CHEMICAL CHARACTERIZATION OF GEM TOURMALINE

Gem-quality tourmaline occurs predominantly in zoned granitic pegmatites, most commonly in LCT-types enriched in B, Li and Al. In instances where sufficient H₂O is present in the melt, fractional crystallization results in late-stage exsolution of a highly fluxed, low-density melt fraction that accumulates to form miarolitic cavities within or adjacent to the pegmatite core zone. The gem-quality tourmaline crystals occur in these miarolitic cavities.

Gem tourmaline occurs in a rainbow of colors and is commonly marketed under a series of color variety names. Gemological studies of suites of faceted tourmaline from worldwide occurrences included electron microprobe analyses to determine mineral species and the chromophores responsible for tourmaline colors. Most gem tourmaline is Na-dominant elbaite, except for that from Madagascar, Nigeria and Siberia. A large proportion of faceted tourmaline from Madagascar and some from Siberia is Ca-dominant liddicoatite. Nigerian tourmaline is split between elbaite and liddicoatite with red, green and colorless dominantly liddicoatite and blue elbaite.

The dominant chromophores for all gem tourmaline analyzed are Fe, Mn and Ti. The colors are related to the relative amounts of Fe, Mn, Ti, rarely Cu and, in tourmaline with a higher dravite component, Cr and V. In general, the most abundant and strongest chromophore is Fe²⁺, producing colors from black (schorl) to blue to green, with decreasing concentration. Light colored tourmaline has the lowest content of Fe. Tourmalines with high Fe are blue; those with moderate Fe and Ti are green; those with elevated Mn and Ti and negligible Fe are yellow. Pink and red colors are due to Mn in the absence of Fe, where natural irradiation has converted Mn from 2+ to 3+. The very rare Paraiba blue to green color is caused by Cu²⁺with elevated Mn and low to absent Fe.

Trace element analyses of tourmaline reveal district-specific features. Cerium and Sc are elevated in Zambia, Madagascar, Siberia and Myanmar gem tourmaline. Tin is elevated in Namibia, Maine and the Congo and Ga is elevated in Mozambique, Siberia, and Vietnam tourmaline. Maine and California tourmaline has the highest Zn content. The combination of major, minor and trace element analyses are useful for insight into the provenance of gem tourmaline.