Winter 2010

Featured Gem News International: “Neon” Blue-to-Green Cu- and Mn-Bearing Liddicoatite Tourmaline

Four greenish blue faceted oval specimens, 1.29–1.45 ct, were recently submitted to the Gübelin Gem Lab (see figure). The stones were similar in hue, tone, and saturation to Paraíba-type tourmaline. Standard gemological examination gave the following properties: RI—n_o = 1.640–1.641 and n_e = 1.621–1.622; birefringence—0.018–0.020; and SG—3.06–3.08; all these are consistent with tourmaline. Microscopic observation revealed parallel tubes, some hollow and others stained yellow to red-brown. Partially healed and unhealed fissures were also seen, and portions of the fissures were either frosted or reflective. Similar patterns have been observed in some heat-treated Paraíba-type tourmaline from Mozambique (B. M. Laurs et al., “Copper-bearing [Paraíba-type] tourmaline from Mozambique,” Spring 2008 G&G, pp. 4–30). All four samples fluoresced moderate yellowish green to long-wave UV radiation and faint yellowish green to short-wave UV.

The samples’ UV-Vis-NIR spectra were also similar to those of Paraíba-type tourmaline (see P. B. Merkel and C. M. Breeding, “Spectral differentiation between copper and iron colorants in gem tourmalines,” Summer 2009 G&G, pp. 112–119, and references therein). Surprisingly, however, LA-ICP-MS analysis showed that though all of the samples were lithium tourmalines, they contained more Ca than alkalis (Na+K). EDXRF analysis also revealed significantly more Ca than typical of Paraíba-type tourmaline. All samples also contained minor amounts of Mn and Cu.

Elbaite is an Na-rich lithium tourmaline; the other lithium tourmalines are liddicoatite and rossmanite. Identification of tourmaline species is complex, as to date there are 13 known end members. When plotting the chemical composition of these specimens on a ternary diagram for lithium-rich tourmaline, all four samples fell in the liddicoatite field. Based on calculations using a recently developed tool (L. Klemm and P. Hardy, “Determination of tourmaline species by advanced chemical analysis,” Proceedings of the 3rd European Gemmological Symposium, Berne, Switzerland, June 5–7, 2009, pp. 58–59) and averaging the four chemical analyses obtained on each stone by LA-ICP-MS, the samples contained a 50–57% liddicoatite component; thus, all four were liddicoatite tourmaline.

Some traders we spoke with believe that these stones are from Mozambique, near the mine that produced Paraíba-type elbaite. We also have been told that liddicoatite rough has been found in this area that does not need heat treatment to produce the desirable greenish blue coloration. This is the first report of Cu-Mn bearing liddicoatite tourmalines of such color.

Stefanos Karampelas (s.karampelas@gubelingemlab.ch) and Leo Klemm
Gübelin Gem Lab, Lucerne, Switzerland