The Mineralogy of the Historical Mochalin Log REE Deposit, South Urals, Russia. Part I. New Gatelite-Group Minerals ( with Allanite, Bastnasite, etc.) from Russia - 2018

Rare Earth Deposits & Mining

Kasatkin, joined by 10 co-authors, reported in 2018 that this 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) and allanite-(La),
• ferriallanite-(Ce) and ferriallanite-(La),
• bastnäsite-(Ce) and bastnäsite-(La),
• ferriperbøeite-(Ce) and perbøeite-(Ce),
• törnebohmite-(Ce) and 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 x 0.5 mm [ferriperbøeite-(La)] and 3 x 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), 2V meas = 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): (Ca_0.95La_1.58Ce_1.30Nd_0.10Pr_0.06)_Σ3.99(Al_2.27Fe³⁺_0.72Fe²⁺_0.46Mg_0.40Mn_0.16Ti_0.03)Σ4.04Si_4.96O₂₀[(OH)_1.94F_0.06];
• for perbøeite-(La): (Ca_0.94Th_0.01La_1.49Ce_1.34Nd_0.14Pr_0.07)_Σ3.99(Al_2.42Fe³⁺_0.52Fe²⁺_0.51Mg_0.38Mn_0.14 Ti_0.03)_Σ4.00Si_4.99O₂₀[(OH)_1.87F_0.13]. 

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.