Optical properties, inclusions, and chemical composition of blue corundum from the Stazh-2 deposit, South-Western Pamir

Background. The Stazh-2 deposit of blue corundum is a representative object in the post- Soviet territory with prospects for mining of jewelry crystal raw materials. In terms of its geological position, the deposit can be classified as a contact-reaction (bimetasomatic) deposit formed in the contacts of granite veins and magnesites, being comparable to the world-famous Kashmir Sumjam deposit. During its study at the end of the last century, no mineralogical studies of blue corundum were conducted using precision methods, such as IR, Raman and absorption spectra, DiamindView luminescence, in order to establish patterns in color distribution, solid and gas-liquid inclusions, deformations and chemical composition. Determination of these parameters could serve as a basis for the involvement of crystal from this deposit in the jewelry industry.Aim. Examination of blue corundum from the Stazh-2 deposit located in Archean magnesite marbles of the Goran metamorphic series of the South-West Pamir using IR, Raman spectra, absorption spectra, and DiamondView luminescence to establish color distribution within crystals, solid and gas-liquid inclusions, deformations, and their chemical composition.Materials and methods. Four polished plates weighing from 0.48 to 0.76 carats with an area of about 1 cm2 and a thickness of 1.5—2 mm, as well as two faceted inserts — “Oval” 2.86 carats, 9.5x7.5 mm, and “Circle” 1.35 carats, 9 mm. The inserts were prepared from samples from the collection of the Department of Mineralogy and Gemology, Russian State Geological Prospecting University (MGRI). The research methodology comprised (1) optical microscopy using a Leica S9D stereomicroscope; (2) infrared spectroscopy in the mid-IR range from 400 cm-1 to 4500 cm-1 with a Nicolet iS50 FT-IR Thermo Scientific FT-IR spectrometer, detector DTGS KBr, aperture 200, resolution 4 cm-1 , PIKE DRIFTS attachment; (3) Raman spectroscopy using a confocal Renishaw InVia Qontor Raman-microspectrometer with an excitation solid-state laser 532 nm, nominal power 100 mW and diffraction grating 1800 lines/mm; (4) UV-Vis-NIR absorption spectroscopy by a PerkinElmer Lambda 1050WB spectrometer in the range of 270—1200 nm, a spectral resolution of 0.5 nm, a scanning speed of 56.7 nm/min; (5) micro-X-ray fluorescence analysis using a Bruker TORNADO M4 micro-XRF spectrometer with an X-ray tube power 20 W and a vacuum mode of 20 mbar, an X-ray beam diameter of 50 μm; (6) luminescence imaging in short-wave radiation 225 nm, recorded using a Diamond View device. Instrumental studies were carried out at the Gemstone Research Laboratory of Gokhran of Russia.Results. (1) For the first time, spectrometric data of blue corundum from the Stazh-2 deposit were obtained by IR-, Raman, and UV-Vis-NIR spectroscopy methods, as well as micro-XRF. (2) In blue corundum, a local (focal) content of chromium, an element alien to granites, was established. Here, metasomatites with corundum were formed. Areas with chromium exhibit sharp rectilinear boundaries. (3) Zircon, diaspore, boehmite, muscovite, calcite, as well as gas-liquid inclusions with CO2, were determined in the composition of blue corundum. (4) Two color generations due to newly formed fractures and minerals were identified. (5) The inclusions of diaspore, boehmite, muscovite, and calcite are the result of the hydration and carbonation reactions that occurred between corundum and anorthite at later development stages. (6) The blue corundum samples contained Ti and Fe with a predominance of Ti over Fe, which is a distinctive feature of the Stazh-2 deposit from other deposits of blue corundum and sapphires.Conclusion. The obtained materials can be used by Gokhran of Russia for identifying sapphires of the contact-reaction (bimetasomatic) genesis, which include Kashmir sapphires of the highest quality, as well as for educational purposes.

South-West Pamir, deposit, Stazh-2, blue corundum, IR spectroscopy, Raman spectroscopy, XRF, zonal coloring, gas-liquid inclusions, diaspore, boehmite, titanium, iron, chromium, vanadium, gallium