The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part I. New gatelite-group minerals ferriperboeite-(La), (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SIO4](3)O(OH)(2) and perboeite-(La), (CaLa3)(Al3Fe2+)[Si2O7][SiO4](3)O(OH)(2)

摘要

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 gatelitegroup minerals, ferriperboeite-(La) (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SiO4](3)O(OH)(2) and perboeite-(La) (CaLa3)(Al3Fe2+)[Si2O7][SiO4](3)O (OH)(2). Both minerals occur in polymineralic nodules and are associated with one another and allanite-(Ce), allanite-(La), bastnasite-(Ce), bastnasite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperboeite-(Ce), perboeite-(Ce), tornebohmite-(Ce) and tornebohmite-(La). The new minerals form isolated anhedral grains up to 0.3 mm [ferriperboeite-(La)] and 0.5 mm [perboeite-(La)] across and granular aggregates up to 1.5 mm x 0.5 mm Eferriperboeite-(La)] and 3 mm x 1 mm perboeite-(La)]. Both minerals are brownish-black with vitreous lustre. D-calc = 4.510 [ferriperboeite-(La)] and 4.483 [perboeite-(La)] g cm(-3). They are optically biaxial (+); ferriperboeite-(La): alpha = 1.788(5), beta = 1.790(5), gamma = 1.810(5) and 2V(meas) = 40(10)degrees; perboeite-(La): alpha = 1.778(8), beta = 1.783(8), gamma = 1.805(8) and 2V(meas) = 40(10)degrees. Chemical data [wt.%, electron-microprobe; Fe3+:Fe2+ ratio from charge-balance requirements, H2O by stoichiometry; ferriperboeite-(La)/perboeite-(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 O-20(OH,F)(2) are for ferriperboeite-(La): (Ca0.95La1.58Ce1.30Nd0.10Pr0.06)(Sigma 3.99)(Al2.27Fe0.723+Fe0.462+Mg0.40Mn0.16Ti0.03)(Sigma 4.04)Si(4.96)O20[(OH)(1.94)F(0.1)3]; and for perboeite-(La): (Ca0.94Th0.01La1.49Ce1.36Nd0.14Pr0.07)(Sigma 3.99) (Al(2.4)2Fe(0.52)(3+)Fe(0.51)(2+)Mg(0.38)Mn(0.14) Ti-0.03)(Sigma 4.00)Si4.99O20[(OH)(1.87)F-0.13].( )Both minerals are monoclinic, P2(1)/m; the unit-cell parameters Iferriperboeite-(La)/perboeite-(La)] are a = 8.9458(2)/8.9652(4), b= 5.72971(13)/5.7306(2), c= 17.6192(3)/17.6770(9) angstrom, beta = 115.9497(19)/116.053(6)degrees, V= 812.06(3)/815.88(6)angstrom(3) and Z = 2/2. The crystal structures are solved on a single crystal and converged to R= 0.0355 [ferriperboeite-(La)] and 0.0750 [perboeite-(La)]. Ferriperboeite-(La) and perboeite-(La) are named as the La-analogues of ferriperboeite-(Ce) and perboeite-(Ce), respectively.