GSA Annual Meeting: Gem Research Session
The Geological Society of America held its 2020 annual meeting as a virtual conference October 26–30 (see above). The session on gemological research took place on Thursday, October 29, and featured presentations on various gemological topics.
Nelson Eby from the Department of Environmental, Earth, and Atmospheric Sciences of the University of Massachusetts in Lowell presented a chemical classification of emeralds from 24 deposits in 10 countries. Using data on major and minor chemical elements in emeralds obtained by the instrumental neutron activation analysis, he demonstrated how these emerald occurrences could be grouped into deposits related to igneous processes, to those related to low-to-moderate temperature solutions in sedimentary environments, and to metamorphic processes. Rhiana Henry of the Department of Earth, Ocean, and Atmospheric Sciences of the University of British Columbia in Vancouver described a method that could be used to calculate the water content of an emerald based on measured values of either Na+ atoms per formula unit (apfu) or weight percent Na2O. Cisil Badur of the Department of Geosciences at Auburn University in Alabama investigated plagioclase megacrysts containing macroscopic inclusions of native copper from the Dust Devil mine in south-central Oregon. She concluded that the homogeneous distribution of major and trace elements (including Cu) in the feldspar resulted from a rapid rate of cooling of the host basalt (which prevented substantial internal chemical diffusion following initial crystallization). The megacrysts display anomalously young argon-argon (or 40Ar/39Ar) ages, which she attributed to the loss of radiogenic argon. Shiyun Jin of GIA in Carlsbad, California, investigated the thin, oriented, ribbon-like inclusions of magnetite and hematite whose presence creates an aventurescence effect in some rainbow lattice sunstone from Australia. The iron in these inclusions is thought to have been initially dissolved in the feldspar lattice and then expelled during exsolution of albite (as lamellae) and ordering of the crystal structure during cooling of the host orthoclase.
A study of zircon inclusions in unheated sapphires from four important commercial metamorphic deposits was carried out by Wenxing Xu from the Gübelin Gem Lab in Lucerne, Switzerland. She demonstrated how Raman spectroscopic features can help distinguish geologically younger sapphires from Kashmir and Myanmar from geologically older samples from Sri Lanka and Madagascar. Wim Vertriest of GIA in Bangkok discussed the identification of opaque sulfide inclusions in rubies. Inclusions in marble-hosted rubies from Mogok, Myanmar, proved to be pyrrhotite and sphalerite, while those from amphibole rocks in Montepuez, Mozambique, were complex mixtures of Fe-Cu-Ni sulfide minerals, which suggests differences in the geological conditions of ruby formation. Evan Smith from GIA in New York discussed the genetic relationship between type IIb and certain type IIa diamonds. Both types appear to have formed in the sublithospheric mantle with the involvement of subducted serpentinized peridotite.
Ping Ma of the China University of Geosciences in Wuhan discussed the value of using three-dimensional fluorescence spectra to help differentiate untreated and various treated jadeite samples in the marketplace. Di Cui of the Engineering Research Center of Gems and Technological Materials of Tongji University in Shanghai described the chemical composition and crystal structure of both the emeralds from the Davdar deposit in Xinjiang Province of northwest China, and in a separate presentation, the high vanadium-content emeralds from the Dayakou deposit of Malipo County in southwestern China. Elina Myagkaya of GIA in New York described an unusual brownish yellow diamond that was found to be a IIa + IIb mixed-type diamond. Investigation using cathodoluminescence and photoluminescence hyperspectral imaging revealed a complex growth pattern distinguished by differences in luminescence colors and various optical defects.
