Magnetism in cassiterite

Abstract

Large cassiterite crystals from the Pelepah Kanan tin mine in central Johore differ from those at Aberfoyle and Renison Bell, Tasmania, in showing strong colour zoning and possessing a small but conspicuous magnetic fraction. By separating crystals from these localities into fractions of varying magnetic susceptibility, there is found to be a distinct relationship between colouration and the magnetic properties of cassiterite. Furthermore, Mossbauer spectroscopy reveals that ferroan inclusions in the dark brown magnetic fraction contain a higher ferrous to ferric ion ratio than in the colour-less fraction and that only undetectable amounts (less than about 10%) of the iron atoms in these inclusions can have a magnetically ordered environment. Although no significant differences in unit cell dimensions between the two fractions are apparent, electron diffraction d-spacings not assignable to cassiterite are found to compare with paramagnetic FeSn(OH)6 in the dark magnetic fraction and probably with lepidocrocite in the colourless non-magnetic fraction. When the magnetic fraction is heated in air to 600°C, however, the finely dispersed particles of hydrated ferrous stannate seem to coagulate and form a rhombohedral phase which closely resembles ilmenite. It is suggested that this phase is anhydrous ferrous stannate and that it forms a series of compounds with haematite which are analogous to Fe2−xTixO3. For x near 2/3, the latter series becomes appreciably ferrimagnetic and it is concluded that the magnetisation in these cassiterite samples is primarily due to the presence of a similar tin phase which may occur in small dehydrated regions of the primary ferrous stannate hydrate. These conclusions are further substantiated by thermogravimetric and infrared studies for both the Aberfoyle and Pelepah Kanan samples and by thermal demagnetisation analyses for the dark magnetic cassiterite. The thermal demagnetisation data indicate that it is possible to have in cassiterite a series of magnetic compounds with Curie points in the range of 50–250°C which corresponds closely to the magnetic region of the Fe2−xTixO3 series. — It is shown that many features associated with cassiterite paragenesis are readily explicable in terms of colloidal systems following condensation of the primary gaseous ore fluids. Based on this hypothesis and confirmatory experimental evidence, a mechanism for the origin of zoning in cassiterite is outlined