Microscopical Investigations

Abstract

Under the microscope the cassiterite was found to be faintly pleochroic from impure white to faint red, this pleochroism being more easily perceptible in the darker coloured places. The variations in colour may be seen from photo 68, a four-fold magnification of a slide (No 82 B, Ruechimarra) under reflected light. The darker colour extends particularly along the twinning planes and it is clear that not only these planes but also the grain boundaries and the cleavage determine the location of those darker patches. It is possible, therefore, that this colouring is due to infiltration of foreign matter, subsequent to the formation of the cassiterite itself. The same photo also clearly shows a perfect cleavage parallel to ooI and a less perfect cleavage parallel to III. Further, there is the peculiar twinning which also characterises rutile, the twinning plane being probably IOI. For the rest the cassiterite shows the usual high relief and high birefringence.

The erratum of this chapter is available at http://dx.doi.org/10.1007/978-94-015-7553-9_48