Tanzanite and other zoisites from Merelani (NE Tanzania): new gemmological and chemical data

Zoisite is an orthorhombic (space group: Pnma) polymorph of clinozoisite and is not considered a member of the epidote-group, which includes only monoclinic minerals. Tanzanite is the gem name for the violetish-blue coloured vanadium-bearing zoisite occurring in the Merelani mining deposit (northeastern Tanzania) located at the center of the Great Rift Valley region. The more than 5 km long area of the Merelani mineralization is occupied by late Proterozoic metasedimentary rocks and is divided into four government-controlled main blocks. Most gem-quality tanzanites occur in fault zones within outcrops of graphitic gneisses and schists, together with small quantities of transparent zoisite crystals of various colours (brown, yellow, green, pink, colourless). The colour of tanzanite and zoisite on the market today has generally produced by heat-treatment, although the response of the stones changes with the varying amounts of vanadium, chromium, and titanium. In the present work we have investigated five rough samples of zoisite from the Merelani area, consisting of one specimen with the typical violetish-blue colour of tanzanite and four stones ranging in colour from yellow-green to yellow-brown. From this material, we obtained five faceted gems weighting from 0.22 to 0.52 ct (zoisites) up to 2.44 ct (tanzanite). The gems and the rough grains have been characterized by traditional gemmological tests combined with EMPA-WDS and LA-ICP-MS measurements in order to determine their optical, physical and chemical properties. The faceted gems have been also heated at temperature up to 800°C with steps of 50°C. The annealing time for every steps varied from 15 to 120 min, with the increase of the temperature. The gemmological tests have been repeated after each step. Both tanzanite and zoisites examined have a composition close to the idealized formula Ca2Al3[Si2O7][SiO4]O(OH), but contain some other minor and trace elements substituting for calcium and aluminium. In particular, the tanzanite sample is enriched in vanadium and chromium (V=2625 ppm; Cr=224 ppm) respect to zoisites (V=1139-1571 ppm; Cr=81-140 ppm) whereas is depleted mainly in titanium and strontium (tanzanite: Ti=43 ppm, Sr=1266 ppm; zoisites: Ti=95-268 ppm, Sr=1451-2549 ppm). All the examined samples show comparable rare earth elements (REE) distribution patterns with a marked enrichment of LREE relatively to M-HREE, but the determined concentration is higher in tanzanite (ΣREE=384 ppm) than in zoisites (ΣREE=123-364 ppm). Before the heat treatment, all the examined gems were biaxial positive (refractive indices and birifringence ranging over 1.687-1.692 to 1.698-1.702 and 0.008-0.013, respectively) with a strong trichroism; the density varied from 3.20 to 3.46 g/cm3. When viewed with a gemmological microscope, most gems contain several inclusions such as fluid veils, opaque solid grains (probably graphite) and fractures. The four yellow-green to yellow-brown zoisites responded to heat treatment by turning violet-blue and dichroic at approximately 450/550°C. The violetish-blue tanzanite became dichroic, but showed no change in colour. According to the most recent data reported in literature, colour variations are probably due to a change of the oxidation state of vanadium and titanium (Ti3++V4+ converting to Ti4++V3+) with heating.