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Core-Rim Zoning in Watermelon Tourmaline: Elemental and Redox Controls on Chromophore Variation

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Nancy McMillan

Authors:

Gibson, Marie¹, McMillan, Nancy J.²

(1) Geological Sciences, New Mexico State University, Las Cruces, NM, USA; Energy and Critical Materials Team/Subsurface Science Group, Pacific Northwest National Laboratory, Richland, WA, USA, (2) Geological Sciences, New Mexico State University, Las Cruces, NM, USA,

Abstract:

Watermelon tourmaline zoning patterns, typically characterized by a pink core and green rim —but occasionally the reverse coloration—provide insights into pegmatite processes. Nine samples, including one reversely zoned watermelon tourmaline, from eight mines in six countries were analyzed. Electron Probe Micro-Analysis (EPMA) was used to quantify major element concentrations, providing a baseline for understanding core-rim transitions; Laser-Induced Breakdown Spectroscopy (LIBS) was utilized for a comprehensive elemental analysis, detecting subtle compositional changes near or below EPMA detection limits. X-ray Absorption Near Edge Structure (XANES) spectroscopy was used to evaluate the oxidation states of influential chromophores, Fe and Mn, in the core and rim zones. LIBS results were generally consistent with those obtained from EPMA via geochemical exchange vector diagrams, when analyzed through Principal Component Analysis (PCA). Mn, Na, and Fe concentrations were higher in the green zones, whereas Si and Al concentrations were elevated in the pink zones across both normally and reversely zoned tourmalines. The FeO concentration difference was distinct: below detection – 0.39 wt% in pink zones, and 0.27 - 4.20 wt% in green zones. Although MnO has been considered a key chromophore responsible for the pink coloration, green zones (0.74 – 4.54 wt%) generally exhibited higher MnO concentrations than pink zones (below detection – 1.46 wt%). Key geochemical exchanges driving these trends include AlLi(Fe²⁺)-2, AlO(R²⁺OH)-1, and Al□(R²⁺Na)-1. Mn XANES revealed increased oxidation in the pink zones of samples consistent with the role of oxidized Mn as a dominant chromophore responsible for the pink coloration. Fe concentrations in pink zones were too low to obtain reliable Fe XANES in most samples, but the few viable measurements indicate that pink zones crystallized under more oxidizing conditions than green zones.

These findings support a model illustrating dynamic interactions between pegmatite melt, elbaite crystal growth, and external fluids. During pegmatite crystallization, the melt becomes depleted in Fe and Mn, and natural ionizing radiation induces oxidizing conditions (Ertl et al., 2010; Kurtz et al., 2020), resulting in pink elbaite crystallization in pegmatite pockets. Following pegmatite pocket rupture, reduced Fe- and Mn-rich external fluids enter the system, leading to resorption of existing crystal faces and the subsequent crystallization of more Fe-rich, reduced tourmaline. Reversely zoned watermelon tourmaline forms when pocket rupture is followed by additional Si- and Al-rich pegmatitic fluids.