ZONED CASSITERITE FROM SW ENGLAND – AN EXAMPLE OF ORTHOROMBIC OR MONOCLINIC SNO2?

Large (100-800 µm), euhedral grains are observed as lining on clusters of fine-grained tourmaline in a quartz-tourmaline rock at Nanjizal bay, SW England. In plane polarized light, cassiterite is colorless to light brown, with patchy zoning of darker brown. The dark brown patches are clearly linked to W content, showing up to 6000 µg/g in dark brown zones, and mostly below detection limit (~800 µg/g) in colorless zones. Cathodoluminescence (CL) textures reveal concentric growth zoning around the c-axis. A weak positive correlation between Ti and CL intensity and a negative correlation between W and CL intensity can be observed. Particularly, the Ti-CL relationship is inconsistent, and other defects are likely to be the controlling CL activators.

In cross polarized light, sections intersecting the c-axis at high angles show distinct sector zoning. Interference colors are typically 2^nd order blue and 1^st order yellow/purple. This zoning is not related to any of the analyzed trace elements (Fe, W, Ti, Nb) or to cathodoluminescence zoning. The EBSD data showed no orientation difference across the sectors, in contrast to across the elbow twin developed by reflection on the [011] plane, which corresponds to a 67.8° difference in orientation. Only very minor variations in orientation across grains normal to the c-axis were detected by EBSD, and unrelated to the interference color sectors. The sectors represent the growth traces of the pyramidal {h0ℓ} plane. The 4-fold symmetry along the c-axis in cassiterite indicates that the pyramidal faces are symmetrical equivalents. Only a difference in c-axis orientation would produce the variation in interference colors, and two sets of different {h0ℓ} faces is not sufficient. One grain clearly showed biaxial optical properties, indicating that the crystal structure may not be perfectly tetragonal. If, however, the sector zoning is compositional rather than strictly crystallographic, the difference in retardation observed in the optical microscope must be produced by elements other than Fe, W, Ti and Nb. However, incorporating different trace elements at symmetrically equivalent faces requires other properties, such as polarity differences. At the present, most evidence supports the interpretation that the investigated cassiterite is actually orthorhombic or monoclinic.