Halloysite

Halloysite is a hydrous aluminum silicate mineral that belongs to the kaolinite group. Its idealized formula is Al2Si2O5(OH)4·nH2O, reflecting variable hydration. Like kaolinite, halloysite consists of 1:1 aluminosilicate sheets, but it is most notable for its tubular morphology. Halloysite nanotubes, or HNTs, have outer diameters typically 40–70 nm, inner diameters 10–20 nm, and lengths up to several micrometers. The tubes are hollow cylinders with a laminated wall, and their lumen can host guest molecules.

Halloysite occurs in two crystalline forms, halloysite-7Å and halloysite-10Å, named for their characteristic basal spacings of about 7 and 10 Å, which relate to interlayer water content. Natural halloysite forms by weathering of aluminum silicate minerals such as feldspars and is commonly found in soils, as well as in some hydrothermal and volcanic settings. It can be separated from sediments and purified to yield nanotubes or platelet fractions.

Applications of halloysite nanotubes derive from their biocompatibility, hollow lumen, and surface chemistry. They are used as nanocarriers for controlled release of drugs, pesticides, and other actives. They are also employed as reinforcing fillers in polymers, as adsorbents for heavy metals and organic dyes, and as supports for catalysts. In addition, halloysite is studied for environmental remediation and functional coatings.

Safety and handling: halloysite is generally considered low in toxicity, but inhalation of dust should be avoided and standard handling precautions for clays applied.