Rippite, K 2 (Nb,Ti) 2 (Si 4 O 12 )O(O,F), a New K-Nb-Cyclosilicate from Chuktukon Carbonatite Massif, Chadobets Upland, Krasnoyarsk Territory, Russia

: Rippite K 2 (Nb,Ti) 2 (Si 4 O 12 )(O,F) 2 , a new K-Nb-cyclosilicate, has been discovered in calciocarbonatites from the Chuktukon massif (Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia). It was found in a primary mineral assemblage, which also includes calcite, ﬂuorcalciopyrochlore, tainiolite, ﬂuorapatite, ﬂuorite, Nb-rich rutile, olekminskite, K-feldspar, Fe-Mn–dolomite and quartz. Goethite, francolite (Sr-rich carbonate–ﬂuorapatite) and psilomelane (roman è chite ± hollandite) aggregates as well as barite, monazite-(Ce), parisite-(Ce), synchysite-(Ce) and Sr-Ba-Pb-rich keno- / hydropyrochlore are related to a stage of metasomatic (hydrothermal) alteration of carbonatites. The calcite–dolomite coexistence assumes crystallization temperature near 837 ◦ C for the primary carbonatite paragenesis. Rippite is tetragonal: P 4 bm , a = 8.73885(16), c = 8.1277(2) Å, V = 620.69(2) chains and [Si 4 O 12 ] 8 − rings. In structural and chemical aspects it seems to be in close with the labuntsovite-supergroup minerals, namely with vuoriyarvite-(K), K 2 (Nb,Ti) 2 (Si 4 O 12 )(O,OH) 2 · 4H 2 O.


Introduction
The natural analog of the tetragonal synthetic phase KNbSi 2 O 7 , a new structural type among natural K-Nb-cyclosilicates, named rippite, was discovered in calciocarbonatites of the Chuktukon massif, Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia [1,2].

Brief Geological Background for the Chuktukon Massif
The Chadobets alkaline complex is located within the Chadobets upland (basin of the Chadobets River, Krasnoyarsk Territory, Russia) at the southwestern part of the Siberian craton ( Figure 1) [36]. The upland is 2000 km 2 in size and occurs at the intersection of two Neoproterozoic grabens being a part of the Angara-Kotuy large-scale rift system [29,[37][38][39]. The Chadobets alkaline ultramafic carbonatite complex is conventionally subdivided into the Chuktukon alkaline ultramafic carbonatite massif (southern part of the upland) and the Terina alkaline massif (silicate rocks, northern part of the upland). In addition, the Chadobets upland also includes dikes of ultramafic lamprophyres (aillikite-damtjernite series). Carbonatites and their weathering species of the Chuktukon massif host major reserves of Nb and REE [40][41][42]. The U-Pb and Ar-Ar ages for the Chadobets ultramafic alkaline rocks and carbonatites are between 252 and 231 Ma [30,31,43]. For comparison, the Ilbokich aillikite-damtjernite lamprophyre series, which is located nearby the Chadobets upland, has 392 Ma age [43][44][45]. The detailed geology of the Chadobets alkaline ultramafic carbonatite complex was described by Kirichenko et al. [36]. The sequence of the Chuktukon rocks, from the earliest to the youngest, was established by field observations (drillholes description) and comprises alkaline ultramafic rocks, calciocarbonatites and, finally, damtjernites [36]. Alkaline ultramafic rocks (aillikites-damtjernites, olivine melilitites, peridotites and others) of the first emplacement phase form small plugs (up to 1-1.5 km 2 ), dykes and sills ranging in thickness up to 100-120 m. Carbonatites also form small plugs (up to 4 km 2 ), dykes and sills (up to 20 m thick). Damtjernites are presented by pipes (up to 0.5 km 2 ), crosscutting early phases of alkaline ultramafic rocks and carbonatites and typically containing their xenoliths and fragments of the country sedimentary rocks. All rocks of the Chuktukon massif are altered in different degree by hydrothermal and weathering processes [36]. The detailed petrography and mineralogy of the Chadobets alkaline rocks were summarized in [29,30,32,36,43,46,47].

Analytical Methods
Double-polished sections of calciocarbonatites (~50-100 µm in thickness) and individual rippite grains from the Chuktukon massif were used for optical examination in transmitted and reflected light and for other studies. The individual rippite crystals were selected from residue after dissolution of carbonatite samples in the diluted HCl or acetic acid.
The identification of all minerals in the Chuktukon calciocarbonatites was based on energy-dispersive spectra (EDS), back-scattered electron (BSE) images and elemental mapping (EDS system), using a TESCAN MIRA 3MLU scanning electron microscope equipped with an INCA Energy 450 XMax 80 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) at the IGM, Novosibirsk, Russia. The SEM-EDS analyses of minerals were operated at an accelerating voltage of 20 kV and a probe current of 1 nA in high-vacuum mode and at an accumulation time of 20-40 s.
The contents of some light trace elements (Be, Li, B), F and H 2 O in rippite of the Chuktukon carbonatite were analyzed by secondary-ion mass spectrometry (SIMS) on a Cameca IMS-4f ion probe at the Analytical Centre of the Yaroslavl Branch of the Institute of Physics and Technology (YBIPT), Yaroslavl, Russia. For the analysis, grains larger than 20 µm and previously analyzed by EMPA-WDS and SEM-EDS were selected. Analysis of trace elements was carried out by the energy filter method; the operating conditions were as follows: primary O 2beam-20 µm, I = 2-4 nA, energy offset-100 eV, and energy slit-50 eV. Concentrations of elements were determined from the ratios of their isotopes to 30 Si, using calibration curves for standard samples [48]. The hydrogen content was determined from the 1 H mass together with trace elements. Low background content of H 2 O (0.03 wt %) in the mass spectrometer was achieved by storing the samples for 24 h in high-vacuum. The NIST SRM610 glass was used as external standard [49].
The trace element composition of rippite was also determined by laser ablation ICP-MS at the Friedrich-Alexander University, Erlangen-Nürnberg, Germany. The instrumentation includes an Agilent 7500i inductively coupled plasma mass spectrometer combined with an ESI New Wave UP193-FX laser ablation system. Samples were ablated at plasma power of 1350 W in an atmosphere of pure He (0.65 L/min) and Ar (1.10 L/min) as gas carriers. In addition, Ar was used as the plasma (14.9 L/min) and auxiliary gas (0.9 L/min) plasma immediately after the ablation cell. Analyses were conducted in spot mode, each analysis lasting 80 s, including 20 s of background acquisition (laser off) followed by 60 s of ablation. A beam 10-50 µm in diameter (2.7 J/cm 2 ) with a laser frequency of 16 Hz was used. The NIST SRM610 standard [49] was used to calibrate the weights of determined elements. The contents of minor elements were compared to those measured on an electron microprobe. Trace element concentrations were calculated with the GLITTER software [50].
The Raman spectra for rippite were recorded on a LabRAM HR 800 mm (HORIBA Scientific Ltd., Kyoto, Japan) spectrometer equipped with a 1024 pixel LN/CCD detector and coupled to an Olympus BX40 confocal microscope (objective ×100) at the IGM. A semiconductor laser emitting at 514.5 nm with a nominal power output of 50 mW was used for excitation. In each case, a 200 µm confocal hole and integrated. Most spectra were recorded between 100 and 1200 cm −1 , and some spectra were made for the 100-4000 cm −1 and 3000-4000 cm −1 region. The monochromator was calibrated using the 520.7 cm −1 Raman line of elemental Si. A Bruker Vertex 70 FTIR spectrometer equipped with a Hyperon 2000 microscope at the IGM was used for measurements of rippite in the IR region (powder KBr pellets and individual crystals putted on KBr plate). Pellets for infrared absorbance measurements were obtained by pressing a mixture of about 1 mg of powdered rippite diluted in 400 mg of dried KBr. Spectra over 4000-370 cm −1 range were obtained by of 23 scans with a resolution of 2 cm −1 . Interferences from KBr and air were cancelled by subtracting their spectra from the sample spectrum.
The main results are presented in Figures 1-15 and Tables 1-9. Some data are given in Supplementary section (Table S1 and Figures S1-S7).        . The short-wave edge of the rippite transmission spectra at 300 (curve 1) and 80 K (curve 2). In the insert: the Tauc plot for the case when the edge shape is determined by direct allowed electronic transitions. The band gap E g for rippite is 4.37 and 4.50 eV at 300 and 80 K, respectively.     Table 2 and Table S1.      Figure 4 and Figures S1-S4.  The strongest diffraction lines are given in bold.
Detailed studies of the petrography and mineralogy of the highly altered calciocarbonatite are given in [32,40]. These rocks are composed of relict minerals (calcite, fluorcalciopyrochlore, rippite, rutile, and zircon) and minerals formed during alteration. The rocks are fine-grained, banded, and patchy-striped, which is expressed as alternating bands and spots of goethite and rare earth minerals. The groundmass is composed of an aggregate of "goethite" and kaolinite. Florencite-(Ce), monazite-(Ce), "psilomelane" aggregate, churchite-(Y), and daqingshanite-(Ce) are minor and accessory minerals.
"Goethite" aggregate from hydrothermally altered carbonatites contains up to 2.2 wt % of Nb 2 O 5 [32]. Florencite-(Ce) and monazite-(Ce) occur as irregularly shaped grains, up to 0.3 cm in size, as well as aggregates of micron-sized grains of intricately shaped patches, as well as network of vein-like segregations. Typically, both minerals have a porous internal structure. Churchite-(Y) forms radially fibrous 2-5 mm aggregates and individual 2-3 mm crystals. A network of "psilomelane" (microveinlets) dissects the rocks [29]. The petrographic observations of different types of the Chuktukon calciocarbonatites have shown that rippite is very resistant to alteration/weathering processes. In addition to holotype sample 546-193.5 ( Figure 2) this mineral was also found as minor component in calciocarbonatites (including highly altered samples) from 180-200 m depths of neighboring drillholes.

Morphology, Optical and Physical Properties of Rippite
Rippite commonly forms elongated prismatic crystals (up to 0.5-2 mm) and their intergrowths in carbonate matrix (Figures 2 and 3), rarely in the "goethite" or "francolite" aggregates. The major forms are prism {100} and pinacoid {001}. Twinning is none observed. The mineral is colorless and it may be confused with fluorapatite due to elongated crystals.
The color of the powdered mineral is white. Rippite has vitreous luster and weak fluorescence. The second harmonic generation test indicated nonlinear optical properties of the mineral. Under laser the weak glow of rippite powder was steady and green that was comparable with colour characteristics of the reference sample (LiIO 3 ). Cleavage is very perfect on (001) and perfect to distinct on (100); parting is none observed, fracture is stepped to uneven across cleavage (Figure 3). The Mohs' hardness of rippite is 4-5, micro-indentation hardness is VHN 50 = 210-487 kg/mm 2 , mean (n = 19)-307 kg/mm 2 . The density measured by flotation in the Clerici liquid (HCOOTl + Tl 2 [OOCCH 2 COO]) is 3.17(2) g/cm 3 . Density calculated from unit-cell dimensions and results of electron-microprobe analyses assumes 3.198 g/cm 3 . Rippite is optically uniaxial (+); ω = 1.737-1.739; ε = 1.747 (589 nm). The optical orientation is X = c. No dispersion and pleochroism were observed. The mineral is not soluble in concentrated HCl and H 2 SO 4 that is comparable with the synthetic phase. The Gladstone-Dale's compatibility factor for the holotype rippite is 0.005 (superior).
Fluorcalciopyrochlore octahedra and tainiolite blades occasionally occur as inclusions in rippite, and vice versa rippite (± quartz and other phases) sometimes fills the fissures in fluorcalciopyrochlore crystals (Figure 4, Figures S1 and S2). Such relationships indicate that rippite is later Nb-mineral than fluorcalciopyrochlore and crystallized together with tainiolite or later (Figure 4, Figures S1-S3). Most of rippite grains are partially resorpted by quartz especially along cleavage planes and even replaced by quartz in the outer parts of some crystals (Figures 3-5, Figures S3-S5). "Goethite" impregnation is common of some grains occurring in altered carbonatite samples. In general, rippite is resistant to low-temperature alteration or weathering, but may be partially replaced by quartz (plus rutile and other phases) during carbonatite crystallization ( Figure 5 and Figures S3-S5).

SEM and EMPA data
Majority of rippite prismatic crystals are homogeneous in composition and unzoned or weakly-zoned that is virtually invisible in the elemental SEM maps (Figures S3, S6 and S7). The contrast zonation is only fixed for rare grains with narrow Ti-rich outermost zone. Those core-to-rim variations are well indicated in the Nb and Ti elemental maps ( Figure 5). In general the chemical composition of the mineral is close to the ideal structural formula K 2 Nb 2 (Si 4 O 12 )O 2 (Tables 2  and 3 Figure 5). The ZrO 2 amount is up to 1.3 wt %. However, there is no any clear correlation ZrO 2 with TiO 2 and Nb 2 O 5 ( Figure 6). All above-mentioned data strongly suggest the incorporation of (Ti,Zr) and F in the structure of rippite via the isomorphism Nb 5+ + O 2− → (Ti,Zr) 4+ + F 1− (Figure 7). The content of a hypothetical end-member K 2 Ti 2 [Si 4 O 12 ]F 2 may be up to 17 mol. % ( Table 2). The database of chemistry for all rippite grains is given in Supplementary Table S1.
The contents of some light trace elements (Be, Li, B), F and H 2 O in low-Ti rippite were analyzed by SIMS (Table 4). The results indicate very low concentration of light elements (<5 ppm) and H 2 O (0.09-0.23 wt %). The content of F is consistent with EMPA-WDS data.

Cathodoluminescence
The CL images for single rippite crystals show green colour and very complex history during their growth (Figure 8). However, the individual zones and regions fixed in the CL images are weakly documented by the BSE images and elemental maps and do not reflect the variations in chemical composition (Figure 8, Figures S6 and S7). In general, the stages of nucleation, chaotic growth, dissolution and further growths may be observed in the CL images of individual rippite grains (Figure 8).

Transmission Spectra
The color of the powdered rippite is white and its crystals are almost colorless. Figure 9 shows the short-wave edge of the optical transmission spectra obtained for a crystal of about 150 × 150 mm 2 and about 100 mm thick, at 300 and 80 K. It is seen that the crystal becomes transparent from a wavelength of about 275 nm in the UV region of the spectrum. The Tauc analysis [51] indicates that the spectra are straightened in coordinates (α*(hν)) 2 = f(hν), where α is the absorption coefficient and hν is the photon energy. This is evidence that the shape of the fundamental absorption edge is determined by the direct allowed band-to-band electronic transitions. The position of the intersection points of straight lines with the abscissa axis determines the values of the band gap at 300 and 80 K: E g = 4.37 and 4.50 eV, respectively. Under the action of laser radiation with a wavelength of 1052 nm, second-harmonic radiation (526 nm) is generated in the rippite powder. The intensity of the second harmonic is comparable to that obtained from the powder of the reference sample, a well-known nonlinear optical crystal, lithium iodate (LiIO 3 ), for which the second-order nonlinear susceptibility is d 31 = 4.1 ± 0.4 pm/V [52].
Rippite has a weak fluorescence and, in addition, its samples show spontaneous luminescence in the range of 100-400 K when heated or cooled at a rate of~30 deg/min. This glow is typical for pyroelectrics [53] and indicates the absence of an inversion center in the structure. Indeed, rippite with a structure with a point group of 4mm belongs to one of the 10 crystallographic groups with a pyroelectric effect: 1, 2, 3, 4, 6, m, mm, mm2, 3m, 4mm and 6mm [54]. Such crystals have electric polarization even in the absence of an external electric field (spontaneous polarization). Pyroluminescence is the result of modifications in the atomic polarization in the crystal volume as temperature changes [53]. Flashes of light are the result of an electrical breakdown in pyroelectric fields, the intensity of which reaches hundreds of kV/cm.

Infrared and Raman Spectroscopy
Tetragonal rippite and its synthetic analog are related to space group P4bm (No. 100), its point group is C 4v (4mm). Symmetry-representation analysis gives 105 modes of vibrations for rippite, which can be divided as 21A 1 + 16A 2 + 12B 1 + 17B 2 + 39E, where 20A 1 + 38E vibrations are infrared active, 20A 1 + 12B 1 + 17B 2 + 38E vibrations are Raman active, and 16A 2 vibrations are inactive in both regions. Acoustic modes A 1 + E are not included here. Among these, the silicate tetrahedrons will share the following internal vibrations 5A 1 + 4B 1 + 5B 2 + 9E. External vibrations include translation and SiO 4 libration modes. Their irreducible representations can be expressed as: 8A 1 + 7B 1 + 8B 2 + 18E and 7A 1 + B 1 + 4B 2 + 11E, respectively. The C 4v point groups allow the simultaneous activity of some part of vibrations in both IR and Raman spectra [55]. The number of IR and Raman modes observed for rippite is fat fewer than predicted, because of the low intensity of some modes and orientation effects. Crystals of Ti-poor rippite were mainly used for the IR and Raman spectroscopic studies.
The strongest absorption band of the IR spectrum of rippite is 967 cm −1 ( Figure 10) and assigned to be the ν3 internal stretching mode of the SiO 4 tetrahedra.  (Figures 10 and 11). However, heating of rippite in powder KBr pellet up to 380 • C and following IR measurements resulted to diminishing of these bands. Consequently, the presence of the substantial bands in the 1600-1700 and 3400-3500 cm −1 region in unheated rippite samples may be partially related to absorbed H 2 O. Nevertheless, we have to mention that our SIMS data indicate low H 2 O content (0.09-0.23 wt %) in the Ti-poor rippite. Thus, weak bands in the 3000-4000 cm −1 range, which may be related to the vibrations of (OH)-groups, unfortunately cannot outline the presence or absence of H 2 O and as result the true H 2 O concentration. In general, bands below 360 cm −1 are ascribed to vibrations in polyhedra and octahedra as well as librational vibrations of the Si 4 O 12 group. Bands in the 390-700 cm −1 region are related to tetrahedral SiO 4 bending vibrations (ν4 + ν2). The strongest Raman band at 761 cm −1 (Figure 12) is assigned to be the internal stretching mode ν1 of the SiO 4 tetrahedra. The ν3 modes of the SiO 4 tetrahedra seem to be bands in the 900-1200 cm −1 region.
In addition, we tried to obtain data for rippite concerning to the dependence between chemical composition and changing in Raman spectra. For this purpose, we used zoned crystal Nb3-18. All spectra were obtained during one analytical session and in same conditions. The most substantial differences are visible for the bands at 934, 395 and 354 cm −1 (Figure 13). The increasing of the Ti(+Zr) content in rippite lead to left shifting for the 934 cm −1 band and to right shifting for the 395 and 354 cm −1 bands in Raman spectra. Unfortunately, such insufficient deviations (in 1-2 cm −1 ) cannot be used for identification of chemical composition for rippite according to Raman spectra.

X-ray Crystallography and Crystal Structure of Rippite
The second harmonic generation test for rippite indicated the weak glow of powder, which was steady and green. The reference sample (LiIO 3 ), which is used in nonlinear optics, showed the same colour characteristics. All this strongly assumes a noncentro-symmetrical structure for rippite.
In contrast with this K-Nb-silicate, rippite and synthetic K 2 (NbO) 2 Si 4 O 12 display a strong non-linear effect, as the NbO 6 octahedra are linked in columns (Figures 14 and 15).

Discussion and Concluding Remarks
In general the detailed studies for the Chuktukon calciocarbonatites (Chadobets upland, Krasnoyarsk Territory, Russia) gave the possibility of describing the chemical composition and crystal structure for a new mineral, rippite K 2 (Nb,Ti) 2 (Si 4 O 12 )(O,F) 2 , which belongs to a new structural type among the [Si 4 O 12 ]-cyclosilicates and is close to the labuntsovite supergroup. This mineral is related to primary phases, which were crystallized after fluorcalciopyrochlore, the main carrier of Nb in the calciocarbonatites. The primary carbonatite paragenesis may be formed at temperatures near and below 837 • C according to the calcite-dolomite thermometer. The synthesis in the system SiO 2 -Nb 2 O 5 -K 2 O has indicated that the K 2 Nb 2 (Si 4 O 12 )O 2 phase may be stable in the 510-735 • C temperature range [4,6,[8][9][10]13,14].
Rippite is the second main mineral to concentrate Nb in the Chuktukon calciocarbonatites [1,2,29,32]. The presence of this mineral should be into account in processing schemes of carbonatitic Nb-ores, as in some rock species the modal contents of pyrochlore and rippite may be equal. The weathered carbonatites are the basis of the Chuktukon niobium-rare earth elements ore deposit, which is very promising for development in eastern Siberia in a future [40][41][42]. It is quite possible that unweathered calciocarbonatites will be also involved in processing schemes. In general, it is concerning not only to the Chuktukon calciocarbonatites, but and to other pyrochlore-rich carbonatites around the world [64]. At present rippite was found only in calciocarbonatites of the Chuktukon massif. However, we are not sure that this mineral is endemic for the Chuktukon rocks and may be also occur in other pyrochlore-containing carbonatites worldwide.
It should be noted that the mineral phase with ratio of the main components (K:Nb:Si) similar to rippite has been described as minute inclusions in late apatite aggregates from calciocarbonatites of the Araxá alkaline-carbonatite complex, Brazil [65]. According to chemistry (SiO 2 -37.9-39.3 wt %; Nb 2 O 5 -34.7-37.6 wt %; TiO 2 -0.3-1.6 wt %; K 2 O-14.2-14.4 wt %) and low total sums this mineral was identified as nenadkevichite (labuntsovite supergroup). Probably further studies will clearly outline whether the Araxá mineral is rippite or it is vuoriyarvite-K (labuntsovite supergroup).
The discovery of rippite also highlights some problems in the synthesis and growth of high-quality crystals suitable for industrial purposes. It is known that synthetic analog of rippite is a good material for nonlinear optics [3][4][5][6][7][8][9][10][11][12][13][14]. Ideal and large single crystals of KNbSi 2 O 7 (near 1 cm) of high optical quality, which are valid for industrial purpose, are still rarity [14]. The mineral association of rippite-containing calciocarbonatites may be a key in understanding of favorable conditions for synthesis of the perfect KNbSi 2 O 7 crystals (Ca-rich carbonate environment).
The compositional study of rippite strongly suggests the incorporation of (Ti,Zr) and F in the structure via the isomorphism Nb 5+ + O 2− → (Ti,Zr) 4+ + F 1− . As a result the content of a hypothetical end-member K 2  It should be also mentioned that rippite has been successfully used as a geochronometer. The Ar-Ar age estimated for rippite from the Chuktukon calciocarbonatite (sample 546-193.5) is 231.1 ± 2.7 Ma [31].