Crystal structure and thermal behavior of Bi $$_6$$ 6 Te $$_2$$
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ORIGINAL PAPER
Crystal structure and thermal behavior of Bi6Te2O15 : investigation of synthetic and natural pingguite Gwilherm Nénert1 · Owen P. Missen2,3 · Hong Lian4 · Matthias Weil5 · Graeme R. Blake4 · Anthony R. Kampf6 · Stuart J. Mills2 Received: 20 April 2020 / Accepted: 14 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The previously unknown crystal structure of pingguite was determined and refined from laboratory X-ray powder diffraction data using a synthetic sample. Additional single crystal diffraction of natural pingguite confirms that the crystal structure of the synthetic sample is identical to the natural mineral. This new crystal structure calls for a revised chemistry of the rare mineral pingguite to Bi6Te2O15 instead of the previously reported formula Bi6Te2O13 . Pingguite contains TeVI only and not TeIV as previously reported. Pingguite undergoes an irreversible phase transition around 840 ◦ C which is characterized by a loss of oxygen and a reduction from TeVI to TeIV resulting in a 𝛿-Bi2O3 like type structure. In addition, we report the Raman spectroscopic data on the natural pingguite. Keywords Pingguite · Tellurate · Crystal structure · High-temperature transformation · Blue Bell · California · USA
Introduction Pingguite was first reported in 1994 from the Yangjia Au O13 (Zhifu deposit, Beijing municipality, China, as Bi6 TeIV 2 et al. 1994). No crystal structure was determinable and * Gwilherm Nénert [email protected] Stuart J. Mills [email protected] 1
Malvern Panalytical B. V., Lelyweg 1, 7602 EA Almelo, The Netherlands
2
Geosciences, Museums Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia
3
School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, Victoria 3800, Australia
4
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
5
Division of Structural Chemistry, Institute for Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164‑SC, 1060 Vienna, Austria
6
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
the valence of Te was determined by X-ray photoelectron spectroscopy (XPS). Subsequent occurrences of pingguite have been found in four additional countries, a more widespread distribution than many secondary tellurium minerals. Around half of the ∼ 90 secondary Te minerals are found only in North America (Missen et al. 2020). Despite its low planetary abundance, tellurium is found in an anomalously large number of minerals (Christy 2015). This is in part due to its complex chemistry in surface environments, including the stability of two higher oxidation states (TeIV and TeVI ) which may coexist (Missen et al. 2020). Pingguite is the most recently described mineral containing only bismuth, tellurium and oxygen, following smirnite (orthorhombic Bi2TeIVO 5 ) (Spiridonov et
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