IMA 2018-106 and 2018-116 = ferriperbøeite-(La) and perbøeite-(La)

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Marco E. Ciriotti
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IMA 2018-106 and 2018-116 = ferriperbøeite-(La) and perbøeite-(La)

Messaggio da Marco E. Ciriotti » ven 15 mag, 2020 11:26

Prossima pubblicazione.

▪ Kasatkin, A.V., Zubkova, N.V., Pekov, I.V., Chukanov, N.V., Škoda, R., Polekhovsky, Y.S., Agakhanov, A.A., Belakovskiy, D.I., Kuznetsov, A.M., Britvin, S.N., Pushcharovsky, D.Yu. (2020): The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part I. New gatelite-group minerals ferriperbøeite-(La), (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SiO4]3O(OH)2, and perbøeite-(La), (CaLa3)(Al3Fe2+)[Si2O7][SiO4]3O(OH)2. Mineralogical Magazine, 84, (in press).

The paper opens a series devoted to the mineral diversity of the Mochalin Log REE deposit, South Urals, Russia. There is a brief outline of the history of studies and geology of the deposit as well as the description of two new isostructural gatelite-group minerals, ferriperbøeite-(La) (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SiO4]3O(OH)2 and perbøeite-(La) (CaLa3)(Al3Fe2+)[Si2O7][SiO4]3O(OH)2. Both minerals occur in polymineralic nodules and are associated with one another and allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), perbøeite-(Ce), törnebohmite-(Ce), törnebohmite-(La), etc. The new minerals form isolated anhedral grains up to 0.3 mm [ferriperbøeite-(La)] and 0.5 mm [perbøeite-(La)] across and granular aggregates up to 1.5-0.5 mm [ferriperbøeite-(La)] and 3-1 mm [perbøeite-(La)]. Both minerals are brownish-black with vitreous lustre. Dcalc = 4.510 [ferriperbøeite-(La)] and 4.483 [perbøeite-(La)] g·cm-3. They are optically biaxial (+); ferriperbøeite-(La): α = 1.788(5), β = 1.790(5), γ = 1.810(5), 2Vmeas = 40(10)º; perbøeite-(La): α = 1.778(8), β = 1.783(8), γ = 1.805(8), 2Vmeas = 40(10)º. Chemical data [wt.%, electron-microprobe; Fe3+:Fe2+ ratio from charge-balance requirements, H2O by stochiometry; ferriperbøeite-(La)/perbøeite-(La)] are: CaO 4.91/4.81, ThO2 0.00/0.32, La2O3 23.75/22.16, Ce2O3 19.69/20.05, Pr2O3 0.85/1.09, Nd2O3 1.48/2.18, MgO 1.47/1.38, Al2O3 10.68/11.25, MnO 1.07/0.92, FeO 3.04/3.35, Fe2O3 5.31/3.78, TiO2 0.19/0.19, SiO2 27.47/27.35, F 0.11/0.23, H2O 1.61/1.54, –O=F –0.05/–0.10, total 101.58/100.50. The empirical formulae based on O20(OH,F)2 are for ferriperbøeite-(La): (Ca0.95La1.58Ce1.30Nd0.10Pr0.06)Σ3.99(Al2.27Fe3+0.72Fe2+0.46Mg0.40Mn0.16Ti0.03)Σ4.04Si4.96O20[(OH)1.94F0.06]; for perbøeite-(La): (Ca0.94Th0.01La1.49Ce1.34Nd0.14Pr0.07)Σ3.99(Al2.42Fe3+0.52Fe2+0.51Mg0.38Mn0.14 Ti0.03)Σ4.00Si4.99O20[(OH)1.87F0.13]. Both minerals are monoclinic, P21/m; the unit-cell parameters [ferriperbøeite-(La)/perbøeite-(La)] are: a = 8.9458(2)/8.9652(4), b = 5.72971(13)/5.7306(2), c = 17.6192(3)/17.6770(9) Å, β = 115.9497(19)/116.053(6)º, V = 812.06(3)/815.88(6) Å3 and Z = 2/2. The crystal structures are solved on single crystal and converged to R = 0.0354 [ferriperbøeite-(La)] and 0.0750 [perbøeite-(La)]. Ferriperbøeite-(La) and perbøeite-(La) are named as the La-analogues of ferriperbøeite-(Ce) and perbøeite-(Ce), respectively.
Marco E. Ciriotti

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