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Hydroxylbastnäsite-(La), an ‘old new’ bastnäsite-group mineral

Published online by Cambridge University Press:  12 November 2024

Igor V. Pekov*
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
Natalia V. Zubkova
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
Anatoly V. Kasatkin
Affiliation:
Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 18-2, 119071 Moscow, Russia
Nikita V. Chukanov
Affiliation:
Federal Research Center of Problems of Chemical Physics and Medicinal Chemistry of the Russian Academy of Sciences, 142432 Chernogolovka, Moscow region, Russia
Natalia N. Koshlyakova
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
Dmitry A. Ksenofontov
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
Radek Škoda
Affiliation:
Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 611 37, Brno, Czech Republic
Sergey N. Britvin
Affiliation:
St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
Anatoly S. Kirillov
Affiliation:
St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
Anatoly N. Zaitsev
Affiliation:
St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
Aleksey M. Kuznetsov
Affiliation:
Independent researcher, 454071 Chelyabinsk, Russia
Dmitry Yu. Pushcharovsky
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
*
Corresponding author: Igor V. Pekov; Email: igorpekov@mail.ru

Abstract

Hydroxylbastnäsite-(La), the OH- and La-dominant member of the bastnäsite group, in fact known for many years, was studied in detail and has been approved by the IMA–CNMNC as a new mineral species with the ideal, end-member formula La(CO3)(OH). The holotype originates from the Vuoriyarvi (another spelling: Vuorijärvi) alkaline-ultrabasic complex, Northern Karelia, and the cotype from the Mochalin Log REE deposit, Potaniny Mts, South Urals, both in Russia. At Vuoriyarvi, hydroxylbastnäsite-(La) occurs as clusters (up to 1 mm) of light brown, honey-yellow or colourless hexagonal tabular to short-prismatic crystals up to 0.15 mm associated with fluorite and ancylite-(Ce) in cavities of calcite–dolomite carbonatites. At Mochalin Log, hydroxylbastnäsite-(La) forms light brown grains up to 0.2 mm included in massive aggregates of other LREE minerals: bastnäsite-(Ce), bastnäsite-(La), percleveite-(Ce), percleveite-(La), biraite-(Ce), biraite-(La), törnebohmite-(La), ferriperbøeite-(Ce), allanite-(Ce), etc. Dmeas is 4.75(2) and Dcalc is 4.778 g cm–3 (holotype). Hydroxylbastnäsite-(La) is optically uniaxial (+), ω = 1.76(1) and ε = 1.86(1) (holotype). The chemical composition (wt.%, electron microprobe, CO2 and H2O calculated: holotype/cotype) is: CaO 0.23/0.00, SrO 0.07/0.00, La2O3 39.47/39.58, Ce2O3 33.51/31.99, Pr2O3 1.03/1.51, Nd2O3 1.95/2.38, F 0.76/3.33, CO2 20.49/20.34, H2O 3.77/2.58, –O=F 0.32/1.40, total 100.96/100.31. The empirical formulae, calculated based on the sum of metal cations of 1 apfu and one CO3 group pfu, are (La0.52Ce0.44Nd0.02Pr0.01Ca0.01)Σ1.00(CO3)[(OH)0.90F0.09]Σ0.99 (holotype) and (La0.53Ce0.42Nd0.03Pr0.02)Σ1.00(CO3)[(OH)0.62F0.38]Σ1.00 (cotype). Hydroxylbastnäsite-(La) is hexagonal, P6̅, unit-cell parameters (from powder XRD data, holotype/cotype) are: a = 12.537(3)/12.533(1), c = 9.968(2)/9.908(1) Å, V = 1356.8(5)/1347.9(3) Å3 and Z = 18. Strong reflections of the powder XRD pattern [d,Å(I)(hkl)] are (holotype): 4.98(39)(002), 3.616(88)(300), 2.926(100)(302), 2.089(41)(330), 2.052(46)(304) and 1.927(40)(332). The crystal structure of holotype hydroxylbastnäsite-(La) was refined by the Rietveld method, Rwp = 0.0071, Rp = 0.0050, Robs = 0.0466. It is isostructural to hydroxylbastnäsite-(Ce) and synthetic bastnäsite-type hydroxyl-carbonates REE3+(CO3)(OH) (REE = La–Er), but differs from fluorine-dominant bastnäsites which adopt the space group P6̅2c.

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© The Author(s), 2024. Published by Cambridge University Press on behalf of The Mineralogical Society of the United Kingdom and Ireland.

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