Color in Watermelon Tourmaline : The Role of Hydroxyl Diffusion

Abstract

Tourmaline is a semiprecious gemstone known in the geologic community known for its complex chemical formula. This is because the crystal structure of tourmaline can incorporate and contain numerous elements from the periodic table. Tourmaline is also revered by the geologic community as an accurate indicator of its growth conditions due to being stable through a wide range of pressure and temperature conditions with negligible diffusion resulting. Using visible and micro-infrared spectroscopy to measure individual hydroxyl-bearing contaminant species, unexpected prominent diffusion of these species was observed. Significant changes were observed in the peaks around 3660cm-1 and 3480cm-1. The 3660cm-1 and 3480cm-1 peaks are associated with two separate hydroxyl species within the tourmaline structure. Diffusion of the OH1 corresponding to peak 3660cm-1 is more prominent then the OH3 that corresponds to peak 3480cm-1. This is believed to occur due to the structural position of OH1 in a channel parallel to the c-axis within the tourmaline structure. This allows for expedited diffusion compared to OH3 that is not oriented within a channel. Our data shows that tourmaline is compositionally stable, but hydroxyl diffusion does still occur.