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2011 |
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Тезисы международной конференции |
Abstracts of International conference |
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A NEW TECHNIQUE OF THE PYRALSPITE GARNET STRUCTURE ANALYSIS USING AN IR-FOURIER MICROSCOPE G.K.Khachatryan, T.E.Shcherbakova, T.I.Kolesnikova Central Research Institute of Geological Prospecting for Base and Precious Metals (TSNIGRI), Moscow, Russia khachatryan_g_k@mail.ru
Unconventional diagnostic method have been developed for mineral grains identification by IR-Fourier microscope. The method is a high-sensitive and high-speed procedure, and it does not require of special specimen treatment. Investigations of kimberlite indicator minerals have demonstrated applicability of the method to heavy concentrate mineralogical prospecting of diamond deposits.
When working with stream sediment samples, a mineralogist has to deal with individual grains of diamond accessory minerals. A high-sensitive express method is necessary for diagnostics and definition of their typomorphic properties. The IR-Fourier spectroscopy which has been applied for diamond analysis for more than fifty years is one of such methods. However its wide application is complicated as there is no database on monocrystal spectra for the majority of minerals and sample preparation for obtaining of the spectra (manufacturing of the polished plane-parallel plates from mineral grains) is quite difficult. The main purpose of our work was to create an express technique for the structure analysis of variously-formed individual grains of pyrope-almandine-spessartine garnets from stream sediment samples. This technique can be used for separation from the samples the kimberlite and lamproite indicating garnets. The technique is simple to use since it does not demand any special preparation of samples to obtain the spectrum. The sample or a group of grains is placed on a reflecting surface, then the microscope beam is focused on the object and measurement program is started. All operations on spectrum obtaining, processing and the sample identification based on the reference spectra are conducted using the OMNIC computer program. The only restriction is the excessive thickness of the sample (> 100 mkm) and weak transmission of IR-emission connected with it. In such cases you need to split the grain into smaller fragments. 30 reference garnet grains previously analyzed by means of the Camebax SX-50 X-ray-spectrum microanalyzer were used as the objects of examination. The spectra were registered using the Nicolet 380 device with a THERMO Scientific Centaurus microscope within a range of 650-4000 cm-1. In the range of 800-1000 cm-1, "monocrystal" spectra of all garnets have a double maximum of absorption with characteristic frequencies ν1 (870-942 cm-1) and ν2 (826-884 cm-1) which slightly depend on the grain form, coloring and surface area. These parameters consistently decrease in the pyrope ® almandine ® spessartine row in the inverse relationship from the ionic radiuses of the bivalent cathions (Mg, Fe, Mn). Correlation of the ν1 and ν2 values for pyralspite garnets has a linear character. Chromic pyropes, i.e. pyrope-knorringite garnets, are separated into an independent branch as they differ from almandines and almandine-spessartines by their ν1 and ν2 value ratio. All these dependences are presented in the form of the diagram which allows determining the garnet structure based on the IR-spectrum parameters. The correlation between the ν2 value and Cr2O3 mass content in the pyrope has been determined which can then be used for the estimation of the chrome mass content in the pyrope. The technique is applicable when diamond deposit prospecting using the sediment-mineralogical method. This technique helps to distinguish in stream sediment samples visually similar yellow-orange and orange-red pyropes and pyrope-almandines from the dispersion halos associated with kimberlite bodies from vermeils of metamorphic rocks.
References: Khachatryan G.K., Kryazhev S.G. IR-microscope-assisted analysis technique for ore-related main and accessory minerals.Rudy Ores&Metals. 2010. № 5. P.64-73 (in Russian) McAloon B.P., Hofmeister A.M. 1995. Singl-crystal IR-spectroscopy of grossular-andradite garnets. American Mineralogist. V. 80. pp. 1145-115 Shcherbakova T.Е., Kolesnikova T.I. 2008. Mineralogical representation of kimberlite pipes of Zimniy Bereg area in covering Middle Paleozoic, Quaternary glacial deposits and recent alluvium. 9th International Kimberlite Conference Extended Abstract No 9IKC-A-00087 |
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