The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part V. Zilbermintsite-(La), (CaLa 5 )(Fe 3+ Al 3 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 5 O(OH) 3 , a new mineral with ET2 type structure and a definition of the radekškodaite group

The new mineral zilbermintsite-(La), ideally (CaLa 5 )(Fe 3+ Al 3 Fe 2+ )[Si 2 O 7 ][SiO 4 ] 5 O(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. D calc = 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); 2V meas = 40(15)° and 2V calc = 40°. The empirical formula based on O 28 (OH,F) 3 apfu is (Ca 0.94 La 2.56 Ce 2.18 Nd 0.20 Pr 0.10 Th 0.02 ) Σ6.00 (Al 2.96 Fe 3+ 0.90 Fe 2+ 0.64 Mg 0.34 Mn 0.13 Ti 0.03 ) Σ5.00 Si 7.00 O 28 [(OH) 2.42 F 0.58 ]. Zilbermintsite-(La) is monoclinic, P 2 1 / 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) Å 3 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.