Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Halloysite clay minerals — a review

The Noachian terrain west of the Isidis basin hosts a diverse collection of alteration minerals in rocks comprising varied geomorphic units within a 100,000 km2 region in and near the Nili Fossae. Prior investigations in this region by the Observatoire pour l'Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) instrument on Mars Express revealed large exposures of both mafic minerals and iron magnesium phyllosilicates in stratigraphic context. Expanding on the discoveries of OMEGA, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) has found more spatially widespread and mineralogically diverse alteration minerals than previously realized, which represent multiple aqueous environments. Using CRISM near-infrared spectral data, we detail the basis for identification of iron and magnesium smectites (including both nontronite and more Mg-rich varieties), chlorite, prehnite, serpentine, kaolinite, potassium mica (illite or muscovite), hydrated (opaline) silica, the sodium zeolite analcime, and magnesium carbonate. The detection of serpentine and analcime on Mars is reported here for the first time. We detail the geomorphic context of these minerals using data from high-resolution imagers onboard MRO in conjunction with CRISM. We find that the distribution of alteration minerals is not homogeneous; rather, they occur in provinces with distinctive assemblages of alteration minerals. Key findings are (1) a distinctive stratigraphy, in and around the Nili Fossae, of kaolinite and magnesium carbonate in bedrock units always overlying Fe/Mg smectites and (2) evidence for mineral phases and assemblages indicative of low-grade metamorphic or hydrothermal aqueous alteration in cratered terrains. The alteration minerals around the Nili Fossae are more typical of those resulting from neutral to alkaline conditions rather than acidic conditions, which appear to have dominated much of Mars. Moreover, the mineralogic diversity and geologic context of alteration minerals found in the region around the Nili Fossae indicates several episodes of aqueous activity in multiple distinct environments.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009JE003339

Identification of hydrated silicate minerals on Mars using MRO‐CRISM: Geologic context near Nili Fossae and implications for aqueous alteration

Physical chemistry of soil potassium

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

The seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure, X ray diffraction techniques, and electron microprobe backscatter (BSE) imaging. The velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding. Anisotropy, at elevated pressures caused mainly by preferred orientation of clays (illite) parallel to bedding, ranges from 20% (Vp) and 19% (Vs) for a sample of New Albany Shale to 30% (Vp) and 35% (Vs) for a sample of Chattanooga Shale. The degree of clay mineral alignment in the shales is constrained by “orientation indices” produced using simple X ray diffraction techniques. A strong positive correlation is found between the degree of preferred orientation, as expressed in the orientation indices, and seismic anisotropy. BSE images of the shale fabrics confirm in a qualitative manner the results of the X ray study. To investigate wave propagation in the shales, elastic constants of each sample are calculated and used to produce phase and group (wave) velocity surfaces, which describe variation in velocity as a function of angle to the bedding normal. The calculated velocity surfaces, constrained by independent velocity measurements, display a lack of shear wave splitting at “near-normal incidence” in even the most anisotropic shales. For the highly anisotropic Chattanooga shales, group velocity surfaces differ significantly from corresponding phase velocity surfaces.

Seismic anisotropy of shales

Desilication and removal of bases from the basalitic rocks of the Hawaiian Islands produces large amounts of halloysite and gibbsite, together with variable quantities of allophane and nontronite, iron and titanium oxides, and amorphous mineral and gel material.

The nature and relative abundance of intermediate and end products of weathering depend primarily upon the amount of rainfall, angle of slope, and texture of the rock. The formation of clay minerals is the common step in the transformation of primary silicates to oxides and hydroxides, but the amount and relative importance of the clay “stage” depend on the intensity of the weathering process.

Halloysite forms from plagioclase usually by alteration of first the core and subsequently the rim of the feldspar laths. No kaolinite was observed, its absence being attributed to the absence of fine-grained mica as an intermediate weathering product.

Gibbsite is produced by (1) removal of silica from halloysite, (2) dehydration of Al-gel, and (3) precipitation from solution. Although it is possible that the mineral may form directly from feldspar, halloysite is the common crystalline intermediate on both the megascopic and microscopic scales.

An amorphous transition state, probably ranging in composition from allophane to Al-gel, exists as part of the change from halloysite to gibbsite as evidenced by electron microscope and diffraction work on pseudomorphs after halloysite tubes found in certain samples studied in more detail than others.

Volcanic glass is the apparent source of most Al-Fe-Si gel material which upon dehydration becomes allophane, cliachite, or gibbsite depending on the kind and relative proportion of the cations present.

Halloysite and gibbsite formation in hawaii1

Desilication and removal of bases from the basalitic rocks of the Hawaiian Islands produces large amounts of halloysite and gibbsite, together with variable quantities of allophane and nontronite, iron and titanium oxides, and amorphous mineral and gel material The nature and relative abundance of intermediate and end products of weathering depend primarily upon the amount of rainfall, angle of slope, and texture of the rock The formation of clay minerals is the common step in the transformation of primary silicates to oxides and hydroxides, but the amount and relative importance of the clay “stage” depend on the intensity of the weathering process Halloysite forms from plagioclase usually by alteration of first the core and subsequently the rim of the feldspar laths No kaolinite was observed, its absence being attributed to the absence of fine-grained mica as an intermediate weathering product Gibbsite is produced by (1) removal of silica from halloysite, (2) dehydration of Al-gel, and (3) precipitation from solution Although it is possible that the mineral may form directly from feldspar, halloysite is the common crystalline intermediate on both the megascopie and microscopic scales An amorphous transition state, probably ranging in composition from allophane to Al-gel, exists as part of the change from halloysite to gibbsite as evidenced by electron microscope and diffraction work on pseudomorphs after halloysite tubes found in certain samples studied in more detail than others Volcanic glass is the apparent source of most Al-Fe-Si gel material which upon dehydration becomes allophane, cliachite, or gibbsite depending on the kind and relative proportion of the cations present

Halloysite and Gibbsite Formation in Hawaii1

Morphology and crystal chemistry of 1:1 layer lattice silicates

Electron microscope studies of chrysotile show that tubes are present in bulk specimens and that these tubes commonly have fuzzy, amorphous-looking material on both the inside and outside. Similar material is associated with synthetic chrysotile and has been noted previously in halloysite specimens. The existence of such material between and within the tubes, together with apparent irregularities in size, shape and packing of tubes, explains the apparent discrepancy between the measured density of bulk samples and the calculated density of a hypothetical sample consisting of close-packed, regular, hollow capillaries. Replicas of fractured surfaces of halloysite (2H2O) from various localities reveal that the particles occur as curved to flat laths commonly possessing “hexagonal” terminations and surface features indicative of a higher degree of crystallinity than tubes of halloysite (4H2O). It is suggested that a complete morphological series from plates through laths to tubes exists both in platy to fibrous serpentine and in kaolinite to halloysite (4H2O). In each series a number of structural varieties are to be expected between the morphologically distinct “end members.”

Further Observations on the Morphology of Chrysotile and Halloysite1

Improvements in replica techniques have made possible the high magnification study of textural characteristics and surface features of clay aggregrates found either in nature or in the laboratory. The most successful method of sample preparation involves pre-shadowing the specimen with platinum and backing this with a layer of carbon prior to removal of the clay with a suitable solvent.

Thomas F. Bates

Papers

Halloysite and gibbsite formation in hawaii1

Halloysite and Gibbsite Formation in Hawaii1

Morphology and crystal chemistry of 1:1 layer lattice silicates

Further Observations on the Morphology of Chrysotile and Halloysite1

Electron Microscopy of Clay Surfaces