Halloysite

Electron micrograph of halloysite nanotubes.[5]

Halloysite nanotubes intercalated with ruthenium catalytic nanoparticles.[5]

The formation of halloysite is due to hydrothermal alteration, and it is often found near carbonate rocks. For example, halloysite samples found in Wagon Wheel Gap, Colorado, United States are suspected to be the weathering product of rhyolite by downward moving waters.[4] In general the formation of clay minerals is highly favoured in tropical and sub-tropical climates due to the immense amounts of water flow. Halloysite has also been found overlaying basaltic rock, showing no gradual changes from rock to mineral formation.[6] Halloysite occurs primarily in recently exposed volcanic-derived soils, but it also forms from primary minerals in tropical soils or pre-glacially weathered materials.[7] Igneous rocks, especially glassy basaltic rocks are more susceptible to weathering and alteration forming halloysite.

Often as is the case with halloysite found in Juab County, Utah, United States the clay is found in close association with goethite and limonite and often interspersed with alunite. Feldspars are also subject to decomposition by water saturated with carbon dioxide. When feldspar occurs near the surface of lava flows, the CO₂ concentration is high, and reaction rates are rapid. With increasing depth, the leaching solutions become saturated with silica, aluminium, sodium, and calcium. Once the solutions are depleted of CO₂ they precipitate as secondary minerals. The decomposition is dependent on the flow of water. In the case that halloysite is formed from plagioclase it will not pass through intermediate stages.[4]

One of the largest halloysite deposits in the world is Dunino, near Legnica in Poland. It has reserves estimated at 10 million tons of material. This halloysite is characterized by layered-tubular and platy structure.[8]

Halloysite naturally occurs as small cylinders (nanotubes) that have a wall thickness of 10–15 atomic alumosilicate sheets, an outer diameter of 50–60 nm, an inner diameter of 12–15 nm, and a length of 0.5–10 μm. Their outer surface is mostly composed of SiO₂ and the inner surface of Al₂O₃, and hence those surfaces are oppositely charged.[5][9] Two common forms are found. When hydrated, the clay exhibits a 1 nm spacing of the layers, and when dehydrated (meta-halloysite), the spacing is 0.7 nm. The cation exchange capacity depends on the amount of hydration, as 2H₂O has 5–10 meq/100 g, while 4H₂O has 40–50 meq/100g.[10] Endellite is the alternative name for the Al₂Si₂O₅(OH)₄·2(H₂O) structure.[10][11]

Owing to the layered structure of the halloysite, it has a large specific surface area, which can reach 117 m²/g.[12]

Highly pure halloysite is mined from a rhyolite occurrence in New Zealand. Its uses include porcelain and bone china.[13][14][15][16]

Halloysite is an efficient adsorbent both for cations and anions. It has also been used as a petroleum cracking catalyst, and Exxon has developed a cracking catalyst based on synthetic halloysite in the 1970s.[17] Owing to its structure, halloysite can be used as filler in either natural or modified forms in nanocomposites. Halloysite nanotube can be intercalated with catalytic metal nanoparticles made of silver, ruthenium, rhodium, platinum or cobalt, thereby serving as a catalyst support.[5]

Besides supporting nanoparticles, halloysite nanotubes can also be used as a template to produce round well-dispersed nanoparticles (NPs). For example, bismuth and bismuth subcarbonate NPs with controlled size (~7 nm) were synthesized in water. Importantly, when halloysite was not used, large nanoplates instead of round spheres are obtained.[18]