Anisotropic and variable track etching characteristics in natural sphenes

The etching characteristics of fission tracks in sphene are shown to be significantly different before and after thermal annealing. This is due to a progressive modification of the initially anisotropic etching properties of natural sphenes owing to the accumulation of radiation damage from the α-decay of U and Th. As the level of radiation damage increases in sphene, changes occur in the colour, etching rate, track geometry, apparent etching efficiency and angular distribution of the etched tracks. Eventually the sphene behaves like an isotropic material. All these property changes are reversed by thermal annealing and are not reproduced during normal neutron irradiations. The α-damage is annealed at a lower temperature than that required for track-fading. Spontaneous (i.e. fossil) tracks in sphene are therefore not directly comparable in their etching characteristics to tracks induced in the sphene after thermal annealing. Where sphene is used for fission-track dating it is important to use laboratory techniques which do not require such a comparison of tracks before and after annealing.