Course

GEOL-451

Document Type

Student Paper

Publication Date

2020

Disciplines

Geochemistry | Geology | Inorganic Chemistry | Materials Chemistry

Description, Abstract, or Artist's Statement

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 Madagascar 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 conditions. The blue (C1) sapphires either underwent no change of color or became lighter with increasing temperature and some received a yellowing effect resulting in green in oxidizing conditions. In reducing conditions, the blue (C1) sapphires became darker in color, especially from 1400℃ and up. Green/blue (C2) sapphires became lighter and increased the green intensity from 1200 to 1400℃ exclusively and up to 1600℃ in oxidizing conditions. Starting at 1400℃ heat-treatment of the stones in reducing conditions turned almost all of the samples grey/black. Electron microprobe 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. C1 sapphires contained the highest amount of Ti and Fe (Ti > 100ppm, Fe 8000-10000 ppm), C2 sapphires contained much less Ti but the same amount of Fe (Ti 40-50ppm) and C3 samples contained the same amount of Ti as C2 but the least amount of Fe (Fe 400-700 ppm).

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