The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part I. New gatelite-group minerals ferriperbøeite-(La), (CaLa₃)(Fe³⁺Al₂Fe²⁺)[Si₂O₇][SiO₄]₃O(OH)₂ and perbøeite-(La), (CaLa₃)(Al₃Fe²⁺)[Si₂O₇][SiO₄]₃O(OH)₂

Abstract

<jats:title>Abstract</jats:title> <jats:p>The paper opens a series devoted to the mineral diversity of the Mochalin Log rare earth element (<jats:italic>REE</jats:italic>) deposit, South Urals, Russia. There is a brief outline of the history of studies and geology of the deposit as well as the description of two new isostructural gatelite-group minerals, ferriperbøeite-(La) (CaLa<jats:sub>3</jats:sub>)(Fe<jats:sup>3+</jats:sup>Al<jats:sub>2</jats:sub>Fe<jats:sup>2+</jats:sup>)[Si<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub>][SiO<jats:sub>4</jats:sub>]<jats:sub>3</jats:sub>O(OH)<jats:sub>2</jats:sub> and perbøeite-(La) (CaLa<jats:sub>3</jats:sub>)(Al<jats:sub>3</jats:sub>Fe<jats:sup>2+</jats:sup>)[Si<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub>][SiO<jats:sub>4</jats:sub>]<jats:sub>3</jats:sub>O(OH)<jats:sub>2</jats:sub>. Both minerals occur in polymineralic nodules and are associated with one another and allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), perbøeite-(Ce), törnebohmite-(Ce) and törnebohmite-(La). The new minerals form isolated anhedral grains up to 0.3 mm [ferriperbøeite-(La)] and 0.5 mm [perbøeite-(La)] across and granular aggregates up to 1.5 mm × 0.5 mm [ferriperbøeite-(La)] and 3 mm × 1 mm [perbøeite-(La)]. Both minerals are brownish-black with vitreous lustre. <jats:italic>D</jats:italic><jats:sub>calc</jats:sub> = 4.510 [ferriperbøeite-(La)] and 4.483 [perbøeite-(La)] g cm<jats:sup>–3</jats:sup>. They are optically biaxial (+); ferriperbøeite-(La): α = 1.788(5), β = 1.790(5), γ = 1.810(5) and 2V<jats:sub>meas</jats:sub> = 40(10)°; perbøeite-(La): α = 1.778(8), β = 1.783(8), γ = 1.805(8) and 2V<jats:sub>meas</jats:sub> = 40(10)°. Chemical data [wt.%, electron-microprobe; Fe<jats:sup>3+</jats:sup>:Fe<jats:sup>2+</jats:sup> ratio from charge-balance requirements, H<jats:sub>2</jats:sub>O by stoichiometry; ferriperbøeite-(La)/perbøeite-(La)] are: CaO 4.91/4.81, ThO<jats:sub>2</jats:sub> 0.00/0.32, La<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 23.75/22.16, Ce<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 19.69/20.05, Pr<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 0.85/1.09, Nd<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 1.48/2.18, MgO 1.47/1.38, Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 10.68/11.25, MnO 1.07/0.92, FeO 3.04/3.35, Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 5.31/3.78, TiO<jats:sub>2</jats:sub> 0.19/0.19, SiO<jats:sub>2</jats:sub> 27.47/27.35, F 0.11/0.23, H<jats:sub>2</jats:sub>O 1.61/1.54, –O = F –0.05/–0.10, total 101.58/100.50. The empirical formulae based on O<jats:sub>20</jats:sub>(OH,F)<jats:sub>2</jats:sub> are for ferriperbøeite-(La): (Ca<jats:sub>0.95</jats:sub>La<jats:sub>1.58</jats:sub>Ce<jats:sub>1.30</jats:sub>Nd<jats:sub>0.10</jats:sub>Pr<jats:sub>0.06</jats:sub>)<jats:sub>Σ3.99</jats:sub>(Al<jats:sub>2.27</jats:sub>Fe<jats:sup>3+</jats:sup><jats:sub>0.72</jats:sub>Fe<jats:sup>2+</jats:sup><jats:sub>0.46</jats:sub>Mg<jats:sub>0.40</jats:sub>Mn<jats:sub>0.16</jats:sub>Ti<jats:sub>0.03</jats:sub>)<jats:sub>Σ4.04</jats:sub>Si<jats:sub>4.96</jats:sub>O<jats:sub>20</jats:sub>[(OH)<jats:sub>1.94</jats:sub>F<jats:sub>0.06</jats:sub>]; and for perbøeite-(La): (Ca<jats:sub>0.94</jats:sub>Th<jats:sub>0.01</jats:sub>La<jats:sub>1.49</jats:sub>Ce<jats:sub>1.34</jats:sub>Nd<jats:sub>0.14</jats:sub>Pr<jats:sub>0.07</jats:sub>)<jats:sub>Σ3.99</jats:sub>(Al<jats:sub>2.42</jats:sub>Fe<jats:sup>3+</jats:sup><jats:sub>0.52</jats:sub>Fe<jats:sup>2+</jats:sup><jats:sub>0.51</jats:sub>Mg<jats:sub>0.38</jats:sub>Mn<jats:sub>0.14</jats:sub> Ti<jats:sub>0.03</jats:sub>)<jats:sub>Σ4.00</jats:sub>Si<jats:sub>4.99</jats:sub>O<jats:sub>20</jats:sub>[(OH)<jats:sub>1.87</jats:sub>F<jats:sub>0.13</jats:sub>]. Both minerals are monoclinic, <jats:italic>P</jats:italic>2<jats:sub>1</jats:sub>/<jats:italic>m</jats:italic>; the unit-cell parameters [ferriperbøeite-(La)/perbøeite-(La)] are: <jats:italic>a</jats:italic> = 8.9458(2)/8.9652(4), <jats:italic>b</jats:italic> = 5.72971(13)/5.7306(2), <jats:italic>c</jats:italic> = 17.6192(3)/17.6770(9) Å, β = 115.9497(19)/116.053(6)°, <jats:italic>V</jats:italic> = 812.06(3)/815.88(6) Å<jats:sup>3</jats:sup> and <jats:italic>Z</jats:italic> = 2/2. The crystal structures are solved on a single crystal and converged to <jats:italic>R</jats:italic> = 0.0355 [ferriperbøeite-(La)] and 0.0750 [perbøeite-(La)]. Ferriperbøeite-(La) and perbøeite-(La) are named as the La-analogues of ferriperbøeite-(Ce) and perbøeite-(Ce), respectively.</jats:p>

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

“…The Raman spectra of both new minerals are close to those of västmanlandite-(Ce) (Holtstam et al , 2005) and ferriperbøeite-(La) (Kasatkin et al , 2020b) in the range of 300–1000 cm –1 but significantly differ from them in the regions of stretching vibrations of the Si–O–Si fragments (1000 to 1100 cm –1 ), O–H stretching vibrations (above 3200 cm –1 ) and stretching vibrations involving REE and bivalent cations forming low-force-strength bonds (below 300 cm –1 ). In particular, in the spectra of radekškodaite-(La) and radekškodaite-(Ce), the vibrational bands of stretching vibrations of the Si–O–Si fragments are significantly weaker than in the ferriperbøeite-(La) spectrum, which reflects a lower Si 2 O 7 :SiO 4 ratio in their composition.…”

“…As has been mentioned by Kasatkin et al (2020b), one of the most characteristic features of REE minerals from Mochalin Log is that in the majority of samples the gross Ce:La ratio varies widely but is most frequently close to 1:1 so that both Ce- and La-dominant members may occur in the same specimen. Therefore, in the text below, the absence of a particular Levinson suffix means the presence of Ce- and La-dominant mineral species together.…”

“…This article is the second in a series of papers on the mineralogical description and crystal chemistry of minerals containing rare earth elements ( REE ) as species-defining cations (henceforth – ‘ REE minerals’) from polymineralic nodules found at the Mochalin Log deposit, Chelyabinsk Oblast, South Urals, Russia (55°48′42″N, 60°33′46″E). A brief outline of the history of studies, the general data on geology and mineralogy of this deposit, as well as the description of two new isostructural gatelite-group minerals, ferriperbøeite-(La) and perbøeite-(La) are given in the first paper of this series (Kasatkin et al , 2020b) and references therein.…”

The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part III. Percleveite-(La), La2Si2O7, a new REE disilicate mineral

“…According to the distribution scale of RΕΕ minerals found at the Mochalin Log deposit (Kasatkin et al , 2020a), percleveite-(La) should be considered as rare: among 300 nodules with REE -bearing minerals investigated by us, it was found in only 12 of them, in minor amounts.…”

“…This article continues a series of papers on the mineralogy and crystal chemistry of new mineral species containing rare-earth elements ( REE ) as species-defining cations (henceforth ‘ REE minerals’) discovered at the Mochalin Log deposit, Chelyabinsk Oblast, South Urals, Russia (55°48′42″N, 60°33′46″E). A general description and mineralogical data for this deposit were given in the first paper which also contained the characterisation of two new minerals ferriperbøeite-(La) and perbøeite-(La) (Kasatkin et al , 2020a). In the second article we described radekškodaite-(La) and radekškodaite-(Ce), two members of the epidote–törnebohmite polysomatic series with a novel-type structure including one epidote and two törnebohmite modules ( ET2 ) (Kasatkin et al , 2020b).…”