HalloysiteDuring the weathering of intrusive rocks the sodium feldspar contained therein breaks down into kaolin group minerals -- kaolinite and halloysite. Both are hydrated aluminosilicate minerals. Halloysite (Al2Si2O5(OH)4 2H2O) is almost identical to kaolinite (Al2Si2O5(OH)4) with the difference being non-structural water that occurs as a monolayer of water molecules intercalated between the aluminosilicate layers. Both halloysite and kaolinite initially occur as platelets, but imperfections in the crystal lattice of the halloysite due to interference of the interlayer of water causes a misfit between different aluminosilicate layers which in turn causes the halloysite platelet to roll up into a tubular form (halloysite nanotubes or HNTs). HNTs can be as long as 10 microns with the diameter generally in the order of 10's of nanometers. The ratio of the length of the HNT to the diameter, known as the aspect ratio, is an important measure of the quality of a HNT. HNTs with a high aspect ratio are more highly sought and generally more valuable as they can be used in a wider range of applications.
Halloysite has historically been used in the manufacture of porcelain, bone china, and fine china. In these applications the combination of the tubular shape in clay with low iron and titanium content produces ceramic ware with exceptional whiteness and translucency. Its fine particle size enables halloysite to also be used as a suspension agent in glaze preparations as well as in filters and inkjets.
HNTs can be coated with metallic and other substances to achieve a wide variety of electrical, chemical, and physical properties. The hollow tube of HNTs can be filled with a variety of active ingredients including those used for cosmetics, household and personal care products, pesticides, pest repellents, pharmaceuticals and other agents that could benefit from extended release. Preliminary work completed by I-Minerals indicates the HNTs from the Bovill Kaolin deposits have an excellent aspect ratio and the lowest levels of trace elements when compared with HNTs from other halloysite deposits around the world. The low initial levels of trace elements provides a strong starting point for further purification to meet the stringent requirements of the pharmaceutical and cosmetic industries while the high aspect ratio adds strength in plastics and polymers applications.