EVOLVING GROWTH CONDITIONS FOR WATERMELON TOURMALINE: ELEMENTAL ABUNDANCE PATTERNS TELL THE STORY

Due to sluggish diffusion rates, the major and minor element compositions of natural tourmaline crystals are thought to faithfully represent their original mineral chemistries set at the time of growth. Thus, element partitioning in tourmaline has been used to shed light on the pressure, temperature, and chemical surroundings that prevailed throughout crystallization within their host igneous and/or metamorphic environments. The striking green-to-pink coloration of watermelon tourmaline demonstrates that two visibly contrasting environments are associated with growth in their host hydrothermal environment. Using high-resolution electron microprobe, FT-infrared, and visible spectroscopy, we characterize the variation in major and minor elements within watermelon tourmaline crystals that reflect these two contrasting environments. Element distribution maps (using EDS) and high-resolution spot traverses (using WDS) across wafers cut perpendicular to the c-axis show three contrasting behaviors for chemical components in watermelon tourmaline: 1) High concentrations in green regions and low concentrations in pink regions (‘W’ trends, hold for Fe, Mn, Na); 2) Low concentrations in green regions and high concentrations in pink regions (‘M’ trends, hold for Al and Ca); and 3) no significant variation in concentration (‘flatline’ trends, hold for Si, F, Ti, K, and P). Three major bands in the infrared (at 3660, 3580, and 3480 cm^-1) vary both radially and vertically within our crystals. The absorption intensity of the IR bands all follow the ‘M’ distribution trend (similar to Ca and Al). Because total Mn inversely correlates with pink color, we note that either Mn is not the agent for the pink coloration in watermelon tourmaline (contrary to long established wisdom), or (likely) that the oxidation state of the host fluid of our crystals changed inversely with respect to Mn content during growth, and adoption of pink coloration is related to enhanced uptake of relatively small amounts of oxidized Mn. Although some of the variability in the elemental abundance patterns may result from minor post-crystallization element mobility within the crystals, we document that first-order variations in chemical abundances are related to the evolving conditions within the hydrothermal environment during growth.