Gem-quality zoisite from Merelani (Northeastern Tanzania): Review and new data (Review)

摘要

The Merelani area (NE Tanzania) is the unique locality in the world for tanzanite, a violetish blue gem-quality variety of vanadium-rich zoisite. However, other coloured (brown, yellow, orange, etc.) zoisite occurs in this deposit, closely associated with tanzanite. This study reports a review and new data on physical and chemical properties of this material, obtained investigating five gem-quality samples, ranging in colour from yellowish brown and greenish yellow to violetish blue, by means of classical gemmological methods and by XRD, EMP and LA-ICP-MS analyses. The results confirm that the major element concentration of all the samples is almost identical, so their different colour is mainly dependent on the variation of some minor and trace elements. In particular, the main chromophore element is V but also the V/Ti ratio plays a significant role in the colour characteristics. All the faceted gems have been subjected to heat treatment in order to observe a possible change of their colour. The various coloured zoisite gems become permanently blue and change their pleochroic scheme from trichroic to dichroic at ～ 500 °C, except for the blue specimen which is dichroic before and after heating and does not show any change of colour. The oddness of this sample could be due to an undeclared previous heat treatment, made to enhance the colour. All these characteristics observed in zoisite from Merelani derive from the geological history that makes it one of the most interesting and significant gem deposit of the world.