Publication details
The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part III. Percleveite-(La), La2Si2O7, a new REE disilicate mineral
| Authors | |
|---|---|
| Year of publication | 2020 |
| Type | Article in Periodical |
| Magazine / Source | Mineralogical Magazine |
| MU Faculty or unit | |
| Citation | |
| Web | https://doi.org/10.1180/mgm.2020.81 |
| Doi | http://dx.doi.org/10.1180/mgm.2020.81 |
| Keywords | crystal structure; Mochalin Log; new mineral; percleveite-(La); Raman spectrum; rare-earth disilicate; South Urals |
| Description | The new mineral percleveite-(La) (IMA2019-037), ideally La2Si2O7, was found in polymineralic nodules of the Mochalin Log REE deposit, Chelyabinsk Oblast, South Urals, Russia. It is associated with allanite-(Ce), allanite-(La), bastnasite-(Ce), bastnasite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbOeite-(Ce), ferriperbOeite-(La), fluorbritholite-(Ce), hydroxylbastnasite-(Ce), perbOeite-(Ce), perbOeite-(La), tornebohmite-(Ce) and tornebohmite-(La). Percleveite-(La) occurs as isolated anhedral grains commonly up to 0.2 mm x 0.4 mm and very rarely up to 1 mm x 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 (+), omega = 1.825(10) and = 1.835(10). The Raman spectrum is given. Chemical data (wt.%, electron-microprobe) are: La2O3 36.80, Ce2O3 31.22, Pr2O3 1.57, Nd2O3 2.96, SiO2 26.73, total 99.28. The empirical formula based on 7 O apfu is (La1.02Ce0.86Nd0.08Pr0.04)(Sigma 2.00)Si2.00O7. Percleveite-(La) is tetragonal, P4(1); the unit-cell parameters are: a = 6.8482(3), c = 24.8550(13) angstrom, V = 1165.64(11) angstrom(3) and Z = 8. The strongest reflections in the powder X-ray diffraction pattern [d, angstrom(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 > 2 sigma(I)]. The new mineral is named as an analogue of percleveite-(Ce) with La predominance over the rare-earth elements. |