Chromite typology and composition characterized through multispectral specular reflectance |
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| Institution: | 1. Universidad Politécnica de Madrid, Center for Computational Simulation, Department of Geological and Mining Engineering, Ríos Rosas, 21, 28003 Madrid Spain;2. University of Granada, Department of Mineralogy and Petrology, Av. Fuentenueva s/n, 18002 Granada Spain;3. Universidad Politécnica de Madrid, Department of Geological and Mining Engineering, Ríos Rosas, 21, 28003 Madrid Spain;1. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China;2. Department of Earth Sciences, University of Adelaide, SA 5005, Australia;3. Chengdu Center, China Geological Survey, Chengdu 610081, China;4. Luanchuan Burcan of Geology and Resources, Luoyang 417500, China;5. Shandong Lunan Geology and Exploration Institute, Jining 272100, China;1. Department of Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran, Iran;2. School of Geosciences, Shahrood University of Technology, Shahrood, Iran;3. Chrono-Environnement, Université de Franche-Comté/CNRS, 25030 Besançon cedex, France;4. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;1. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. The Bureau of Geo-exploration Guangxi and Mineral Development, Nanning 530023, China;3. Trading Center of Land and Resources of Guangxi Zhuang Autonomous Region, Nanning 530023, China;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Science, Guiyang, 550081 Guiyang, China;2. School of Earth Sciences and Resources, China University of Geosciences, Beijing, Haidian, 100083 Beijing, China;3. University of Chinese Academy of Sciences, Beijing, Haidian, 100049 Beijing, China |
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| Abstract: | Determination of multispectral specular reflectance is an important tool for ore identification in reflected light microscopy, and may be used for automated characterization of ores. However, reflectance values can be affected by compositional variations in a way that is seldom understood. The aim of the present work is to investigate this problem in chromite, an ore whose composition may show typically large natural variations as member of the spinel family, and whose relatively high reflectance variations are apparently unpredictable. For this research, eighteen samples of chromian spinel covering a large range of compositions in the base of the Hagerthy prism were selected for microprobe analysis and reflectance measure on polished sections. The samples belong to a variety of deposits and types (ultramafic massif, ophiolitic, and metamorphosed types: Ojén and Ronda Massifs, Spain; Mayarí, Moa-Baracoa, Camagüey and Sagua de Tánamo, Cuba; Golyamo Kemenyane, Avren and Yakovitsa, Bulgaria). The specular reflectances are characterized as multispectral values, measured at thirteen intervals (50 nm each) in the VNIR region (Visible and Near-Infrared: 400–1000 nm), using the automated CAMEVA System.The relationship between compositional and reflectance values is studied by multivariate analysis and subsequently tested on independent samples. For this purpose, the samples have been grouped in two sets: a larger population of fifteen samples constitutes the initial data set for mathematical processing, while a selection of three samples of widely differing compositions is used to test the resulting functions, so as to gain a critical appraisal of their validity.The results obtained show that the specular reflectance of chromite depends on composition and can be used to estimate compositional parameters, as #Cr = Cr/(Cr + Al) or #Mg = Mg/(Mg + Fe2+), but this relationship is complex and does not allow simple direct determinations, due to the multiplicity of possible changes and coupled substitutions (e.g. Al–Cr–Fe3+, or Mg-Fe2+, Ni, Zn, Ti …) in the chromian spinel family. On the other hand, the relationship of chromite composition with deposit type is also detectable through the reflectance values, but with a higher uncertainty. Reflectance increases with increasing Cr and Fe3+ contents in all cases, but the relationship of reflectance with Mg seems to be specific of the deposit type: while reflectance increases with increasing #Mg in the Ojén chromites, it shows the opposite behaviour in podiform chromites. Although these results should be regarded as preliminary until further studies on larger sample populations can be achieved, they are suggestive of possible practical applications for exploration, e. g. to approach Cr-spinel compositions or chromite deposit typology, early in an exploration campaign, by inexpensive reflected light determination of specular reflectance in a few samples. |
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| Keywords: | Chromite Typological characterization Multispectral reflectance VNIR CAMEVA System Image analysis |
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