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

Abstract

 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 (tanzanite) 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 do not show any change of colour. The existence in the Merelani area of “natural heated” blue gems confirms the complexity both of the geological evolution  and of the tanzanite mineralization.