Halloysite naturally occurs as small cylinders (nanotubes) that have a wall thickness of 10–15 atomic aluminosilicate sheets, an outer diameter of 50–60 nm, an inner diameter of 12–15 nm, and a length of 0.5–10 μm.[5] Their outer surface is mostly composed of SiO2 and the inner surface of Al2O3, and hence those surfaces are oppositely charged.[6][7] 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.[8] Endellite is the alternative name for the Al2Si2O5(OH)4·2(H2O) structure.[8][9]
Owing to the layered structure of the halloysite, it has a large specific surface area, which can reach 117 m2/g.[10]
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.[11] Halloysite occurs primarily in recently exposed volcanic-derived soils, but it also forms from primary minerals in tropical soils or pre-glacially weathered materials.[12] 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.[18] It has reserves estimated at 10 million tons of material. This halloysite is characterized by layered-tubular and platy structure.[19]
The Dragon mine, located in the Tintic district, Eureka, Utah, US deposit contains catalytic quality halloysite. The Dragon Mine Deposit is one of the largest in the United States. The total production throughout 1931–1962 resulted in nearly 750,000 metric tons of extracted halloysite. Pure halloysite classified at 10a and 7a are present.[20]
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.[21] 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, platinumorcobalt, thereby serving as a catalyst support.[6]
Halloysite has been evaluated for use in the sorption of CO2[22] and CH4.[23]
Due to its nanostructure, halloysite is used as the main nanostructured filler in multifunctional mixed matrix membranes (MMMs), opening up new possibilities in the separation of gaseous and liquid mixtures [24] and water purification.[25]
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.[26]
Halloysite is also used to purify water, e.g. from two azo dyes were removed from aq. solutions. by adsorption on a Polish halloysite from Dunino deposit.[27]
Halloysite have many advantages and reported as a nanocontainer.[28][29]
Halloysite can also be used to produce porous silicon nanotubes as anode materials for Li-ion batteries through the selective etching of aluminium oxide and thermal reduction.[30]
As a nanofiller in nanocomposite e.g. thermoplastic polyurethane acting on the mechanical, physicochemical and biological properties.[31]
^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1995). "Halloysite"(PDF). Handbook of Mineralogy. Vol. II, 2003 Silica, Silicates. Chantilly, VA, US: Mineralogical Society of America. ISBN978-0962209710.
^Yang, Y. Zhang; J. Ouyang (2016). "Physicochemical Properties of Halloysite". Nanosized Tubular Clay Minerals - Halloysite and Imogolite. Developments in Clay Science. Vol. 7. pp. 67–91. doi:10.1016/B978-0-08-100293-3.00004-2. ISBN9780081002933.
^Sakiewicz, P.; Lutynski, M.; Soltys, J.; Pytlinski, A. (2016). "Purification of Halloysite by Magnetic Separation". Physicochemical Problems of Mineral Processing. 52 (2): 991–1001. doi:10.5277/ppmp160236.
^Patterson, S., & Murray, H. (1984). Kaolin, refractory clay, ball clay, and halloysite in North America, Hawaii, and the Caribbean region. Professional Paper, 44-45. doi:10.3133/pp1306
^Robson, Harry E., Exxon Research & Engineering Co. (1976) "Synthetic halloysites as hydrocarbon conversion catalysts" U.S. patent 4,098,676
^Ortiz-Quiñonez, J.L.; Vega-Verduga, C; Díaz, D; Zumeta-Dubé, I (June 13, 2018). "Transformation of Bismuth and β‑Bi2O3 Nanoparticles into (BiO)2CO3 and (BiO)4(OH)2CO3 by Capturing CO2: The Role of Halloysite Nanotubes and "Sunlight" on the Crystal Shape and Size". Crystal Growth & Design. 18 (8): 4334−4346. doi:10.1021/acs.cgd.8b00177. S2CID103659223.
