A NEW CHEVKINITE-GROUP MINERAL FROM THE EIFEL AREA,
Nikita Chukanov, Sergey Aksenov, Ramiza Rastsvetaeva, Dmitriy Belakovskiy, Jörg Göttlicher, Sergey Britvin, Steffen Möckel
New Data on Minerals, 47, 33-42, 2013
A new mineral, christofschäferite‑(Ce), was discovered at the famous outcrop Wingertsbergwand (Wingertsberg Mt.) of the Laacher See volcano near Mendig, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany. It occurs in a Mn-rich volcanic ejectum, in the association of metasomatic origin with orthoclase, rhodonite, bustamite, tephroite, zircon, fluorapatite, pyrophanite, and jacobsite at the contact of an alkali basalt magma and Mn-rich rock. The new mineral forms coarse crystals or isolated anhedral grains up to 3 mm in size. Christofschäferite‑(Ce) is black, resinous, and translucent with a brown streak. It is brittle, with conchoidal fracture. Cleavage or parting were not observed. Mohs hardness is 6. Dmeas = 4.8(1), Dcalc = 4.853 g/cm3. The mineral is optically biaxial (–), α = 1.945(10), β = 2.015(10), γ = 2.050(10), 2Vmeas = 70(10)°, 2Vcalc = 68°. It is strongly pleochroic (very dark brown to light brown), Z > Y > X. The IR spectrum of christofschäferite‑(Ce) is similar to those of other chevkinite‑group minerals with the main absorption bands (cm–1, s = strong, sh = shoulder): 1119, 1035s, 1010sh, 904s, 849s (Si‑O stretching vibrations), 671, 609, 562 (Ti‑O stretching vibrations and mixed vibrations of Si2O7 groups), 511s, 469s, 410sh, 390sh, 374s (combination of Si‑O‑Si bending vibrations and stretching vibrations in octahedra centered by Mn and Fe). The bands related to H-, B-, or C-bearing groups are absent in IR spectrum. The average (range) of five microprobe chemical analyses (WDS, valency of Mn by XANES data, wt%) is: CaO 2.61 (2.24–2.98), La2O3 19.60 (19.20–19.83), Ce2O3 22.95 (22.84–23.06), Pr2O3 0.56 (0.43–0.68), Nd2O3 2.28 (2.01–2.50), MgO 0.08 (0–0.20), MnO 4.39 (4.27–4.51), total Fe as FeO 6.98 (6.74–7.26) (apportioned in the proportions Fe2+:Fe3+ = 3:2, by structural data: FeO 4.18, Fe2O3 3.11), Al2O3 0.08 (0–0.19), TiO2 19.02 (18.64–19.39), Nb2O5 0.96 (0.83–1.11), SiO2 19.38 (19.16–19.52), total 99.20. The empirical formula based on 22 O is
(Ce1.72 La1.48 Nd0.17 Pr0.04 Ca0.57)Σ3.98 Mn2+0.76 Fe2+0.72 Mg0.02 Ti2.935 Fe3+0.48 Al0.02 Nb0.09 Si3.98 O22. The simplified formula is (Ce,La,Ca)4Mn(Ti,Fe2+)3(Fe3+,Ti)(Si2O7)2O8. Powder X-ray diffraction data were obtained by Gandolfi method (MoKα radiation). The strongest lines of the powder diffraction pattern [dobs (Å) (Iobs%, hkl)] are: 10.96 (24, 001), 4.90 (39, 111), 4.64(65, 202), 3.480 (78, 310), 3.169 (81, 311, 312), 3.095 (43, 113), 2.730 (100, 004), 2.169 (46, 421, 513), 1.737 (46, 603, 504, 315, 025, 622). The monoclinic unit-cell parameters are: a = 13.3722(4), b = 5.7434(1), c = 11.0862(2) Å, β = 100.580(2)°, V = 836.97(4) Å3, Z = 2. Space group P21/m. The crystal structure of christofschäferite-(Ce) is similar to that of other chevkinite-group minerals. The sheets of distorted Ti-centred octahedra (with minor Fe3+, Al, and Nb) are parallel to the (100). These sheets are interleaved with a layer consisting of Si2O7 groups and larger octahedra centered by Mn2+. The REE ions and subordinate Ca fill the channels formed through the linkage of heteropolyhedral and octahedral layers. Those sites have coordination numbers from 8 to 10. The crystal-chemical formula of christofschäferite-(Ce) is
X(Ce,REE,Ca)2 VIII(Ce,REE,Ca)2 VI(Mn,Fe2+) VI(Ti,Fe3+,Nb)3 VI(Fe3+,Fe2+,Ti) (Si2O7)2O8. The idealized end-member formula is Ce3CaMn Ti3Fe3+ (Si2O7)2O8. The mineral was named in honor of Christof Schäfer (b. 1961), a prominent German mineral collector. Both the mineral and its name have been approved by the IMA CNMNC (IMA 2011-107). Type specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia. D.B.