The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part V. Zilbermintsite-(La), (CaLa₅)(Fe³⁺Al₃Fe²⁺)[Si₂O₇][SiO₄]₅O(OH)₃, a new mineral with ET2 type structure and a definition of the radekškodaite group

Abstract

The new mineral zilbermintsite-(La), ideally (CaLa5)(Fe3+Al3Fe2+)[Si2O7][SiO4]5O(OH)3, was found in a single polymineralic nodule from the Mochalin Log REE deposit, Chelyabinsk Oblast, South Urals, Russia. Zilbermintsite-(La) forms anhedral grains up to 0.65 × 0.20 mm at the contact of ferriperbøeite-(La), törnebohmite-(La) and ferriallanite-(Ce). Other associated minerals include bastnäsite-(La), biraite-(La), ferriallanite-(La), ferriperbøeite-(Ce), fluorbritholite-(Ce), monazite-(La), perbøeite-(La), percleveite-(Ce), percleveite-(La), perrierite-(Ce), perrierite-(La), thorianite, thorite and quartz. The new mineral is light brown, translucent in thin fragments with a vitreous lustre. It is brittle, with good {100} cleavage. Mohs hardness is ca. 6. Dcalc = 4.684 g cm–3. Optically, zilbermintsite-(La) is biaxial (+), α = 1.805(7), β = 1.812(7) and γ = 1.867(8) (589 nm); 2Vmeas = 40(15)° and 2Vcalc = 40°. The empirical formula based on O28(OH,F)3 apfu is (Ca0.94La2.56Ce2.18Nd0.20Pr0.10Th0.02)Σ6.00(Al2.96Fe3+0.90Fe2+0.64Mg0.34Mn0.13Ti0.03)Σ5.00Si7.00O28[(OH)2.42F0.58]. Zilbermintsite-(La) is monoclinic, P21/m; the unit-cell parameters are: a = 8.9605(5), b = 5.7295(2), c = 25.1033(13) Å, β = 116.616(7), V = 1152.21(12) Å3 and Z = 2. The crystal structure of zilbermintsite-(La) is solved from the single-crystal X-ray diffraction data [R = 0.0757 for 2857 unique reflections with I > 2σ(I)]. The new mineral is isotypic to radekškodaite-(La) and radekškodaite-(Ce) and together with them forms the newly defined radekškodaite group. All members of this group possess the ET2 type structure where one epidote-type module (E) regularly alternates with two törnebohmite-type modules (T). The new mineral honours Professor Veniamin A. Zilbermints (1887–1939) who was a pioneer of the study of the Mochalin Log deposit. The Levinson suffix-modifier -(La) indicates the predominance of La among rare-earth elements in the mineral.