Experimental Formation of Chlorites from Montmorillonite1

Chlorites and hydroxy-interlayered vermiculite and smectite

Summary
The results obtained by the author in the study of clay-minerals diagenesis are compared critically with the principal publications in this field, giving a general picture of the transformation of sheet silicates.

Kaolinite minerals are related to the surficial zones of the earth's crust where they are formed. They are characterized by the hexacoordination of aluminium. They furnish paleogeographic indications in ancient sediments. During diagenesis they are very sensitive to the geochemical environment, stable in acid conditions, unstable in alkaline conditions. However, the increase in temperature by burial causes their destruction sooner or later. In the transitional zone to metamorphism (anchizone), kaolinite is not present. Only dickite and nacrite can be observed, provided that the environment is acid.

Montmorillonites are hydrated minerals. The rise in temperature and above all in pressure during burial expels water from the interlayers. Concentrated interstitial solutions of diagenesis provide cations which replace molecules of water between the layers. It is an irreversible reaction which produces 14-A minerals (chlorites) or 10-A minerals (illites), passing generally through mixed-layer structures. The lack of montmorillonite is normal in formations which have undergone a marked burial.

Mixed-layers are intermediate stages which occur during degradation by weathering and during aggradation by deep diagenesis. This aggradation is the result of an incorporation of certain cations taken up from interstitial solutions, and of a rearrangement within the lattice. There are two major pathways: a potassium and sodium pathway, which produces the illites, then the micas, passing possibly by regular mixed-layering of the allevardite-rectorite type; and a magnesium pathway, which produces the chlorites, passing possibly by a regular mixed-layering of the corrensite type. These mixed-layers can remain stable until the border of meta-morphism (anchizone).

Micaceous clay minerals or illites form a very heterogenous group in the sediments which have been hardly diagenetized. Particles of diverse origin are found. They become more regular during burial. In deep diagenesis and the anchizone, crystallo-graphic parameters of the illite are sufficiently well defined to serve as a scale of recrystallization, a zoneographic index. The morphology of the particles changes. Polymorphic types 1Md and 1M are replaced by the 2M-type. The sharpness of the 10-A peak, conventionally called “crystallinity”, is an interesting quantitative criterium, together with the intensity ratio of the 5-A and 10-A peaks, which is related to the chemical composition of the octahedral layer.

Micas in low-grade metamorphism, called sericites by petrographers, replace the illites discussed above. They are different from the true micas by a weaker layer charge, less than 0.9 by half-cell. They often contain sodium (paragonitic muscovites and paragonites). The octahedral charge (zero for the muscovite) is generally high, due to the replacement of Al by Fe2+ and Mg (phengites). These transformations should not obscure the fact that metamorphism is also accompanied by crystalline growth and massive neoformation.

Chlorites are the least well-known clay minerals in diagenesis. Detrital particles can be aggraded to chlorite during early diagenesis by passing through the mixed-layer stage of corrensite. A massive growth of chlorite is observed in late diagenesis and the anchizone. Illite and chlorite slates give place to sericite and chlorite schists. At present, general data are not available on the crystal chemistry of chlorites in the anchizone and the greenschist facies.

The stages in the diagenetic evolution of clay minerals are too little understood to be able to give them precise limits. However, the following provisional scheme can be proposed:

(1) Early diagenesis (= “diagenesis” of Russian authors; = the “shallow-burial stage” of Muller, 1967a). In this stage all the clay minerals are stable; some undergo aggradation by adsorption of Mg, K and Na (various mixed-layers); some are neoformed (montmorillonites).

(2) Middle diagenesis (= “early catagenesis or epigenesis” of Russian authors; the “deep-burial stage” of Muller, 1967a, includes this stage and all the following until metamorphism). In this stage the sediment becomes compact. It has lost at least 50% of its connate water. Porosity is high and circulation still plays an essential part. Some detrital minerals, such as biotite, are unstable. All the clay minerals are still stable, but many types of replacement take place, due to interstitial circulation. Dickitization of kaolinite and illitization of montmorillonite can already be observed.

(3) Deep or late diagenesis (= “late catagenesis or epigenesis” of Russian authors). In this stage the temperature is greater than 100 °C, pressure increases and porosity becomes very weak. Montmorillonites and irregular mixed-layers disappear. Kaolinite recrystallizes as dickite in acid environment. These changes are irreversible.

(4) Anchizone (= “metagenesis” of Russian authors; = “zone anchimetamorphique” of Kubler, 1964). This is the transitional zone to metamorphism. It agrees with temperatures around 200 °C. Illite and chlorite are almost the only sheet silicates. However, dickite can be observed as well as pyrophyllite generally associated with allevardite. The crystallographic parameters of illite define the limit of the following zone, the metamorphic epizone or greenschist facies.

The crystallochemical processes that take place during the diagenetic evolution of clay minerals are schematically the following:

(1) Gradual tetracoordination of aluminium.

(2) Filling of octahedral sites either by interlayer cations, either by cations derived from outside the lattice, without the distinction dioctahedral-trioctahedral becoming very clear.

(3) Interlayer exchange between crystal lattice and interstitial solution. Gradual closing of the layers by alkaline cations or octahedral brucite-like sheets.

(4) Massive crystalline growth in the anchizone and the epizone.

These processes are roughly symmetrical with those which occur during weathering.

This review is a summary of the conclusions drawn in a Docteur-es-Sciences thesis (G. Dunoyer de Segonzac, 1969: Les Mineraux argileux dans la Diagenese. Passage au Metamorphisme, 339 p., 45 tables, 110 illus.) to be published as part of the series Memoires du Service de la Carte Geologique d'Alsace et de Lorraine. Most of the evidence on which these conclusions have been based is not cited directly in this article, but can be found in the thesis mentioned above, to which the reader is referred.

The transformation of clay minerals during diagenesis and low-grade metamorphism: a review

A stable porous system consisting of montmorillonite cross-linked by Al-hydroxide oligomers was synthesized by reacting at room temperature an aqueous solution of such oligomers with a unit-layer dispersion of montmorillonite. The resulting cross-linked montmorillonite (Al-CLM) is a nonswelling material, showing basal spacings of 14.4 to 18.8 A after air drying and between 14.2 to 18.0 A after treatment at 110°C. The basal spacing is found to depend on the age and OH/Al ratio of the Al-hydroxide solution, as well as on the relative amounts of the two reactants. A specific surface area of 160 m2/g and a diffraction pattern with a dominant basal spacing of 17.5 A is obtained by using Al-hydroxide with OH/Al = 1.85, aged for at least 5 days, and by applying an Al/montmorillonite ratio greater than 1.5 in the cross-linking process. The basal spacing of Al-CLM remains essentially unchanged after heating at 220°C, while the specific surface area is not affected by heat treatment up to 480°C. Two possible configurations of Al-hydroxide oligomers, homogeneously distributed between parallel montmorillonite unit-layers, were considered in order to account for the basal spacing of 17.5–18.8 A, viz. (a) stacking of two oligomeric ring units in parallel orientation relative to the clay lamellae and (b) perpendicular orientation of individual oligomeric units.

Cross-linked smectites; I, Synthesis and properties of hydroxy-aluminum-montmorillonite

Vermiculites and smectites in soils and sediments are frequently partially interlayered or “chloritized”. Dioctahedral expansible layer silicates are those most frequently interlayered, and hydroxy-Al appears to be the principal component of the non-exchangeable interlayer material. The most favorable soil conditions for interlayer formation appear to be: moderate pH (4·6–5·8), frequent wetting and drying cycles, and low organic matter content. In marine sediments, hydroxy-Mg interlayering may be significant. Soil-derived clays containing partially filled hydroxy-Al “brucite” sheets may be filled out with hydroxy-Mg. Under reducing conditions, hydroxy-Fe interlayers may be important. Depending on the OH/Al ratio and Al content of hydroxy-Al interlayers, expansible layer silicate may either promote or retard the formation of gibbsite. Interlayered expansible layer silicates also may be precursors to kaolinite.

Hydroxy Interlayers in Expansible Layer Silicates

The geothermal area on Reykjanes, Iceland has been investigated mineralogically. The temperature within the studied area is very variable from 30–300° C. Mineral zones corresponding to the temperature conditions in the area are found. Accidental changes in the geothermal system are also reflected in the mineralogy by formation of anhydrite. Changes in temperature conditions in the field are indicated by epidote occurrence at 40° C and retrograd formation of montmorillonite.

Experimental Formation of Chlorites from Montmorillonite1

Citations

Chlorites and hydroxy-interlayered vermiculite and smectite

The transformation of clay minerals during diagenesis and low-grade metamorphism: a review

Cross-linked smectites; I, Synthesis and properties of hydroxy-aluminum-montmorillonite

Hydroxy Interlayers in Expansible Layer Silicates

High temperature alteration minerals and thermal brines, Reykjanes, Iceland

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Hydroxy Interlayers in Expansible Layer Silicates

Aluminum in Interlayers of Vermiculite1

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Interlayering of Expansible Layer Silicates in Soils by Chemical Weathering

Preparation and Properties of Some Hydroxy-Aluminium Beidellites