Gem Session at 2014 GSA Meeting
The morning presentations focused mainly on diamonds. Thomas Stachel (University of Alberta) summarized work to constrain the geological conditions of diamond formation. It is thought to have formed in the mantle from an oxidized carbonate-bearing fluid with high carbon and low nitrogen concentrations. Eloïse Gaillou (Natural History Museum of Los Angeles County) discussed pink diamonds whose color originates from narrow lamellae created by plastic deformation parallel to the octahedral growth planes. These lamellae contain a high density of luminescent defects along with localized strain. As a result of plastic deformation in the mantle, these diamonds accommodate stress along the lamellae by mechanical twinning. The unidentified optical defects responsible for the pink coloration are also created by this deformation. Richard Wirth (GeoForschungsZentrum) described the use of a transmission electron microscope with a focused ion beam to characterize microstructures in materials. Blue color in quartz is caused by Rayleigh scattering from submicron- and nanometer-sized inclusions of mica, ilmenite, and rutile. The black color of carbonado diamond results from its polycrystalline nature and partially open grain boundaries that cause internal total reflection of light. Cloud-like inclusions in Madagascar sapphires are due to nanocrystalline titanium-rich oxide inclusions. Wuyi Wang (GIA) discussed the use of carbon isotope analysis to help distinguish natural diamonds from CVD-grown synthetic diamonds. He found no overlap in isotope composition between them, but within the synthetic diamonds there was variation between manufacturers and even from the same manufacturer. Karen Smit (GIA) described interesting features of diamond crystals from Marange, Zimbabwe, which display both cuboid and octahedral growth sectors and distinctive center-cross patterns (often highlighted by graphite-particle clouds). Mike Breeding (GIA) reviewed the properties of certain treated-color diamonds. Electron irradiation yields green colors in diamonds with higher nitrogen content, and blue colors in diamonds with lower amounts of nitrogen. Characterization of their optical defects provides a means of distinguishing them from natural-color diamonds. Troy Ardon (GIA) characterized some optical defects in hydrogen-rich diamonds from Zimbabwe. Spectral features associated with certain defects could be correlated with noticeable clouds in these diamonds. Robert Luth (University of Alberta) discussed a model of diamond formation in the mantle. He suggested that it takes place as carbon-hydrogen-oxygen (CHO) fluids rise in the lithosphere during isobaric cooling, or combined cooling and decomposition. This model helps explain a number of observations regarding the geologic occurrence of diamonds. Mandy Krebs (University of Alberta) discussed size frequency distribution as a predictive tool for macro-diamond grade at Canada’s Ekati mine. She found that Ekati diamonds of different size fractions display broadly similar nitrogen and carbon-13 isotope compositions. Mederic Palot (University of Alberta) studied diamonds from the Kankan deposit in Guinea that are believed to have formed at great depths (300 km in the mantle). Characterization of nitrogen and carbon isotopes indicates rapid ascent from the mantle to shallower depths. Charles Kosman (University of British Columbia) studied the chemical composition of mineral inclusions in alluvial diamonds from the Kasai area of Congo. These diamonds originated from the kimberlites around Lucapa in neighboring Angola, which contain a significant proportion (~ 20%) of eclogitic diamonds.
The afternoon session covered a range of topics. Lee Groat (University of British Columbia) reviewed Canadian gem minerals. In addition to diamond, the country is an important source of nephrite jade, ammolite, and amethyst. Recent discoveries of ruby, sapphire, spinel, and emerald also show promise. David Newton (University of British Columbia) analyzed peridotite and pyroxenite xenoliths from the Muskox kimberlite in northern Canada to better understand the rock types in the underlying mantle. Howard Coopersmith (Fort Collins, Colorado) described a placer deposit at Mount Carmel in Israel that has yielded diamond, ruby, sapphire, and natural moissanite crystals and fragments that appear to have originated from different igneous and metamorphic source rocks. Aaron Palke (GIA) presented a trace-element study of demantoid garnet as the basis for distinguishing this important gem material from different geologic and geographic sources. Cigdem Lule (Glenview, Illinois) discussed the need for a more systematic approach to gem nomenclature, which is often determined by jewelry trade practices, as opposed to the more rigorous procedure used for assigning new mineral names. Nancy McMillan (New Mexico State University) presented a multivariate chemical analysis of tourmaline, a mineral that can be an exceptional provenance indicator in detrital sediments. This analysis has proven successful in assigning tourmaline samples to different geologic environments. David Turner (University of British Columbia) conducted a study of sapphires and associated minerals from the marble-hosted Beluga occurrence in northern Canada by hyperspectral imaging over the 550–2500 nm range. This technique could be useful in the field for analyzing rock samples and in airborne surveys over terrain with good rock exposures. Philippe Belley (University of British Columbia) studied blue calcite skarn deposits in Ontario and Quebec. Their geologic setting and low iron content make them potential sources of grossular garnet and other gem minerals. J.N. Das (Geological Survey of India) reviewed the exploration and mining of diamonds and colored gemstones throughout India. Andrew Fagan (University of British Columbia) described the Aappaluttoq ruby and pink sapphire deposit in Greenland. Based on field studies, it appears to be the world’s largest geologically defined gem corundum deposit, with an estimated 400 million carats of material within 65 meters of the surface. Commercial mining is expected to begin in 2015. Sytle Antao (University of Calgary) studied optical anisotropy in cubic garnets, which has been recognized for more than a century but remains incompletely understood. X-ray diffraction study of several anisotropic samples revealed they contain two or three cubic phases with slightly different structural parameters. The intergrowth of these phases on a nano-domain scale produces the strain-induced optical anisotropy. Barbara Dutrow (Louisiana State University) discussed how tourmaline, an important accessory mineral in many geologic environments, can be used to help locate gem deposits.
In the separate poster session, Ellen Svadlenak (Oregon State University) and co-authors used electron microprobe analysis to study variations in the trace elements in emeralds from Muzo, Colombia. Lauren Forbes (Western Washington University) and co-authors analyzed emeralds from four different Muzo mines using SEM-CL (cathodoluminescence) along with LA-ICP-MS to better understand the chemical and physical conditions for their growth. All emeralds tested showed a strong red luminescence. The researchers concluded that Muzo emeralds are compositionally unique. Gena Philibert-Ortega (Murrieta, California) and co-authors described the scientific influence of James Sowerby’s rare five-volume British Mineralogy in the early 19th century. Elise Skalwold (Cornell University) and coauthors reviewed their research on a highly unusual blue inclusion in a diamond. While it was identified as olivine, the reason for the blue color remains a mystery.
An overview of the gem research session from the 2014 GSA meeting is available at https://gsa.confex.com/gsa/2014AM/webprogram/Session35272.html.