Publication details
Hydroxylbastnäsite-(La), an ‘old new’ bastnäsite-group mineral
| Authors | |
|---|---|
| Year of publication | 2024 |
| Type | Article in Periodical |
| Magazine / Source | Mineralogical Magazine |
| MU Faculty or unit | |
| Citation | |
| web | https://doi.org/10.1180/mgm.2024.65 |
| Doi | http://dx.doi.org/10.1180/mgm.2024.65 |
| Keywords | hydroxylbastn & auml;site-(La); new mineral; bastn & auml;site group; rare-earth carbonate; crystal structure; Vuoriyarvi complex; Mochalin Log <italic>REE</italic> deposit |
| Description | Hydroxylbastn & auml;site-(La), the OH- and La-dominant member of the bastn & auml;site group, in fact known for many years, was studied in detail and has been approved by the IMA-CNMNC as a new mineral species with the ideal, end-member formula La(CO3)(OH). The holotype originates from the Vuoriyarvi (another spelling: Vuorij & auml;rvi) alkaline-ultrabasic complex, Northern Karelia, and the cotype from the Mochalin Log REE deposit, Potaniny Mts, South Urals, both in Russia. At Vuoriyarvi, hydroxylbastn & auml;site-(La) occurs as clusters (up to 1 mm) of light brown, honey-yellow or colourless hexagonal tabular to short-prismatic crystals up to 0.15 mm associated with fluorite and ancylite-(Ce) in cavities of calcite-dolomite carbonatites. At Mochalin Log, hydroxylbastn & auml;site-(La) forms light brown grains up to 0.2 mm included in massive aggregates of other LREE minerals: bastn & auml;site-(Ce), bastn & auml;site-(La), percleveite-(Ce), percleveite-(La), biraite-(Ce), biraite-(La), t & ouml;rnebohmite-(La), ferriperb & oslash;eite-(Ce), allanite-(Ce), etc. Dmeas is 4.75(2) and Dcalc is 4.778 g cm-3 (holotype). Hydroxylbastn & auml;site-(La) is optically uniaxial (+), omega = 1.76(1) and epsilon = 1.86(1) (holotype). The chemical composition (wt.%, electron microprobe, CO2 and H2O calculated: holotype/cotype) is: CaO 0.23/0.00, SrO 0.07/0.00, La2O3 39.47/39.58, Ce2O3 33.51/31.99, Pr2O3 1.03/1.51, Nd2O3 1.95/2.38, F 0.76/3.33, CO2 20.49/20.34, H2O 3.77/2.58, -O=F 0.32/1.40, total 100.96/100.31. The empirical formulae, calculated based on the sum of metal cations of 1 apfu and one CO3 group pfu, are (La0.52Ce0.44Nd0.02Pr0.01Ca0.01)Sigma 1.00(CO3)[(OH)0.90F0.09]Sigma 0.99 (holotype) and (La0.53Ce0.42Nd0.03Pr0.02)Sigma 1.00(CO3)[(OH)0.62F0.38]Sigma 1.00 (cotype). Hydroxylbastn & auml;site-(La) is hexagonal, P6, unit-cell parameters (from powder XRD data, holotype/cotype) are: a = 12.537(3)/12.533(1), c = 9.968(2)/9.908(1) & Aring;, V = 1356.8(5)/1347.9(3) & Aring;3 and Z = 18. Strong reflections of the powder XRD pattern [d,& Aring;(I)(hkl)] are (holotype): 4.98(39)(002), 3.616(88)(300), 2.926(100)(302), 2.089(41)(330), 2.052(46)(304) and 1.927(40)(332). The crystal structure of holotype hydroxylbastn & auml;site-(La) was refined by the Rietveld method, Rwp = 0.0071, Rp = 0.0050, Robs = 0.0466. It is isostructural to hydroxylbastn & auml;site-(Ce) and synthetic bastn & auml;site-type hydroxyl-carbonates REE3+(CO3)(OH) (REE = La-Er), but differs from fluorine-dominant bastn & auml;sites which adopt the space group P62c. |