T172 Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

THE CAUSE OF COLOR VARIATION IN WATERMELON TOURMALINE: INSIGHTS FROM INFRARED SPECTROSCOPY

Paper number: T172-18
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Watermelon tourmaline is known for its dramatic and vivid pink-to-green color transition within single gemmy crystals. To date, there is no consensus within the scientific community regarding the mechanism causing the color variation. Two contrasting mechanisms can account for its cause. One theory advocates that the different colors reflect the changing chemical conditions while the crystals were growing. That is, pink regions in the core of the crystal incorporated pink coloring agents early in their growth, while later, outer regions of the crystal grew while incorporating green coloring agents. A second potential mechanism that accounts for color variation invokes chemical diffusion after crystal growth. That is, the crystal grew with pink coloring agents, and subsequent thermal soaking then resulted in either the progressive loss or gain of contaminants to generate the zoned color sequence. Here, we present results using high-resolution infrared and visible spectroscopy and microprobe analyses to offer insights into this question. Our results on two crystals (collected from Afghanistan and Brazil) show a correlation between observed color and species absorption at 3660 cm-1. All data collected from pink regions of the crystals show Abs/mm > 2.1 and all data from green regions are <2.1, indicating that the abundance of contaminant species at 3660 correlates with the abundance of the species responsible for absorbance in the visible spectrum. This result is consistent with the notion that partial diffusive loss of contaminants generated the observed dramatic variation in color. However, we additionally observe ~constant values in absorption at 3660 throughout the green regions of the crystal, with no additional diffusive loss toward crystal edges. Our results suggest that crystal color is correlated with a distinguishable contaminant fingerprint incorporated during crystal growth and can reflect changes in environmental factors that prevailed during this time.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308641

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.