The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part I. New gatelite-group minerals ferriperbøeite-(La), (CaLa 3 )(Fe 3+ Al 2 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 3 O(OH) 2 and perbøeite-(La), (CaLa 3 )(Al 3 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 3 O(OH) 2

The paper opens a series devoted to the mineral diversity of the Mochalin Log rare earth element ( 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) (CaLa 3 )(Fe 3+ Al 2 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 3 O(OH) 2 and perbøeite-(La) (CaLa 3 )(Al 3 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 3 O(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) and törnebohmite-(La). 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 mm × 0.5 mm [ferriperbøeite-(La)] and 3 mm × 1 mm [perbøeite-(La)]. Both minerals are brownish-black with vitreous lustre. D calc = 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) and 2V meas = 40(10)°; perbøeite-(La): α = 1.778(8), β = 1.783(8), γ = 1.805(8) and 2V meas = 40(10)°. Chemical data [wt.%, electron-microprobe; Fe 3+ :Fe 2+ ratio from charge-balance requirements, H 2 O by stoichiometry; ferriperbøeite-(La)/perbøeite-(La)] are: CaO 4.91/4.81, ThO 2 0.00/0.32, La 2 O 3 23.75/22.16, Ce 2 O 3 19.69/20.05, Pr 2 O 3 0.85/1.09, Nd 2 O 3 1.48/2.18, MgO 1.47/1.38, Al 2 O 3 10.68/11.25, MnO 1.07/0.92, FeO 3.04/3.35, Fe 2 O 3 5.31/3.78, TiO 2 0.19/0.19, SiO 2 27.47/27.35, F 0.11/0.23, H 2 O 1.61/1.54, –O = F –0.05/–0.10, total 101.58/100.50. The empirical formulae based on O 20 (OH,F) 2 are for ferriperbøeite-(La): (Ca 0.95 La 1.58 Ce 1.30 Nd 0.10 Pr 0.06 ) Σ3.99 (Al 2.27 Fe 3+ 0.72 Fe 2+ 0.46 Mg 0.40 Mn 0.16 Ti 0.03 ) Σ4.04 Si 4.96 O 20 [(OH) 1.94 F 0.06 ]; and for perbøeite-(La): (Ca 0.94 Th 0.01 La 1.49 Ce 1.34 Nd 0.14 Pr 0.07 ) Σ3.99 (Al 2.42 Fe 3+ 0.52 Fe 2+ 0.51 Mg 0.38 Mn 0.14 Ti 0.03 ) Σ4.00 Si 4.99 O 20 [(OH) 1.87 F 0.13 ]. Both minerals are monoclinic, P 2 1 / 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)°, V = 812.06(3)/815.88(6) Å 3 and Z = 2/2. The crystal structures are solved on a single crystal and converged to R = 0.0355 [ferriperbøeite-(La)] and 0.0750 [p