The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia; Part II, Radekskodaite-(La), (CaLa5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 and radekskodaite-(Ce), (CaCe5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3, two new minerals with a novel structure-type belonging to the epidote-törnebohmite polysomatic series

Two new isostructural minerals radekskodaite-(La) (CaLa5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 and radekskodaite-(Ce) (CaCe5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 were discovered in polymineralic nodules from the Mochalin Log REE deposit, South Urals, Russia. Radekskodaite-(La) is associated with allanite-(Ce), allanite-(La), bastnasite-(Ce), bastnasite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperboeite-(La), fluorbritholite-(Ce), tornebohmite-(Ce) and tornebohmite-(La). Radekskodaite-(Ce) is associated with ancylite-(Ce), bastnasite-(Ce), bastnasite-(La), lanthanite-(La), perboeite-(Ce) and tornebohmite-(Ce). The new minerals form isolated anhedral grains up to 0.35×0.75 mm [radekskodaite-(La)] and 1 mm ×2 mm [radekskodaite-(Ce)]. Both minerals are greenish-brown with vitreous lustre. Dcalc=4.644 [radekskodaite-(La)] and 4.651 [radekskodaite-(Ce)] g cm-3. Both minerals are optically biaxial (+); radekskodaite-(La): α=1.790(7), β=1.798(5), γ=1.825(8) and 2Vmeas=60(10)°; radekskodaite-(Ce): α=1.798(6), β=1.806(6), γ=1.833(8) and 2Vmeas=65(10)°. Chemical data [wt.%, electron-microprobe; FeO:Fe2O3 by charge balance; H2O by stochiometry; radekskodaite-(La)/radekskodaite-(Ce)] are: CaO 3.40/2.74, La2O3 27.68/22.23, Ce2O3 20.39/24.30, Pr2O3 0.94/1.48, Nd2O3 1.71/3.18, ThO2 0.23/0.24, MgO 0.85/1.04, Al2O3 10.35/10.84, MnO 0.64/0.69, FeO 2.55/2.76, Fe2O3 3.12/2.57, TiO2 0.13/0.04, SiO2 26.03/26.10, F 0.10/0.09, H2O 1.62/1.63, -O=F -0.04/-0.04, total 99.70/99.89. The empirical formulae based on O28(OH,F)3 are: radekskodaite-(La): (Ca0.98Th0.01La2.75Ce2.01Nd0.16Pr0.09)Σ6.00 (Al3.28Fe3+0.63Fe2+0.57Mg0.34Mn0.15Ti0.03)Σ5.00 Si7.00O28[(OH)2.91F0.09]; radekskodaite-(Ce): (Ca0.79Mn0.16Th0.01Ce2.39La2.20Nd0.30Pr0.14)Σ5.99 (Al3.43Fe2+0.62Fe3+0.52Mg0.42Ti0.01)Σ5.00 Si7.00O28[(OH)2.92F0.08]. Both minerals are monoclinic, P21/m; the unit-cell parameters [radekskodaite-(La)/radekskodaite-(Ce)] are: a=8.9604(3)/8.9702(4), b=5.7268(2)/5.7044(2), c=25.1128(10)/25.1642(13) Å, β=116.627(5)/116.766(6)°, V=1151.98(7)/1149.68(11) Å3 and Z=2/2. The crystal structures are solved based on single-crystal X-ray diffraction data; R=0.0554 [radekskodaite-(La)] and 0.0769 [radekskodaite-(Ce)]. Both minerals belong to the epidote-tornebohmite polysomatic series and represent first members of ET2-type: their structure consists of regular alternating modules, one slab of the epidote (E) structure and two slabs of tornebohmite (T). The rootname radekskodaite is given in honor of the Czech mineralogist R