GEOCHEMICAL ANALYSIS AND HEAT-TREATMENT OF NATURAL SAPPHIRES FROM MOZAMBIQUE AND TANZANIA IN OXIDIZING AND REDUCING CONDITIONS

Natural or synthetic sapphires can be heat-treated to improve the clarity by removing “silk” (inclusions) and to change color by introducing color-inducing elements (i.e, chromophores) into the lattice structure or changing their valency. Due to these reasons, sapphires can be heat-treated to increase their monetary value. Twenty natural blue (C1), 20 green (C2) and 20 clear (C3) sapphires from Mozambique and Tanzania were heat-treated in a muffle furnace in oxidizing and reducing conditions, from 1200 to 1600℃, for 10-hour soak time. In total, 5 experiments were conducted in which soak time remained constant: experiment 1 was performed at 1200℃, exp. 2 at 1300℃, exp. 3 at 1400℃, exp. 4 at 1500℃, and exp. 5 at 1600℃. Each experiment contained 4 sapphires from C1, C2, and C3 respectively, and half of the sapphires were exposed to oxidizing conditions, while the other half to reducing conditions. To achieve reducing conditions, graphite slabs were used to create the CCO buffer; the oxidizing conditions were achieved simply with air. The clear (C3) sapphires remained clear under both conditions from 1200 to 1500℃ exclusively and up to 1600℃ in oxidizing. The blue (C1) sapphires either underwent no change of color or became lighter and some received a yellowing effect resulting in green from 1200 to 1500℃ exclusively and up to 1600℃ in oxidizing. Green/blue (C2) sapphires became lighter and increased the green intensity from 1200 to 1500℃ exclusively and up to 1600℃ in oxidizing. These changes were the same regardless of whether the stones were heat-treated in reducing or oxidizing conditions. At 1500℃ and 1600℃ heat-treatment of the stones in reducing conditions turned all of the samples grey/black. Electron micro probe analyses were used to determine concentrations of chromophores at minor and trace element concentrations, including: Ca, Ti, Zn, Mg, Si, Ga, Fe, Mn, and Cr to evaluate the geochemical effects of heat-treating sapphires. Chemical compositions reveal that the most abundant trace and minor elements are: Fe, Ca, Si, and Mg. The concentrations of Ti and Fe increased after being heat-treated in reducing conditions and decreased in oxidizing conditions.