Up until about 2005, the main application of halloysites had been as an alternative raw material to kaolinite for ceramics. Since then, however, there has been an exponential increase in studies aimed at applications of halloysite nanotubes, now widely referred to as HNTs. The readily available and relatively cheap nanotubular forms of halloysite have potential uses in nanocomposites with polymers, as carriers for active agents, e.g. in medicine, agriculture, cosmetics and environmental remediation, as well as in nanotemplating, as supports for catalyst immobilization and as heterogeneous catalysts.

Halloysite with tubular morphology is formed in a wide range of geological environments from the alteration of various rock types. Intrusive acidic coarse-grained rocks, such as granites, pegmatites and anorthosite, with large potash and sodic feldspars contents, are subsequently altered to kaolinite, halloysite and other clay minerals by weathering or shallow hydrothermal fluid activity. Processing to separate the halloysite-kaolinite fraction from the altered host rock provides a product which can be used as a paper filler and in ceramics and fibreglass, among other uses, with various deposits in Brazil, China, Thailand and elsewhere. In the Kerikeri-Matauri Bay district of Northland, North Island, New Zealand, volcanic alkali rhyolite was extruded as domes and cooled rapidly with fine-grained feldspar subsequently altered to halloysite. The IMERYS plant in Matauri Bay separates the clay from the quartz-cristobalite matrix with an ∼20% yield of halloysite. The principal market is for high-quality porcelain and bone china that require low levels of Fe2O3 and TiO2. Deposits with high levels of halloysite occur in China, Turkey and the USA. The Dragon mine in Utah, USAwas recently reopened by Applied Minerals Inc. and now produces halloysite from zones of up to 100% white halloysite. Smaller occurrences of tubular halloysite are mined in China, Turkey and elsewhere from masses of comparatively pure clay that appear to have crystallized directly from solutions in which Al and Si were soluble.

Twenty one samples of relatively pure tubular halloysites (HNTs) from localities in Australia, China, New Zealand, Scotland, Turkey and the USA have been investigated by X-ray diffraction (XRD), infrared spectroscopy (IR) and electron microscopy. The halloysites occur in cylindrical tubular forms with circular or elliptical cross sections and curved layers and also as prismatic tubular forms with polygonal cross sections and flat faces. Measurements of particle size indicate a range from 40 to 12,700 nm for tube lengths and from 20 to 600 nm for diameters. Size distributions are positively skewed with mean lengths ranging from 170 to 950 nm and mean diameters from 50 to 160 nm. Cylindrical tubes are systematically smaller than prismatic ones. Features related to order/ disorder in XRD patterns e.g. as measured by a ‘cylindrical/prismatic’ (CP) index and IR spectra as measured by an ‘OH-stretching band ratio’ are related to the proportions of cylindrical vs. prismatic tubes and correlated with other physical measurements such as specific surface area and cation exchange capacity. The relationships of size to geometric form, along with evidence for the existence of the prismatic form in the hydrated state and the same 2M1 stacking sequence irrespective of hydration state (i.e. 10 vs. 7 Å) or form, suggests that prismatic halloysites are the result of continued growth of cylindrical forms.

Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3 = 5.2 wt.%) than tubes (Fe2O3 = 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10–20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite.

The cation exchange capacity (CEC) of seven well characterized halloysite nanotubes (HNTs) in the dehydrated 7 Å form has been measured using a method based on cobalt hexammine exchange. In addition to unbuffered measurements, which varied between 2.9 and 9.3 cmol(+)kg−1, CECs were also determined over a wide pH range and proton titration measurements were conducted on two samples. The data were fitted using a constant capacitance model based on the presence of two sites: permanently charged sites and pH-dependent variable charged sites. Normalization of CEC to the average specific surface area (BET) of the halloysite samples reduces considerably the variation of CEC values for the different samples particularly over the intermediate pH range (5–9) with the average value at pH 7 equal to 8.5 cmol(+)kg−1 and a standard deviation of 1.17. Overall the CEC behaviour of the seven samples appears reasonably consistent throughout the set. Calculations based on proton titrations suggest a ratio of variable charge to basal sites for the dehydrated halloysite nanotubes of ∼3:1.

The present study investigated the efficiency and mechanisms of aqueous Pb(II), Cd(II), Zn(II) and As(V) adsorption on natural (H), calcined (HC), and acid-activated halloysite (HA). The XRD and FTIR measurements indicated that the aluminosilicate framework was not affected by high-temperature treatment, in contrast to acid activation, which led to structural changes mainly in the tetrahedral sheet. The sorption of cations on H sample was low, though it was most effective for As(V). The X-ray photoelectron spectroscopy results suggested that removal of As(V) might be related to its reduction to As(III) involving oxidation of Fe(II) present in the mineral structure and/or iron minerals. The calcination enhanced halloysite sorption capacity for cations, while the As(V) sorption decreased. This was due to partial dehydroxylation and the subsequent formation of additional active sites. The acid treatment induced selective adsorption of Pb(II).

New insights from the recent literature are summarized and new data presented concerning the formation, structure and morphology of halloysite. Halloysite formation by weathering always requires the presence of water. Where substantial drying occurs, kaolinite is formed instead. Halloysite formation is favoured by a low pH. The octahedral sheet is positively charged at pH < ∼8, whereas the tetrahedral sheet is negatively charged at pH > ∼2. The opposing sheet charge would facilitate interlayer uptake of H2O molecules. When halloysite intercalates certain polar organic molecules, additional (hkl) reflections appear in the X-ray diffraction pattern, suggesting layer re-arrangement which, however, is dissimilar to that in kaolinite. Associated oxides and oxyhydroxides of Fe and Mn may limit the growth of halloysite particles as does incorporation of Fe into the structure. Particles of different shape and Fe content may occur within a given sample of halloysite.

Tubular halloysite has many applications as a nanomaterial. Spheroidal halloysite (SPH) is the other most common form of halloysite. Its mode of formation has had different explanations, including association with allophane, or more generally, following weathering of volcanic glass. Some SPHs have formed from minerals in crystalline rocks, sometimes as an early stage of evolution into plates and/or tubes of halloysite and ultimately to kaolinite. Spheroidal halloysites can show a range of Fe contents and can occur with other forms of halloysite; they have often formed in confined environments whereas tubular halloysites apparently form in more open spaces. They have also formed on microbes or where there is a significant amount of organic matter. Generally, SPHs have often formed by rapid dissolution of volcanic glass and primary minerals. The SPHs can persist over time. They have fewactive edges, so interparticle interaction is poor, causing low viscosities in clay-water suspensions, poor soil stability and low adsorption capacities.

Halloysite clay nanotubes are safe and biocompatible nanomaterials and their application in biomaterials is very promising. The microencapsulation of yeast cells in the shell of clay nanotubes modifying their properties was demonstrated here. Each cell was coated with a 200–300 nm-thick tube shell and this coating was not harmful for these cells’ reproduction. Synthesis of magnetic nanoparticles on the surfaces of the nanotubes allowed for magnetic-field manipulation of the coated cells, including their separation. Providing nano-designed shells for biological cells is a step forward in development of ‘cyborg’ microorganisms combining their intrinsic properties with functions added through nano-engineering.

The present study explores the effect of a magnetically modified halloysite (mHNT) surface on the structure and properties of biodegradable polymer nanocomposites based on poly ɛ-caprolactone (PCL). Halloysite nanotubes (HNTs) have been modified by a scalable and tunable procedure using magnetic Fe oxide particles prepared by microwave-assisted synthesis fromferrous sulfate at high pH. The HNT content in composites prepared in melt varied from 5 to 30 wt.%. Application of magnetically modified HNT to PCL resulted in the formation of soft magnetic materials. Analyses of the nanocomposite structure revealed that both natural and magnetized HNTs, as well as free magnetite particles are dispersed uniformly in the polymer matrix. Investigation of the mechanical and physical properties confirmed that the reinforcing ability of HNTs was not affected by magnetic modification.

Hybrid material based on halloysite nanotubes (HNTs) and sodium perfluorooctanoate (NaPF8) was used as a consolidant for paper treatment. The consolidation efficiency was determined by thermogravimetry as well as by paper grammage determination. Morphological analysis of the treated paper was performed by means of scanning electron microscopy while the effect of modified HNTs on the thermal behaviour of the cellulose fibres was investigated by differential scanning calorimetry which determined the combustion enthalpy of the paper.Water contact angle measurements were performed to study the paper wettability. The physico-chemical properties investigated (mesoscopic structure, thermal stability and wettability) of the treated paper were correlated successfully with the consolidation loading and, consequently, to the affinity between the fluorinated modified HNTs and the fibrous cellulose structure. This study proposes a new green protocol for paper consolidation based on natural tubular nanoparticles with a flame retardant effect.

As a unique tubular nanoclay, halloysite nanotubes (HNTs) have recently attracted significant research attention. The HNTs have outer diameters of ∼50 nm, inner lumens of ∼20 nm and are 200–1000 nm long. They are biocompatible nanomaterials and widely available in nature, which makes them good candidates for application in biomedicine. Compared with other types of nanoparticles such as polymer nanoparticles and carbon nanotubes, the drawbacks associated with HNTs include brittleness, difficulty with fabrication, low fracture strength, high density and inadequate biocompatibility. Preparation of polysaccharide-HNT composites offer a means to overcome these shortcomings. Halloysite nanotubes can be incorporated easily into polysaccharides via solution mixing, such as with chitosan (CS), sodium alginate, cellulose, pectin and amylose, for forming composite films, porous scaffolds or hydrogels. The interfacial interactions, such as electrostatic attraction and hydrogen bonding, between HNTs and the polysaccharides are critical for improvement of the properties. Morphology results show that HNTs are dispersed uniformly in the composites. The mechanical strength and Young's modulus of the composites in both the dry and wet states are enhanced by HNTs and the HNTs can also increase the storage modulus, glass-transition temperature and thermal stability of the composites. Cytocompatibility results demonstrate that the polysaccharide-HNT composites have low cytotoxicity even for HNT loading >80%. Therefore, the polysaccharide-HNT composites show great potential for biomedical applications, e.g. as tissue engineering scaffold materials, wound-dressing materials, drug-delivery carriers, and cell-isolation surfaces.

New drug-delivery systems have remained a challenge for pharmaceutical scientists due to the use of expensive polymers and the low loading capacity of prepared nanoparticles. There is pressure to develop formulations that contain not only cheaper materials but also have controlled-release properties. Halloysite nanotubes (HNTs) are a naturally occurring clay mineral similar to kaolin, possessing a special particle shape in the form of an ultramicroscopic multilayered hollow cylinder. Its uses encompass a wide range in anticancer therapy, sustained- and controlled-release drug-delivery systems, cosmetics, delivery of proteins, vaccines and genes. These advantages are due to its biocompatibility, significant mechanical strength and natural availability. The surfaces of the tubules can be modified by coating different polymers for application in the drug-delivery system. This review is focused on the various aspects of HNTs such as structure, properties, loading methods, applications and characterizations.

There is increasing research interest in potential applications of halloysite as fillers for polymer composites, controlled drug delivery, carriers for the supply and sustained release of active agents for anticorrosion coatings, in nanoreactors or nanotemplates, and for the uptake of contaminants or pollutants and support for catalysts. In this review, recent findings in terms of the prospects and challenges of using halloysite nanotubes (HNTs) in different polymeric matrices targeting a range of old and new applications are discussed and evaluated. The compositions include chitosan/halloysite membranes as bone-tissue scaffolds, polylactic acid (PLA)/halloysite membranes for food-packaging applications and their antimicrobial activities, instrumented impact properties of epoxy/halloysite nanocomposites and the role of halloysite in the self-healing of epoxy composites, polyacryronitrile (PAN)/halloysite membranes for use in water filtration as well as a review of some recent applications of halloysite/alginate beads in the adsorption of contaminants such as lead.

The effects of increasing nickel contamination of soil on the update of selected microelements by Brassica juncea L. in the presence of raw halloysite (RH) and halloysite modified by thermal treatment (calcination) at 650°C (MH) were investigated experimentally. Such treatment causes partial dehydroxylation and enhances mineral-adsorption properties towards cations. In a vegetative-pot experiment, four different levels of Ni contamination, i.e. 0 (control), 80, 160, 240 and 320 mg kg−1 were applied in the form of an analytical-grade NiSO4·7H2O solution mixed thoroughly with the soil. Among the minerals which were added to soil to alleviate the negative impact of Ni on plant biomass, MH had a particularly beneficial effect on the growth of B. juncea L. The amount of Ni, Zn, Cu, Mn, Pb and Cr in Indian mustard depended on the Ni dose and type of accompanying mineral structure. The average accumulation of trace elements in B. juncea L. grown in Ni-contaminated soil follow the decreasing order Mn > Zn > Cu > Ni > Pb > Cr.

Ka Loko Dam, in Kauai, Hawaii, failed suddenly and catastrophically on March 14, 2006. The resulting breachwas marked by three topographic benches, the lowest of which exposed native volcanic deposits once resident in the dam foundation. These deposits were found to contain outcrops of a waxy, gel-like material that appeared to result from in situ weathering processes. This unusual material was found to be highly enriched in halloysite. Gravel-size pieces in the hydraulic fill of the embankment derived from these materials also exhibited significant in situ weathering and significant halloysite content. Engineers and geologists generally recognize that bedrock materials weather progressively into soil constituents over ‘geological time’, and that this process is accelerated in tropical environments. Still, the strength, stiffness and durability of bedrock, earth and embankment materials are not expected to vary significantly over the geologically short life of a dam. In the case of Ka Loko Dam, however, the volcaniclastic sediments that comprise the local bedrock experienced substantial in situ weathering over its geologically brief 115-year operational lifetime. Prolonged exposure to seepage of anoxic water weathered the sediments completely to saprolite, including weak, sensitive, fine, spherical halloysite.

Halloysite-rich soils derived from in situ weathering of volcanic materials support steep stable slopes, but commonly fail under triggers of earthquakes or rainfall. Resulting landslides are slideflow processes, ranging from small translational slides to larger rotational failures with scarps characteristic of sensitive soils. Remoulding of failed materials results in high-mobility flows with apparent friction angles of 10–16°. The materials characteristically have high peak-friction angles (∼25– 37°), low cohesion (∼12–60 kN m−2) and plasticity ( plasticity index ∼10–48%), and low dry bulk density (∼480–1,080 kg m−3) with small pores due to the small size of the halloysite minerals. They remain saturated under most field conditions, with liquidity indexes frequently >1. Remoulded materials have limited cohesion (<5 kN m−2) and variable residual friction angles (15°–35°). Halloysite mineral morphology affects the rheology of remoulded suspensions: tubular minerals have greater viscosity and undrained shear strength than spherical morphologies.