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Falgarite, K4(VO)3(SO4)5, a new mineral from sublimates of a natural underground coal fire at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, Tajikistan

Published online by Cambridge University Press:  01 April 2020

Leonid A. Pautov ,
Mirak A. Mirakov ,
Oleg I. Siidra Open the ORCID record for Oleg I. Siidra [Opens in a new window] ,
Abdulkhak R. Faiziev ,
Еvgeny V. Nazarchuk ,
Vladimir Yu. Karpenko  and
Saimudasir Makhmadsharif
Leonid A. Pautov
Affiliation:
Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskiy pr. 18-2, 119071Moscow, Russia Institute of Mineralogy UB RAS, Miass, Chelyabinsk district, 456317, Russia
Mirak A. Mirakov
Affiliation:
Institute of Geology, Earthquake Engineering and Seismology, Academy of Sciences of the Republic of Tajikistan, Aini 267, 734063, Dushanbe, Tajikistan
Oleg I. Siidra*
Affiliation:
Department of Crystallography, St. Petersburg State University, University Emb., 7/9, St. Petersburg, 119034, Russia Kola Science Center, Russian Academy of Sciences, Apatity, Murmansk Region, 184200 Russia, 683006, Russia
Abdulkhak R. Faiziev
Affiliation:
Institute of Geology, Earthquake Engineering and Seismology, Academy of Sciences of the Republic of Tajikistan, Aini 267, 734063, Dushanbe, Tajikistan
Еvgeny V. Nazarchuk
Affiliation:
Department of Crystallography, St. Petersburg State University, University Emb., 7/9, St. Petersburg, 119034, Russia
Vladimir Yu. Karpenko
Affiliation:
Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskiy pr. 18-2, 119071Moscow, Russia
Saimudasir Makhmadsharif
Affiliation:
Institute of Geology, Earthquake Engineering and Seismology, Academy of Sciences of the Republic of Tajikistan, Aini 267, 734063, Dushanbe, Tajikistan
*
*Author for correspondence: Oleg I. Siidra, Email: [email protected]
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Abstract

A new mineral falgarite, K4(V+4O)3(SO4)5 was discovered at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, ca. 75 km N of Dushanbe, Tajikistan. The new mineral is named after the Falgar, an ancient Sogdian name for an area around the Zeravshan riverhead. Falgarite is a fumarolic mineral formed directly from a gas emitted by a natural underground coal fire. Associated minerals are anhydrite, baryte, molybdite, an unidentified Tl-vanadyl sulfate, K–Mg sulfate and an anhydrous Mg-sulfate. Falgarite forms small isometric or pseudo-octahedral individual crystals (10–60 μm) of turquoise colour and spherical aggregates up to 0.5 mm in diameter. Mohs hardness is ~ 2.5, Dmeas = 2.87(2) and Dcalc = 2.89 g/cm3. Refractive indices are: α = 1.588(3), β(calc.) = 1.600(3) and γ = 1.609(2) (590 nm). In transmitted light falgarite is transparent green with a weak pleochroism. The mineral is non-soluble in H2O and 5% HNO3 at room temperature. Infrared spectra support the absence of H2O and OH–. The chemical composition determined by electron-microprobe analysis is (wt.%): Na2O 0.55, K2O 20.76, Tl2O 1.83, VO2 29.38 and SO3 46.78, total 99.29. The empirical formula (based on 23 O apfu) is: (K3.76Na0.15Tl0.07)Σ3.98V3.02S4.99O23.0. The strongest lines of the powder X-ray diffraction pattern are [d,Å(I,%)(hkl)]: 3.20(70)(202); 3.17(80)024; 3.14(70)$\bar{2}$04; 3.01(50)$\bar{1}$51; and 2.88(100)151. Falgarite is monoclinic, P21/n, a = 8.7209(5), b = 16.1777(6), c = 14.4614(7) Å, β = 106.744(5)°, V = 1953.77(17) Å3, Z = 4 and R1 = 0.05. VO6 octahedra and SO4 tetrahedra link together by sharing corners thus forming a [(VO)3(SO4)5]4– framework. K+, Na+ and Tl+ cations are located in the channels of the framework. The synthetic K4(VO)3(SO4)5 analogue is known.

Keywords

falgariteanhydrous sulfatesexhalative mineralscoal firesfumarolesKukhi-MalikRavat
Type
Article
Information
Mineralogical Magazine , Volume 84 , Issue 3 , June 2020 , pp. 455 - 462
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland, 2020

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Footnotes

Associate Editor: David Hibbs

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