Fluorluanshiweiite, KLiAl 1.5 □ 0.5 (Si 3.5 Al 0.5 )O 10 F 2 , a New Mineral of the Mica Group from the Nanyangshan LCT Pegmatite Deposit, North Qinling Orogen, China

: A new mineral species of the mica group, fluorluanshiweiite, ideally KLiAl 1.5 □ 0.5 (Si 3.5 Al 0.5 )O 10 F 2 , has been found in the Nanyangshan LCT (Li, Cs, Ta) pegmatite deposit in North Qinling Orogen (NQO), and V = 465.37(7) Å 3 . The strongest eight lines in the X-ray diffraction data are [ d in Å( I )(hkl)]: 8.427(25) (001), 4.519(57) (020), 4.121(25) (021), 3.628(61) (1 (cid:3364) 12), 3.350(60) (022), 3.091(46) (112), 2.586(100) (1 (cid:3364) 30), and 1.506(45) (312).


Introduction
A new mineral species of the mica group, KLiAl1.5□0.5(Si3.5Al0.5)O10F2, has been found in the Nanyangshan LCT pegmatite deposit in North Qinling Orogen (NQO), Central China (33°52′58″ N, 110°43′55″ E). It is named fluorluanshiweiite based on its relationship to luanshiweiite. It is characterized as F-dominant at the A site of luanshiweiite [1] or K-dominant at the I site of voloshinite [2]. The new mineral (IMA2019-053) has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA-CNMNC) [3]. The material was deposited in the mineralogical collection of the Geological Museum of China, No. 16, Xisi Yangrou Hutong, Xicheng District, Beijing 100034, People's Republic of China (catalogue number: M16085). This paper describes the physical, chemical, and spectroscopic data of fluorluanshiweiite and its crystal structure.

Chemical Composition
The chemical compositions of the fluorluanshiweiite and associated luanshiweiite and polylithionite were determined at the Beijing Research Institute of Uranium Geology with a JXA-8100 electron microprobe at 15 kV and 10 nA with a beam diameter of 10 μm. The FKα was determined with an LDE1 crystal (JEOL, 2d ≈ 6 nm), which is suitable for the analysis of 6 C-10 Ne, and the P/B ratio is much higher than that of the TAP crystal for F. The standards include phlogopite for K, Fe and Al; pollucite for Rb and Cs; albite for Na; tantalite-(Mn) for Mn; sanidine for Si and fluorapatite for F. The fluorine migration and peak overlap have been fully considered in the factors affecting the accurate quantification of F in the EPMA process to ensure that the results are reliable. The lithium content analysis was carried out using an LA-MC-ICP-MS instrument (Neptune, Thermo Fisher Scientific) equipped with a (ESI) NewWare 193 FX ArF Excimer laser ablation system with a 193-nm wavelength. The operating conditions were as follows: beam diameter = 30 μm, alongside a laser pulse rate of 8 Hz with an energy density of approximately 4 J/cm 2 ; N = 20. The obtained results were consistent with the data (3.95% Li2O) determined by atomic absorption spectroscopy (AAS). Scanning electron microscopy investigations were performed with an FEI Nova Nano scanning electron microscope (SEM) (10 μs; 15 kV). The anhydrous nature of the studied mineral was indicated by the absence of OH stretching vibration absorption in the ATR-FT/IR spectrum, as shown below.

Spectroscopic Features
Spectroscopic analyses were performed at the Beijing Research Institute of Uranium Geology. Infrared spectroscopic data of fluorluanshiweiite were obtained using a Bruker LUMOS spectrometer with an ATR model in the range 600-4000 cm −1 . Raman spectra were recorded on a LabRAM HR Raman microscope with a laser excitation wavelength of 532 nm. The power of the excitation radiation was 20 mW, and the lateral resolution was estimated to be 1 μm. The results were acquired in the range 100-2000 cm −1 .

Infrared Spectroscopy
The vibrations of mica group minerals can be roughly separated into the vibrational region of hydroxyl groups and the lattice vibrational region, including the vibrations of Si(Al)O4 tetrahedra and octahedrally coordinated cations [13]. In the IR spectrum of fluorluanshiweiite (Figure 2), the bands at 747 and 791 cm −1 are due to the Al-O fundamental modes, and the bands at 955, 979, and 1085 cm −1 are due to the Si-O fundamental modes, resembling the spectrum of luanshiweiite (760, 798, 891, 988, and 1115 cm −1 ) [1] and similar to the infrared spectral reference cards (Sil59, Sil60) of the lepidolite series [14]. There is no evidence for the high-energy OH stretching vibrations that commonly occur in the region from 3750-3550 cm -1 for mica group minerals. The defining characteristic of the infrared spectrum is the absence of the high-energy hydroxyl stretching vibration absorption band at −3620 cm -1 [1], which is the most significant difference between fluorluanshiweiite and luanshiweiite. The chemical analyses confirm that the A site in the new mineral is fully occupied by F (Table 1).
Raman spectra of mica minerals show several peaks in the low-wavenumber region: ~100, ~160, ~195, ~220, and ~240 cm -1 , of which peaks at ~195 and ~240 cm -1 are generally strong in diocatahedral lepidolite [19]. On the basis of the previous results of Loh [20], the peaks at 182 and 245 cm -1 for fluorluanshiweiite may be assigned to the internal vibrations of the MO6 octahedron ( Figure 3).

Crystal Structure
Both powder and single-crystal X-ray diffraction data of fluorluanshiweiite were collected on a Rigaku Oxford diffraction XtaLAB PRO-007HF microfocus rotating anode X-ray source (1.2 kW, MoKα, λ = 0.71073 Å) and a hybrid pixel array detector single-crystal diffractometer using MoKαradiation in the Laboratory of Crystal Structure, China University of Geosciences (Beijing, China).
X-ray powder diffraction data are given in A colorless flake of fluorluanshiweiite was selected for the single-crystal X-ray diffraction data collection. All reflections were indexed on the basis of a monoclinic unit cell. The structure was solved and refined using SHELX Software [21] based on the space group C2/m. Unit-cell parameters were as follows: a = 5.2030(5), b = 8.9894(6), c = 10.1253(9) Å, β = 100.68(1)° and V = 465.37(7) Å 3 . Anisotropic refinement using all measured independent data and reflections with Fo ≥ 4σ resulted in an R1 factor of 0.091. The relatively large R1 could be due to the crystal quality being imperfect; such values for the R factor are relatively often reported for the lepidolite series. In addition, the high R1 factor of micas may also be affected by the Durovič effect [22]. The crystal structure refinement details for fluorluanshiweiite are reported in Table 3. A view of the structure is presented in Figure 4. Refined coordinates and anisotropic-displacement parameters are presented in Tables 4 and 5, and selected bond lengths and angles are given in Table 6.   (2) Notes: I site is fixed by 0.85K + 0.12Rb, M2 site is fixed by 0.71Al and T site is fixed by 0.88Si + 0.12Al. Symmetry codes: (i) −1 + x, y, z; (ii) 1 + x, y, z.

Discussion
The structure of fluorluanshiweiite is similar to that of other species of the mica group. The general formula of mica group minerals may be written as IM2-3□1−0T4O10A2 [23]; for fluorluanshiweiite, I is K, M is Li + 1.5Al + 0.5□, T is 3.5Si + 0.5Al, and A is 2F. Regarding the interlayer cation, the difference between the inner (mean 2.970 Å) and outer (mean 3.237 Å) <I-O> distances is similar to that in the luanshiweiite-2M1 structure (2.938 and 3.251 Å, respectively). The mean <M1-O> distance in fluorluanshiweiite is 2.108 Å, including the Li-O distance, 2.112 Å, and the Li-F distance, 2.101 Å. These distances are comparable to the sum of the ionic radii of Li + , O 2-and F -(0.76, 1.40 and 1.38 Å, respectively) [24] and are slightly shorter than the luanshiweiite Li-O distance, 2.140 Å. The <M2-O> distance (mean 1.973 Å) is similar to that of luanshiweiite (mean 1.957 Å) and longer than the values given by Shannon and Prewitt [25] for Al-O (1.91 Å). The octahedral distance from the center of a vacant site to the nearest six oxygen atoms should be considered in reference to other mica minerals, e.g., montdorite and yangzhumingite [26][27][28][29]. In this case, the occurrence of 0.5 vacancies at the M2 site in fluorluanshiweiite must be considered. On the basis of the mean □−O distance in reported dioctahedral micas of 2.174 Å [30,31], the ideal <M2−O> distance in fluorluanshiweiite should be 1.976 Å, which is similar to the distance 1.973 Å observed. The mean <T−O> distance (1.626 Å) in fluorluanshiweiite is similar to that in luanshiweiite (mean 1.628 Å), longer than that in polylithionite (1.621 Å) [32] and shorter than that in trilithionite (1.638 Å) [33,34] due to substitution of smaller Si 4+ and larger Al 3+ in the tetrahedral layer, which is consistent with the occupancy of Si and Al at the T site in the lepidolite species above. Octahedral vacancies can be introduced by a substitution mechanism such as Li + + 0.5Si 4+ ↔ Al 3+ + 0.5□.
Bond-valence analysis (BVS): The bond valence (vu) was calculated from the interatomic distance following the procedure of Brown and Altermatt [35]. The bond-valence sums for the I(K), M1(Li), M2(Al1.5□0.5), and T(Si3.5Al0.5) positions are 0.95, 0.97, 2.31, and 3.97 valence units (vu), respectively, which is in agreement with the expected values given that various cations are present at the same sites. The low BVS for M2 suggests that it has a vacant cation position. The bond-valence sums for the O1, O2, and O3 positions are 2.11, 2.12, and 2.06 vu, respectively. The bond-valence sum for the A site, which is occupied by a hydroxyl group in luanshiweiite [1], is 0.89 vu for F in fluorluanshiweiite. As shown in Table 7, all the resulting BVS values are comparable to ideal values, and the model basically matches the charge balance requirement. Notes: Bond valence sums were calculated with the site-occupancy factors given in Table 4. Calculations were done using the equation and constants of [36], S = exp[(R0-d0)/b]. The symbols → and ↓ mean that the value must be multiplied by a factor 2 or 4 horizontally and vertically, respectively.
In conclusion, fluorluanshiweiite is the third light mica with substantial lithium and a stoichiometry intermediate between diocatahedral and trioctahedral; only two other Li micas have this stoichiometry: luanshiweiite [1] and voloshinite [2]. Fluorluanshiweiite can be considered as the F-dominant analogue at the A site of luanshiweiite or a K-dominant analogue at the I site of voloshinite (Table 8). Notes: Data reference from * Reference [1], † Reference [2].
Author Contributions: K.Q., X.S., G.L., G.F, and G.S. designed the experiments and wrote the paper; G.L. collected the single-crystal X-ray diffraction data and analyzed the crystal structure data; L.Q. obtained the IR spectra and analyzed IR and Raman data; Y.W., X.L., and Z.X. obtained the lithium content data; and G.F. and H.G. collected the electron-microprobe data. All authors have read and agreed to the published version of the manuscript.
Funding: Financial support for this research was received from the Natural Science Foundation of China (NSFC Grant 41502033) and the China Geological Survey Project (DD20160129-3, DD20190813, 1212011120771).