The 21st General Meeting of the International Mineralogical Association (IMA) was held September 1–5 in Johannesburg, South Africa. Several oral and poster presentations were presented in a session on gem materials.
G. Agnosi (University of Bari, Italy) discussed preliminary results of an X-ray diffraction topography study of Colombian trapiche emeralds, which revealed a consistent crystallinity between the arms and the hexagonal core. She presented a model of trapiche formation in which the growth of the hexagonal core occurred first, followed by the six arm sections. Ulrika D’Haenens-Johansson (GIA, New York) outlined the status of synthetic diamond production. She reviewed the means of identification based on visual observations, structure-related ultraviolet fluorescence reactions, and distinctive spectroscopic features. In concluding, she noted that synthetic diamonds can be unequivocally recognized by major gem laboratories.
Andrew Fagan (University of British Columbia, Vancouver) presented the geologic setting and a model of formation of the Fiskenaesset corundum district in southwest Greenland. Estimates of ore reserves suggest that this could become a commercial ruby deposit. Gaston Giuliani (Centre de Recherches Pétrographiques et Géochimiques, Nancy, France) studied the oxygen isotope and trace-element chemistry of sapphire xenocrysts in basalts from Changle, China, and of corundum from the Mbuyi-Mayi kimberlite in the Democratic Republic of Congo, to trace their primary sources. In both cases, the host basalt and kimberlite transported the corundum crystals from the lower crust and upper mantle.
Daniel Ichang'i (University of Nairobi) described efforts by the Kenyan government to document and better understand the geologic setting of the country’s numerous gem deposits. He discussed two main occurrence lithologies: the metamorphic rocks of the Neoproterozoic Mozambique Orogenic belt, and the Paleogene-Neogene basaltic volcanics in the Northern and Central Kenya Rift regions.
Stefanos Karampelas (Gübelin Gem Lab, Lucerne, Switzerland) detailed the gemological characteristics of emeralds from Itatitia in Minas Gerais, Brazil. The emeralds formed along the contact between phlogopite schists and highly evolved granitic pegmatites. Based on minor and trace-element chemistry, they can be distinguished from emeralds from other Brazilian and world deposits. Vincent Pardieu (GIA, Bangkok) gave a talk about Montepuez in northern Mozambique, which is currently the world's largest source of high-quality rubies. He also presented a short movie about GIA expeditions he has led to gem deposits in eastern Africa. Wuyi Wang (GIA, New York) presented a study of carbon isotopes of synthetic and natural diamonds. The latter displayed δ13C values from 0 to -20‰, while the former ranged from -25 to -75‰.
Christopher M. Breeding (GIA, Carlsbad, California) described the interesting features of alluvial diamonds from the Marange deposit in eastern Zimbabwe. These type Ia diamonds display surface radiation staining, aggregated nitrogen impurities, and elevated hydrogen impurity contents, but these features do not provide a reliable indicator of geographic origin. Julien Feneyrol (Centre de Recherches Pétrographiques et Géochimiques, Nancy) presented a model of the metamorphic formation of tsavorite nodules in primary deposits. The tsavorite nodules are always contained within graphitic gneiss and calc-silicates with intercalations of marble. Elena Sorokina (Fersman Mineralogical Museum, Moscow) described a model of ruby and sapphire formation in marbles at Snezhnoe in Tajikistan.
ABOUT THE AUTHOR
Dr. James E. Shigley is a distinguished research fellow at GIA in Carlsbad, California.