Известия Томского политехнического университета: Инжиниринг георесурсов (Dec 2024)
Qualitative characteristics and color causes of dolomite type nephrite, Voimakan deposit, Meadle‐Vitim mountain country
Abstract
Relevance. Nephrite is a highly valuable but insufficiently researched gemstone. The article is devoted to the previously unexplored Voimakan deposit of apodolomite nephrite, the main attention is paid to the causes of nephrite coloring, which remain controversial in relation to this stone in general. Aim. To determine the quality characteristics of the Voimakan deposit nephrite and the causes of its color. Methods. An organoleptic method using binoculars, a gemological flashlight and a magnifying glass of 20x magnification. Silicate and ICP-MS analyses were performed, colorimetric parameters were determined, optical absorption spectroscopy was applied, and Mossbauer measurements were carried out. Results. Greenish-white, light green, grayish-green and brown nephrite forms segregations in calcite-tremolite skarn bodies at the contact of dolomite marble and amphibolite transformed into epidote-tremolite skarn. The optical absorption spectra of nephrite are characterized by a wide low-intensity absorption band in the visible region in the region of 650–670 nm, associated with the charge transfer mechanism Fe2+VI→Fe3+VI in neighboring octahedral positions. Conclusions. Nephrite meets the requirements for gemstone raw materials. Diopsidite with nephrite lenses and interlayers can be used for carving multicolored products or inlays. Iron in nephrite is found in the form of Fe3+ and Fe2+ ions in various positions in the structure of the mineral, namely 58,4% Fe2+ is located in octahedral positions M1–M3, 30,9% Fe2+ is located in the position of octal coordination M4, 8,4% Fe3+ is located in octahedral positions M1–M3. These data are consistent with the bands in the optical absorption spectra, which creates the coloration in the studied nephrites. Electronic paramagnetic resonance and luminescent spectroscopy have shown that Mn2+ ions are located in two nonequivalent positions. The degree of green shade of nephrite increases with growth in Fe2+ content. The brown color of nephrite is determined by Fe3+ in the tremolite structure.
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