The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part III. Percleveite-(La), La 2 Si 2 O 7 , a new REE disilicate mineral

The new mineral percleveite-(La) (IMA2019–037), ideally La 2 Si 2 O 7 , was found in polymineralic nodules of the Mochalin Log REE deposit, Chelyabinsk Oblast, South Urals, Russia. It is associated with allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), ferriperbøeite-(La), fluorbritholite-(Ce), hydroxylbastnäsite-(Ce), perbøeite-(Ce), perbøeite-(La), törnebohmite-(Ce) and törnebohmite-(La). Percleveite-(La) occurs as isolated anhedral grains commonly up to 0.2 mm × 0.4 mm and very rarely up to 1 mm × 1 mm. The new mineral is transparent with greasy lustre. The mineral is very pale yellow to colourless in thin fragments to light yellow in aggregates. It is brittle, with imperfect {001} cleavage and an uneven fracture. Mohs’ hardness is ca . 6. D calc = 5.094 g cm –3 . Under the microscope, percleveite-(La) is non-pleochroic, optically uniaxial (+), ω = 1.825(10) and ɛ = 1.835(10). The Raman spectrum is given. Chemical data (wt.%, electron-microprobe) are: La 2 O 3 36.80, Ce 2 O 3 31.22, Pr 2 O 3 1.57, Nd 2 O 3 2.96, SiO 2 26.73, total 99.28. The empirical formula based on 7 O apfu is (La 1.02 Ce 0.86 Nd 0.08 Pr 0.04 ) Σ2.00 Si 2.00 O 7 . Percleveite-(La) is tetragonal, P 4 1 ; the unit-cell parameters are: a = 6.8482(3), c = 24.8550(13) Å, V = 1165.64(11) Å 3 and Z = 8. The strongest reflections in the powder X-ray diffraction pattern [ d , Å( I )( hkl )] are: 4.194(18)(113), 3.564(16)(106), 3.349(16)(201,202), 3.157(100)(203,116,008), 3.043(22)(211), 2.934(39)(122), 2.893(29)(213) and 2.864(21)(117). The crystal structure of percleveite-(La) is solved from the single-crystal X-ray diffraction data [ R = 0.0617 for 2831 unique reflections with I > 2σ( I )]. The new mineral is named as an analogue of percleveite-(Ce) with La predominance over the rare-earth elements.