Bismuthinite Inclusions in Rose Quartz from Madagascar
The material was recovered from the Itongafeno pegmatite (also known as Tsaramanga), located 23 km west of Antsirabe near the village of Mahaiza. This deposit is part of the Analalava pegmatite district and has been worked for asteriated rose quartz and dark blue aquamarine for almost 100 years. In addition to common pegmatite minerals, it has produced rare examples of emerald and native bismuth. Several vertically disposed zones of mineralization have been identified in the pegmatite, including rose quartz, smoky quartz adjacent to a thin Nb-Ta oxide and biotite layer, and rose quartz with metallic inclusions. About 60 kg of the included rose quartz was extracted between September 2003 and June 2004, although only a small percentage was suitable for cutting.
The inclusions in 10 faceted stones were examined, and with magnification they appeared metallic gray to black and had peculiar shapes (figure 13). They occurred in parallel rows of strongly elongated, sometimes striated crystals about 0.5 mm thick. Powder X-ray diffraction (XRD) analysis of an extracted crystal and chemical analyses of several surface-reaching inclusions in one sample with a Jeol 5800LV scanning electron microscope equipped with a Princeton Gamma-Tech energy-dispersive IMIX detector identified the inclusions as bismuthinite (Bi2S3). These were sometimes accompanied by smaller, more equant, yellow-to-black chalcopyrite grains. XRD also suggested the possible presence of a Cu-Bi-sulfosalt close to gladite in composition, which probably was present as thin metallic needles seen in some samples.
In addition to the inclusions described above, the rose quartz also contained abundant tiny colorless needles possibly rutile; again, see figure 13) oriented in three directions at angles of 120°. These needles are apparently the cause of asterism in samples that are cut as spheres or cabochons.
To our knowledge, this is the first confirmed report of bismuthinite in faceted quartz from a pegmatite. Previously, bismuthinite was identified in quartz from tintungsten deposits and from Alpine-type fissures (see J. Hyrsl and G. Niedermayr, Magic World: Inclusions in Quartz, Bode Verlag, Haltern, Germany, 2003, p. 48).