corundum, Madagascar.12, Andranondambo, gems

Milisenda and Henn describe the geological setting, gemological properties, inclusions and chemical composition of blue sapphires from the newly commercialized Andranondambo occurrence in southeastern Madagascar. The deposit lies approximately 100 km north of Fort Dauphin, within altered marbles and calc-silicate gneisses of the Tranomaro formation. These rocks belong to a high-grade metamorphic terrain extensively reworked during the Pan-African event. Regional geochronological evidence leads the authors to infer that corundum formation accompanied granulite-facies metamorphism approximately 600 million years ago. Sapphires are recovered with simple hand tools from small pits in residual gravels, and the occurrence is considered promising for commercial development. Studied samples are slightly to moderately included and commonly display conspicuous colour zoning, with intense blue concentrated in broad central bands. Alternating darker and paler layers and growth structures follow basal and bipyramidal planes. Heat treatment can substantially reduce the visible zoning and improve appearance. Crystal fragments preserve rhombohedral and pyramidal forms consistent with metamorphic sources, contrasting with the etched or partly dissolved surfaces often associated with basalt-transported corundum. Refractive indices are 1.761–1.763 and 1.769–1.771, maximum birefringence is approximately 0.008, and specific gravity ranges from 3.91 to 3.98. Distinct to strong dichroism combines green-blue and violet-blue hues. All samples are inert under long-wave ultraviolet radiation; most are also inert under short-wave radiation, although weak greenish-blue or uneven orange fluorescence occurs occasionally. Polarized absorption spectra show ferric-iron features and iron–titanium intervalence charge transfer responsible for blue colour. The approximately 850 nm absorption commonly observed in sapphires from basaltic sources is absent. Preliminary microprobe work identifies apatite, calcite, dolomite and an incompletely characterized clay mineral among the inclusions. Other features include probable mica platelets, apatite associated with parallel hollow tubes, negative crystals, healed fractures containing fluid inclusions and occasional lamellar twinning. Electron microprobe analyses of three sapphires identify iron as the principal trace constituent, followed by titanium, with smaller measurable amounts of silicon, gallium and chromium. The combined evidence closely resembles that of metamorphic sapphires from Sri Lanka and Myanmar, supporting the classification of Andranondambo as a significant new source of metamorphic blue sapphire.