Hausmannite (Mn3O4), a manganese oxide with a tetragonally distorted spinel structure, is considered to be ferrimagnetic with a very low Curie temperature of 42.5 K. However, strongly magnetic hausmannite has been discovered in some of the hydrothermally altered high-grade manganese ores of the giant Kalahari manganese deposit in South Africa. EDS-electron microprobe analyses indicate magnetic hausmannite to contain on average between 3 and 11.3 wt.% Fe2O3. In contrast non-magnetic hausmannite contains on average about 1–3 wt.% Fe2O3. X-ray powder diffraction analyses reveal small changes in cell dimensions of the magnetic hausmannite related to the high iron content. Mössbauer spectroscopy suggests that all iron is in the trivalent state. Optical microscopy and scanning electron microscopy (electron back-scatter imaging) proved the magnetic hausmannite to be homogeneous in composition, containing only a few minute inclusions of hematite. Magnetic blocking temperatures of the iron-rich hausmannite, approximating the Curie temperature, are of the order of 750 K. It is suggested that the ferrimagnetic state of hausmannite is stabilized and enhanced by replacement of Mn3+ by Fe3+.

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.