. Sci.— Nat. Res. Counc. 273. 

Interstratified or mixed-layer minerals represent a special case of intergrowths. The simplest case is that in which there exist layers that are more or less hydrated, and this is a case that occurs frequently in clay minerals [Figure 1(a)]. Montmorillonites and vermiculites are, for example, essentially units of hydrated mica that is to say, they contain between their lamellae water molecules. For this reason, it may be supposed a priori that the most frequent type of interstratification will be that of mica-montmorillonite and mica-vermiculite. 

Dahl [1965] has studied Permian bentonites from Texas and found illite-montmorillonite mixed layers, as well as mixtures of these minerals with chlorite. A study has been made of illites and mixed-layer illite-montmorillonites (in Montana, Illinois, Indiana, Colorado, Oklahoma) by Hower and Mowatt [1966]. These authors conclude that the nonexpandable layers are of lower change than true mica and contain lenses of trapped water. 

In sandstone from the river Nionzi-Lubunzi in Lower Congo, Vanderstappen and CORMIL [1958] have found what they consider to be a complicated interstratification of mica, hydromica, and montmorillonite in equal proportions. Kobayashi and Oinuma [1961] have found random mixed-layer minerals together with montmorillonite, chlorite, and illite in tertiary sediments of the Chichibu basin of Saitama Prefecture, Japan. 

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