Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Riccardo Luppi, Henrik Skogby, Federico Pezzotta, Ulf Hålenius, Francesco Di Benedetto, Alfonso Zoleo, Ferdinando Bosi

From multicolored to Paraíba: the role of Mn in heat-induced color change in Cu-bearing tourmalines

Journal of Geosciences, volume 71 (2026), issue 1, 21 - 37

DOI: http://doi.org/10.3190/jgeosci.0058.25

Tourmalines, with their wide range of colors, are among the most sought-after and highly valued minerals in the gemstone market. For this reason, they are often subjected to laboratory treatments to enhance their color. Among the rarest and most expensive tourmaline varieties, the Cu-bearing Paraíba tourmaline stands out for its distinctive and unique neon blue color. In this study, we investigate a multicolored Cu-bearing tourmaline from the Mavuco deposit in Mozambique. Unlike most Paraíba tourmalines, which are typically classified as fluor-elbaite, this sample is compositionally identified as elbaite. Electron microprobe analysis, optical absorption, Fourier-transform infrared, and electron paramagnetic resonance spectroscopy are used to clarify the causes of its natural color zoning and the mechanisms underlying color changes induced by heat treatment. The untreated crystal exhibits distinct color zones: a dark green root (R1), a yellowish light green band (R2), intermediate light blue (I1) and blue violetish (I2) zones, and a greyish violet terminal portion (T). Chemical analyses show that Fe, Mn, Cu, and Ti concentrations change systematically along the crystal, with Fe²⁺/ Fe³⁺ exchange-coupled pairs interactions and Fe²⁺- Ti⁴⁺ intervalence charge transfer (IVCT) controlling the dark green color of the crystal root (R1), while Mn²⁺- Ti⁴⁺ IVCT and Fe²⁺- Ti⁴⁺ IVCT producing the yellowish light green color (R2). Cu²⁺ is the only chromophore in the light blue (I1) zone, classifying it as a natural Paraíba-type tourmaline. In the blue violetish (I2) and greyish violet (T) zones, the presence of Mn³⁺, is likely the result of an interaction with localized natural radioactive fluids, and in a combination with Cu²⁺ absorptions produces violetish color. The heat treatment under reducing conditions (~500 °C, 18 h) selectively reduces Mn³⁺ to Mn²⁺, homogenizing the color of the intermediate (I2) and terminal (T) zones to light blue. The resulting Paraíba-like color is therefore attributed solely to Cu²⁺.
This study demonstrates that the color of Cu-bearing tourmalines is governed by the combination, concentration and oxidation state of Cu, Mn, Fe and Ti, as well as by post-crystallization redox processes. It also highlights the importance of tourmaline color as a petrogenetic recorder of subtle physicochemical changes in its crystallization environment. These findings provide interesting insights into pegmatitic evolution, the role of natural irradiation in gemstone coloration, and the gemological distinction between natural and treated Paraíba-type tourmalines.