Tobias Häger

Confocal Micro-Raman Spectroscopy: A Powerful Tool to Identify Natural And Synthetic Emeralds

More than 300 natural and synthetic emeralds from various sources were examined with confocal Raman spectroscopy. This method identifies different water types in the beryl channel sites, making it possible to determine whether an emerald is natural or synthetic. In addition, this approach can provide information regarding geographic origin or synthesis technique (flux or hydrothermal).

EMERALDS FROM THE FAZENDA BONFIM REGION ,RIO GRANDE DO NORTE ,BRAZIL

In 2006, emeralds were discovered in the Fazenda Bonfim region of northeastern Brazil. Emerald mineralization occurs in association with small recrystallized pegmatitic bodies hosted by metamorphosed ultramafic rocks within the Santa Monica Shear Zone. Prospecting and exploration have been carried out in a few small pits and tunnels, producing emerald crystals with transparent areas that typically range between 2 and 5 mm. Polished samples typically show a saturated bluish green color with a medium-light to medium tone. The most common internal features are partially healed fissures with two-phase (liquid-gas) fluid inclusions and a variety of fine, parallel-oriented growth tubes. The emeralds contain moderate amounts of the chromophore elements Cr and Fe, and traces of V; they also show relatively high K and low Li. FTIR spectroscopic features are consistent with alkali-bearing emeralds that contain considerable CO 2 and a small amount of deuterated water. Emeralds from Fazenda Bonfim can be distinguished from those of other schist- and pegmatite-related commercial deposits.

Gemstones from Vietnam: An Update

GEMS & GEMOLOGY FALL 2012 Vietnam, with an area of 335,000 km 2, occupies the eastern side of the Indochinese peninsula. Most of the country’s northern and central regions are mountainous, reaching an elevation of 3,142 m in the Fan Si Pan massif, near the Chinese border. The country is endowed with some 70 gem deposits and 160 different occurrences (Nguyen et al., 1995). Present gem production includes ruby, sapphire, spinel, tourmaline, peridot, garnet, aquamarine, topaz, quartz, and green orthoclase (e.g., figure 1). With more than 3,400 km of coastline, the country is also a source of saltwater cultured pearls, and several farms have emerged in recent decades. In addition, Melo pearls are retrieved by fishermen on the southern coast and in Ha Long Bay in the north. Compared with the country’s gem wealth, however, the Vietnamese mining industry remains undeveloped. Although it has been nearly 30 years since colored stones were discovered in Vietnam, mining and pearl farming activities are mostly smalland medium-scale operations run by private individuals or small companies. In March 1988, state-owned Vinagemco (Vietnamese Gems Company) was established to direct the exploration, mining, and trading of gem materials (Kane et al., 1991; Pham et al., 2004b). Two subsidiaries, Yen Bai Gemstone Company and Nghe An Gemstone Company, were set up in those provinces that same year. Ultimately, management problems led to the company’s downfall in July 2003. Since then, no state-owned company has been active in the gem sector. Mining, processing, cutting, and trading are all organized by private and joint-stock companies or private individuals. Scientific investigations of Vietnamese gem materials, including their properties and the genesis of the deposits, have resulted in several publications, with special attention to ruby and sapphire (e.g., Kane et al., 1991; Koivula and Kammerling, 1991; Kammerling et al., 1994; Smith et al., 1995; Pham et al., 2004a,b; Nguyen et al., 2011). This article updates the occurrence, production, and gemological features of Vietnam’s major gem materials, and outlines the geology of the deposits.

Anhydrous Amorphous Calcium Oxalate Nanoparticles from Ionic Liquids: Stable Crystallization Intermediates in the Formation of Whewellite

The mechanisms by which amorphous intermediates transform into crystalline materials are not well understood. To test the viability and the limits of the classical crystallization, new model systems for crystallization are needed. With a view to elucidating the formation of an amorphous precursor and its subsequent crystallization, the crystallization of calcium oxalate, a biomineral widely occurring in plants, is investigated. Amorphous calcium oxalate (ACO) precipitated from an aqueous solution is described as a hydrated metastable phase, as often observed during low-temperature inorganic synthesis and biomineralization. In the presence of water, ACO rapidly transforms into hydrated whewellite (monohydrate, CaC2 O4 ⋅H2 O, COM). The problem of fast crystallization kinetics is circumvented by synthesizing anhydrous ACO from a pure ionic liquid (IL-ACO) for the first time. IL-ACO is stable in the absence of water at ambient temperature. It is obtained as well-defined, non-agglomerated particles with diameters of 15-20 nm. When exposed to water, it crystallizes to give (hydrated) COM through a dissolution/recrystallization mechanism.

Morphological and chemical evolution of corundum (ruby and sapphire): Crystal ontogeny reconstructed by EMPA, LA-ICP-MS, and Cr3+ Raman mapping

Abstract The term “ontogeny,” which is commonly used in biology, was introduced into the Earth sciences in 1961 to include the genesis and evolution of single crystals and crystal aggregates. The term encompasses nucleation, growth, alteration, and destruction. We present results of studies concerning the ontogeny of natural corundum (rubies and sapphires), and the chemical and morphological evolution of corundum crystals from deposits in Africa (Kenya, Tanzania, Madagascar) and Southeast Asia (Vietnam). Trace-element compositions indicative for different corundum habits were determined by rim-to-rim LA-ICP-MS and electron microprobe analyses. Raman spectroscopy was applied for Cr3+ photoluminescence mapping. Results traced the development of corundum crystals and the evolution of their chemistry and morphology, and helped to clarify the geological processes within particular deposits. These variations of corundum morphology are directly correlated with Cr and Fe contents and varying P-T conditions that prevailed during crystal growth. Dipyramidal habits combined with white color in corundum from two deposits in the Mangari area in Kenya have Cr concentrations of ~200–700 μg/g in crystals that grew under high P-T conditions. Prismatic habit of bright red ruby crystals was linked to Cr concentrations of ≥1500 μg/g in samples from Luc Yen (Vietnam) and Mangari (Kenya), formed under lower P-T. Concentrations of Cr between 700–1500 μg/g are associated with pink color and combinations of different habits (dipyramidal, prismatic, or dipyramidal-prismatic) in these samples. Contents of Fe ~700 μg/g and Cr ~1200 μg/g in sapphire crystals from the Morogoro area of Tanzania caused pink color that correlated with dipyramidal habit and elongation along the c axis. Rhombohedral habit and blue-violet color were observed at Cr ~600 μg/g and Fe ≥2000 μg/g in sapphires from Andranondambo in Madagascar, formed during the final stage of contact metamorphism.

Rubies and sapphires from snezhnoe, Tajikistan

GEMS & GEMOLOGY SUMMER 2015 Corundum—α-Al2O3—is a common, though minor, component of many metamorphic rocks. Gem-quality varieties of ruby and sapphire occur in only a few primary metamorphic and magmatic rock types depleted in silica and enriched in alumina (Giuliani et al., 2007) and in the secondary placers formed by the erosion of these rocks (e.g., Hughes, 1997; Kievlenko, 2003). The traditional supplier of ruby and sapphire to the world markets is Southeast Asia. Burmese rubies from the Mogok deposit have historically held the highest value, stemming from their “pigeon’s blood” color, a bright red with a slightly purple hue (Hughes, 1997; Smith, 1998). The supply situation has changed dramatically in recent years, with the discovery of new deposits in East Africa and the continued mining of the Central and Southeast Asian occurrences. Situated within the Pamir Mountains, Tajikistan produces a variety of gemstones, including ruby (figures 1 and 2), sapphire, spinel, aquamarine, chrysoberyl, tourmaline, topaz, clinohumite, garnet, scapolite, lazurite, and variscite (Litvinenko and Barnov, 2010). The occurrence of ruby in the Pamirs was first suggested by the Soviet mineraloRUBIES AND SAPPHIRES FROM SNEZHNOE, TAJIKISTAN

The Phase Transition of Corundum-Eskolaite Mineral Series Probed by X-Ray Absorption Spectroscopy

The red color of ruby is caused by Cr3+ impurities substituting Al3+ in Al2O3 structure. The complete substitution of Al3+ by Cr3+ produces Cr2O3 that has the green color. In this study, we focused on the phase transition of solid solution between corundum (Al2O3) and eskolaite (Cr2O3) mineral series. A series of Al2O3-Cr2O3 samples were prepared by mixing fine powder of Al2O3 and Cr2O3 in various ratios. They were ground together and then recrystallized. The obtained crystals were ground again in powder form for X-ray absorption near edge structure (XANES) measurements to avoid polarization dependence. The measurements were carried out at the Cr K-edge absorption energy using a 13-channel array Ge detector in the fluorescence mode. The XANES spectra showed line-shape transitions for 0.125 to 100% variation of Cr2O3. Significant variations in transitions were found when the Cr2O3 content was between 20% and 80%. With Cr2O3 concentration increasing, the sample colors apparently turned from pink to dark green. CIELAB color index measurements were performed. The variations of line-shape spectra of the Al2O3-Cr2O3 samples were found to be consistent with the CIELAB color index results. This Cr K-edge XANES study could be useful for the research in mineral sciences.

High Temperature Study of Metamict Rinkite

Metamict minerals contain radioactive elements that degrade their crystal structure. During the annealing process under ambient pressure in argon atmosphere the sample of metamict rinkite recrystallizes to the primary crystalline form at 700°C. Further heating leads to the formation of a new crystalline phase.

High Temperature Study of Metamict Steenstrupine

Metamict minerals contain radioactive elements that degrade their crystal structure. Unlike other metamict minerals, annealing process under ambient pressure do not lead to direct recrystallization of steenstrupine.