Nöggerathite-(Ce), (Ce,Ca)2Zr2(Nb,Ti)(Ti,Nb)2Fe2+O14, a New Zirconolite-Related Mineral from the Eifel Volcanic Region, Germany

The new mineral nöggerathite-(Ce) was discovered in a sanidinite volcanic ejectum from the Laach Lake (Laacher See) paleovolcano in the Eifel region, Rhineland-Palatinate, Germany. Associated minerals are sanidine, dark mica, magnetite, baddeleyite, nosean, and a chevkinite-group mineral. Nöggerathite-(Ce) has a color that ranges from brown to deep brownish red, with adamantine luster; thel streak is brownish red. It occurs in cavities of sanidinite and forms long prismatic crystals measuring up to 0.02 × 0.03 × 1.0 mm, with twins and random intergrowths. Its density calculated using the empirical formula is 5.332 g/cm3. The Vickers hardness number (VHN) is 615 kgf/mm2, which corresponds to a Mohs’ hardness of 51⁄2. The mean refractive index calculated using the Gladstone–Dale equation is 2.267. The Raman spectrum shows the absence of hydrogen-bearing groups. The chemical composition (electron microprobe holotype/cotype in wt %) is as follows: CaO 5.45/5.29, MnO 4.19/4.16, FeO 7.63/6.62, Al2O3 0.27/0.59, Y2O3 0.00/0.90, La2O3 3.17/3.64, Ce2O3 11.48/11.22, Pr2O3 1.04/0.92, Nd2O3 2.18/2.46, ThO2 2.32/1.98, TiO2 17.78/18.69, ZrO2 27.01/27.69, Nb2O5 17.04/15.77, total 99.59/99.82, respectively. The empirical formulae based on 14 O atoms per formula unit (apfu) are: (Ce0.59La0.165Nd0.11Pr0.05)Σ0.915Ca0.82Th0.07Mn0.50Fe0.90Al0.045Zr1.86Ti1.88Nb1.07O14 (holotype), and (Ce0.57La0.19Nd0.12Pr0.05Y0.06)Σ0.99Ca0.79Th0.06Mn0.49Fe0.77Al0.10Zr1.89Ti1.96Nb1.00O14 (cotype). The simplified formula is (Ce,Ca)2Zr2(Nb,Ti)(Ti,Nb)2FeO14. Nöggerathite-(Ce) is orthorhombic, of the space group Cmca. The unit cell parameters are: a = 7.2985(3), b = 14.1454(4), c = 10.1607(4) Å, and V = 1048.99(7) Å3. The crystal structure was solved using single-crystal X-ray diffraction data. Nöggerathite-(Ce) is an analogue of zirconolite-3O, ideally CaZrTi2O7, with Nb dominant over Ti in one of two octahedral sites and REE dominant over Ca in the eight-fold coordinated site. The strongest lines of the powder X-ray diffraction pattern (d, Å (I, %) (hkl)) are: 2.963 (91) (202), 2.903 (100) (042), 2.540 (39) (004), 1.823 (15) (400), 1.796 (51) (244), 1.543 (20) (442), and 1.519 (16) (282), Minerals 2018, 8, 449; doi:10.3390/min8100449 www.mdpi.com/journal/minerals Minerals 2018, 8, 449 2 of 14 respectively. The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (registration number 5123/1).

This paper describes nöggerathite-(Ce), a new zirconolite-related mineral species from the Laach Lake area situated in the Eifel paleovolcanic region, Germany. This mineral is non-metamict because of the very young geological age of the mother rock [11,12].
The root name of the new mineral is given in honor of Johann Jacob Nöggerath (1788-1877), a prominent German mineralogist and geologist. From 1818 Nöggerath was a professor of mineralogy and geology at the University of Bonn. Among his publications is a geological description of the Laach Lake (Laacher See) paleovolcanic region. The Levinson's modifier -(Ce) in the mineral name reflects the predominance of Ce among rare earth elements, which together are dominant in one structure position. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Associatiob (IMA number 2017-107). The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (registration number 5123/1).

Materials and Methods
The new mineral was found in the In den Dellen (Zieglowski) pumice quarry, 1.5 km to the notrheast of Mendig, in the Laach Lake (Laacher See) paleovolcano of the Eifel region, Rhineland-Palatinate, Germany. Two specimens have been investigated and considered as the holotype and the cotype, which are fragments of the same sanidinite volcanic ejectum. Associated minerals are sanidine, dark mica, magnetite, baddeleyite, nosean, and a chevkinite-group mineral.
The origin of sanidinites of the Laacher See area has been discussed earlier [9,[12][13][14]. These rocks are derivatives of different foyaite magmas which are comagmatic with haüyne-bearing rocks (haüyne foyaite, haüyne syenite, haüyne monzonite, etc.), or with noseane-cancrinite-nepheline syenites. Sanidinites of the latter type are enriched in rare element (Nb, Zr, REE, U, Th) accessory minerals. These rocks are cogenetic with the phonolitic host magma, and the crystallization took place in an intrusive syenite-carbonatite complex at temperatures below 700 • C in the host rock surrounding the top of the magma chamber 5000 to 20,000 years prior to the eruption of the magma chamber [12]. Most probably, nöggerathite-(Ce) and associated minerals forming crystals on the walls of cavities in sanidinite crystallized from above-critical fluid.
Chemical analyses (five for the holotype and three for the cotype) were carried out using an Oxford INCA Wave 700 electron microprobe (WDS mode, 20 kV, 600 pA, 300-nm beam diameter, Oxford Instruments plc, London, UK) housed at the Institute of Experimental Mineralogy RAS. The counting time per peak was 100 s.
The Raman spectrum of a randomly oriented nöggerathite-(Ce) crystal was obtained on the cotype sample using an EnSpectr R532 spectrometer based on an OLYMPUS CX 41 microscope coupled with a diode laser (λ = 532 nm) at room temperature (Enhanced Spectrometry, San Jose, USA). The spectrum was recorded in the range from 100 to 4000 cm −1 with a diffraction grating (1800 mm −1 ) and spectral resolution of about 6 cm −1 . The output power of the laser beam was about 9 mW. The diameter of the focal spot on the sample was less than 10 µm. The backscattered Raman signal was collected with a 40× objective; thel signal acquisition time for a single scan of the spectral range was 2 s, and the signal was averaged over 100 scans.
The Raman spectrum of an oriented laachite crystal used for comparison was obtained with a HORIBA instrument based on the OLYMPUS BX 41/51 microscope (HORIBA Jobin Yvon, Bensheim Germany) with a diode laser (λ = 532 nm) at room temperature. The power of the laser beam at the sample was about 1.5 mW. The spectrum was recorded in a range from 100 to 3800 cm −1 , with a diffraction grating (1800 mm −1 ) and spectral resolution of about 1 cm −1 . The diameter of the focal spot on the sample was less than 7 µm. The backscattered Raman signal was collected with 50× objective; signal acquisition time for a single scan of the spectral range was 10 s and the signal was averaged over five scans.
Both Raman spectrometers were housed at Moscow State University. Maximal and minimal reflectance values (R max /R min ) were measured in air using a MSF-21 micro-spectrophotometer (LOMO company, St. Petersburg, Russia) with a monochromator slit width of 0.4 mm and beam diameter of 0.1 mm. SiC (reflection standard 474251, number 545, Jena, Germany) was used as a standard. The spectrophotometer was housed at St. Petersburg State University.
Powder X-ray diffraction data were collected using a Rigaku RAXIS Rapid II diffractometer (Rigaku Corporation, Tokyo, Japan) with a curved image plate detector and rotating anode in Debye-Scherrer geometry, with an accelerating voltage of 40 kV, current of 15 mA, and exposure time 15 min. The distance between sample and detector was 127.4 mm. Data processing was carried out using osc2xrd software [15]. The diffractometer was housed at St. Petersburg State University.
The single-crystal X-ray diffraction experiment was carried out using a Bruker Kappa APEX DUO CCD diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The diffractometer was housed at Moscow State University. Experimental details are given in Table 1. Table 1. Crystal data, data collection information and structure refinement details for the holotype specimen of nöggerathite-(Ce).

General Appearance and Mechanical Properties
Nöggerathite-(Ce) forms prismatic crystals measuring up to 0.1 × 0.1 × 1.0 mm, elongated along (001), and simple twins, isolated or combined in random aggregates (Figure 1a The new mineral is translucent to transparent, with a color ranging from brown to very dark reddish brown, almost black, with adamantine luster. The streak is brownish red. Nöggerathite-(Ce) is brittle with uneven fractures; no cleavage was observed. Hardness, as determined by the micro-indentation method (Vickers hardness number (VHN) load of 20 g), is equal to 615 kgf/mm 2 which corresponds to a Mohs' hardness of 5 1 2 . The density calculated using the empirical formula is 5.332 g/cm 3 .

Raman Spectroscopy
The Raman spectrum of nöggerathite-(Ce) (Figure 2) shows the absence of absorption bands of H 2 O molecules, OH groups, and CO 3 2- In Raman spectra of laachite (Figure 3), which is a mineral related to nöggerathite-(Ce) but is characterized by much lower REE:Ca and Ti:Nb ratios, the bands of (REE,Ca)-O-and (Ti,Nb,Zr)-O-stretching vibrations are shifted towards higher and lower values, respectively. This regularity is in accordance with mean masses of corresponding groups of atoms.

Optical Properties
In reflected light, nöggerathite-(Ce) is optically anisotropic, with ∆R 589 = 1.27%. The color is light grey, with reddish brown internal reflections. The reflectance values in the visible range are given in Table 2. Mean refractive index calculated from the Gladstone-Dale equation is 2.267.

Chemical Composition
Chemical data for nöggerathite-(Ce) are given in Table 3. Contents of other elements with atomic numbers >8 are below detection limits. Based on structural data (see below) and by analogy with laachite [9], iron and manganese are considered as Fe 2+ and Mn 2+ , respectively.

X-ray Diffraction Data and Crystal Structure
Powder X-ray diffraction data are presented in Table 4. Diffraction peaks are readily indexed in the orthorhombic unit cell, space group Cmca. The unit cell parameters calculated from the powder data are: a = 7.296(1), b = 14.147(2), c = 10.161(1) Å, and V = 1048.9(2) Å 3 .  The crystal structure of the holotype sample (see Tables 5-7) was solved by direct methods based on single-crystal X-ray diffraction data and refined to R = 0.0198 for 574 unique reflections with I > 2σ(I). Nöggerathite-(Ce) is orthorhombic, with space group Cmca. The refined unit cell parameters are: a = 7.2985 (3) Table 6. Selected interatomic distances (Å) in the structure of nöggerathite-(Ce).
Unlike most samples of zirconolites, nöggerathite-(Ce) is non-metamict because it was formed recently: the last eruption of the Laach Lake volcano occurred no later than 13,000 years ago. According to Schmitt [14], the age of zircon from sanidinitic ejecta of the Laach Lake volcano is between 17,000 and 30,000 years. Consequently, sanidinite could have formed 4000-17,000 years before the eruption.
A non-metamict REE-, Nb-, and Mn-rich variety (or, in accordance with the accepted mineralogical nomenclature rules, an REE-analogue) of zirconolite-3O from the Laach Lake eruptive center was described by Della Ventura et al. [7] without structural data. Its chemical composition is close to that of nöggerathite-(Ce) and varies within the following ranges (calculated on the basis of 7 O apfu): Another crystalline mineral related to zirconolite-3O and nöggerathite-(Ce) is laachite (Ca 2 Zr 2 Nb 2 TiFeO 14 ), which originates from sanidinite of the Laach Lake volcano and demonstrates a perfect crystal structure. Laachite is a monoclinic (pseudo-orthorhombic) analogue of zirconolite-3O, with Nb prevailing over Ti in two octahedral sites [9]. Chemical compositions of 24 samples of zirconolite-type minerals from the Laach Lake volcano have been determined by us. Most of them correspond to stefanweissite (Ca,REE) 2 Zr 2 (Nb,Ti)(Ti,Nb) 2 Fe 2+ O 14 (IMA 2018-020), which is an analogue of zirconolite-3O with Nb as a species-defining component. Comparative data for nöggerathite-(Ce), zirconolite-3O, and laachite are given in Table 8 and in Figures 6-8. The main substitution scheme following from the compositional data for these minerals is: Ca 2+ + Ti 4+ + Zr 4+ ↔ REE 3+ + Nb 5+ + Mn 2+ . However, as one can see from Figure 7, the substitution Zr 4+ for Mn 2+ in the M(2) site is to be completed with other (subordinate) schemes involving zirconium, most probably Zr 4+ ↔ Ti 4+ and/or REE 3+ + Zr 4+ ↔ Ca 2+ + Nb 5+ .    REE-dominant zirconolite-type minerals (with REE > Ca in atomic units and Y prevailing among rare-earth elements) have been described in gabbro pegmatite in St. Kilda, Scotland, United Kingdom, in nepheline syenite from Tchivera, Angola, in metamorphic rocks of the Vestfold Hills, East Antarctica, and in lunar rocks [6], but crystal structures of these minerals have not been studied because of their metamict state.
An important specific feature of nöggerathite-(Ce) is its high content of niobium. A review of localities of zirconolites worldwide and a compilation of chemical compositions of about 300 samples were provided by Williams and Gieré [6]. All analyses show significant predominance of Ti over Nb. For most samples the content of Nb 2 O 5 is below 10 wt %. The Nb-richest zirconolite from Vuoriyarvi, Northern Karelia, Russia, was described by Borodin et al. [23] as "niobozirconolite". It contains 0.798 Nb atoms and 1.081 Ti atoms as per a formula calculated on the basis of 7 O atoms. High content of Nb was also detected in zirconolite samples from Kovdor, Kola, Russia (0.652 apfu Nb vs. 0.857 apfu Ti), Kaiserstuhl, Germany (0.687 apfu Nb vs. 0.702 apfu Ti), and Sokli, Finland (0.608 apfu Nb vs. 0.773 apfu Ti) [6].
Author Contributions: N.V.C., N.V.Z., and I.V.P. wrote the paper. C.S. collected the type material. B.T. and W.S. collected other samples of zirconolite-type minerals. S.N.B. obtained single-crystal X-ray diffraction data. N.V.Z. and D.Y.P. solved and refined the crystal structure. I.V.P. obtained powder X-ray diffraction data. V.N.E. and N.V.C. conceived and designed chemical data. M.F.V. obtained Raman spectra. N.V.C. analyzed Raman data. Y.S.P. measured reflectance data and hardness.
Funding: This research was funded by the Russian Science Foundation, under grant no. 14-17-00048 (for part of the chemical study of nöggerathite-(Ce) and other zirconolite-type minerals) and the Russian Foundation for Basic Research, under grant no. 18-29-12007 (for parts of the mineralogical study, Raman spectroscopy and X-ray structural analysis). SNB acknowledges St. Petersburg State University for financial support, grant no. 3.42.741.2017 (in part of single-crystal measurements).