Записки Российского минералогического общества, 2023, T. 152, № 1, стр. 8-17

Nioboixiolite-(Mn2+), $\left( {{\mathbf{N}}{{{\mathbf{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0em} 3}}}}{\mathbf{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0em} 3}}}^{{{\mathbf{2}} + }}} \right){{{\mathbf{O}}}_{{\mathbf{2}}}},$ a New Ixiolite-Group Mineral from the Malkhan Pegmatite Field, Transbaikal Region, Russia

N. V. Chukanov 12*, I. V. Pekov 2, N. V. Zubkova 2, V. O. Yapaskurt 2, Yu. S. Shelukhina 3, S. N. Britvin 3, D. Yu. Pushcharovsky 2

1 Federal Research Center of Problems of Chemical Physics and Medicinal Chemistry RAS
142432 Moscow region, Chernogolovka, Academician Semenov av., 1, Russia

2 Faculty of Geology, Moscow State University
119991 Moscow, Vorobievy Gory, Russia

3 Institute of Earth Sciences, Saint Petersburg State University
199034 Saint Petersburg, Universitetskaya Emb., 7/9, Russia

* E-mail: nikchukanov@yandex.ru

Поступила в редакцию 3.11.2022
После доработки 3.11.2022
Принята к публикации 14.12.2022

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Аннотация

The new ixiolte-group mineral nioboixiolite-(Mn2+), ideally $\left( {{\text{N}}{{{\text{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0em} 3}}}}{\text{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0em} 3}}}^{{2 + }}} \right){{{\text{O}}}_{{\text{2}}}}{\text{,}}$ the niobian analogue of ixiolite-(Mn2+), was discovered in the Sosedka granitic pegmatite vein, Malkhan pegmatite field, Zabaikalsky Krai (Transbaikal Region), Siberia, Russia. The associated minerals are albite, quartz, microcline, elbaite, beryl, bismuthinite, euxenite-(Y), zircon, rutile, cassiterite, and cannonite. Nioboixiolite-(Mn2+) occurs as clusters of dark brown to brown-black prismatic crystals up to 0.8 × 1.5 × 5 mm embedded in albite. The lustre is submetallic to adamantine, and the streak is brown. Cleavage is not observed. The Mohs’ hardness is 4.5–5. Density calculated using the empirical formula is equal to 5.803 g cm–3. The IR spectrum and reflectance spectra in visible range are given. The chemical composition of nioboixiolite-(Mn2+) is (electron microprobe, wt %): MnO 14.94, Sc2O3 1.80, Fe2O3 0.20, Y2O3 1.34, TiO2 7.66, ZrO2 1.74, SnO2 1.01, ThO2 0.26, UO2 1.44, Nb2O5 42.80, Ta2O5 26.77, total 99.96. The empirical formula is (Nb1.59${\text{Mn}}_{{1.04}}^{{2 + }}$Ta0.59Ti0.47Sc0.13Zr0.07Y0.06Sn0.03U0.03${\text{Fe}}_{{0.01}}^{{3 + }}$)Σ4.02O8 (Z = 1). The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0474. The new mineral is isostructural to other ixiolite-group members. Nioboixiolite-(Mn2+) is orthorhombic, space group: Pbcn, a = 4.762(2) Å, b = 5.739(1) Å, c = 5.149(1) Å, V  =  140.7(1) Å3. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.662 (29) (110), 2.984 (100) (111), 2.505 (21) (021), 1.775 (21) (130), 1.748 (28) (202), 1.726 (35) (221), 1.553 (20) (113), 1.473 (19) (023), 1.463 (30) (311, 132).

Keywords: nioboixiolite-(Mn2+), new mineral, ixiolite group, columbite supergroup, crystal structure, Malkhan pegmatite field

1. INTRODUCTION

According to the recently approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA CNMNC) nomenclature of columbite-supergroup minerals (IMA MEMORANDUM 108-SM22; Chukanov et al., 2022), the well-known mineral ixiolite got the new species name ixiolite-(Mn2+) and the idealized formula $\left( {{\text{T}}{{{\text{a}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0em} 3}}}}{\text{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0em} 3}}}^{{{\text{2 + }}}}} \right){{{\text{O}}}_{{\text{2}}}}{\text{.}}$ The new mineral species nioboixiolite-(Mn2+), ideally $\left( {{\text{N}}{{{\text{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0em} 3}}}}{\text{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0em} 3}}}^{{{\text{2 + }}}}} \right){{{\text{O}}}_{{\text{2}}}}{\text{,}}$ described in this paper is isostructural with ixiolite-(Mn2+) being its Nb-dominant analogue. Nioboixiolite-(Mn2+) is also isostructural with other members of the ixiolite group, scrutinyite α-PbO2 (Zaslavskiy and Tolkachev, 1952; Taggart et al., 1988), srilankite, (Ti,Zr)O2 (Willgallis et al., 1983; Willgalis and Hartl, 1983), and seifertite, SiO2 (Dera et al., 2002; El Goresy et al., 2008; Zhang et al., 2016). Thus, nioboixiolite-(Mn2+) is a niobium analogue of all these minerals belonging to the α-PbO2 structure type.

The new mineral nioboixiolite-(Mn2+) and its name were approved by the IMA CNMNC (IMA No. 2021-050a). The type specimen is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with registration number 5721/1.

2. EXPERIMENTAL METHODS

Fourteen electron microprobe analyses were carried out using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter.

In order to obtain infrared (IR) absorption spectrum, powdered nioboixiolite-(Mn2+) sample has been mixed with dried KBr, pelletized, and analyzed using an ALPHA FTIR spectrometer (Bruker Optics) in the range of 360–3800 cm–1 with a resolution of 4 cm–1. A total of 16 scans were collected. IR spectrum of an analogous pellet of pure KBr was used as a reference.

The reflectance values were measured in air by means of the MSF-21 microspectrophotometer (LOMO, Russia) with SiC (Reflexionsstandard 474251, No. 545, Germany) as a reference material.

Powder X-ray diffraction data were collected using a Rigaku R-AXIS Rapid II diffractometer (image plate), CoKα, 40 kV, 15 mA, rotating anode with the microfocus optics, Debye-Scherrer geometry, r = 127.4 mm, exposure 15 min. The raw powder XRD data were collected using program suite designed by Britvin et al. (2017). Calculated intensities were obtained by means of STOE WinXPOW v. 2.08 program suite based on the atomic coordinates and unit-cell parameters.

Single-crystal X-ray studies were carried out using an Xcalibur diffractometer equipped with a CCD-detector (MoKα radiation). Crystal data, data collection information and structure refinement details are given in Table 1.

Table 1.  

Data of the single-crystal X-ray diffraction experiment and crystal structure refinement for nioboixiolite-(Mn2+) Таблица 1.  Данные монокристального рентгенодифракционного эксперимента и уточнения кристаллической структуры ниобоиксиолита-(Mn2+)

Crystal system, space group, Z Orthorhombic, Pbcn, 4
Unit-cell dimensions, Å a = 4.7559(5)
b = 5.7318(5)
c = 5.1344(4)
V, Å3 139.97(2)
Crystal size, mm 0.06 × 0.12 × 0.42
Temperature, K 293(2)
Radiation and wavelength, Å MoKα; 0.71073
Diffractometer Xcalibur S CCD
Absorption correction Gaussian
θ range for data collection, ° 5.572–28.280
Reflections collected 1684
Independent reflections 174 (Rint = 0.0608)
Independent reflections with I > 2σ(I) 149
Refinement method Full-matrix least-squares on F 2
Number of refined parameters 16
Final R indices [I > 2σ(I)] R1 = 0.0474, wR2 = 0.0957
R indices (all data) R1 = 0.0592, wR2 = 0.1008
GoF 1.377
Largest diff. peak and hole, e/Å3 1.96 and –0.80

3. RESULTS

3.1. Occurrence, general appearance and physical properties. The holotype specimen of nioboixiolite-(Mn2+) was collected from the Sosedka granitic pegmatite vein, Malkhan pegmatite field, Krasnochikoisky District, Zabaikalsky Krai, Siberia, Russia. The new mineral forms prismatic, typically lath-like crystals up to 0.8 × 1.5 × 5 mm, elongated along [001] and flattened on [100]. They are commonly crude, divergent and sometimes combined in radiating clusters up to 4 mm × 1 cm embedded in albite (Fig. 1). The other associated minerals are quartz, microcline, elbaite, beryl, bismuthinite, euxenite-(Y), zircon, rutile, cassiterite, and secondary cannonite.

Fig. 1.

Aggregates of nioboixiolite-(Mn2+) in albite. Рис. 1. Агрегаты ниобоиксиолита-(Mn2+) в альбите.

Nioboixiolite-(Mn2+) is dark brown to brown-black, the lustre is submetallic on crystal faces and adamantine on the broken surface. The streak is brown. Cleavage is not observed. The fracture is conchoidal. The mean VHN hardness determined by micro-indentation at load of 100 g is equal to 303 kg/mm2 (range 282–330 kg/mm2, n = 5). The Mohs’ hardness is 4.5–5. Density calculated using the empirical formula and unit-cell parameters obtained from single-crystal X-ray diffraction data is equal to 5.803 g cm–3.

Under the microscope in reflected light, nioboixiolite-(Mn2+) is gray, with strong yellowish-brown internal reflections. Bireflectance is weak, ΔR = 0.7% (589 nm). Anisotropism is weak. Pleochroism is not observed. Reflectance values are given in Table 2 (the reference wavelengths required by the Commission on Ore Mineralogy are given in bold type).

Table 2.  

Reflectance values of nioboixiolite-(Mn2+) Таблица 2.  Коэффициенты отражения ниобоиксиолита-(Mn2+)

λ, nm R1 R2 λ, nm R1 R2
400 17.2 18.0 560 15.1 15.8
420 17.0 17.8 580 15.0 15.7
440 16.6 17.5 589 15.0 15.66
460 16.3 17.1 600 15.0 15.6
470 16.1 16.9 620 14.9 15.6
480 15.9 16.7 640 14.9 15.5
500 15.7 16.4 650 14.9 15.5
520 15.5 16.1 660 14.9 15.5
540 15.3 16.0 680 14.9 15.5
546 15.3 15.9 700 14.9 15.5

The IR spectrum of nioboixiolite-(Mn2+) (Fig. 2) is close to that of ixiolite-(Mn2+) (Chukanov, 2014). IR absorption in the range of 400–700 cm–1 and below 400 cm–1 is due to stretching and bending vibrations of the MO2 octahedral pseudo-framework (M = Nb, Mn2+, Ta, Ti, etc.). Taking into account that Mn2+ is the main low field-strength cation in nioboixiolite-(Mn2+), the band at 480 cm–1 may be tentatively assigned to stretching vibrations of the Nb–O–Mn2+ fragment. The shoulder at 870 cm–1 and the weak band at 1100 cm–1 correspond to combination modes. The absence of absorption bands above 1200 cm–1 indicates the absence of H-, B- and C-bearing groups. The IR absorption bands of nioboixiolite-(Mn2+) are broad and poor-resolved which indicates disordering of cations, in agreement with the structural data (see below).

Fig. 2.

Powder infrared absorption spectrum of nioboixiolite-(Mn2+). Рис. 2. Инфракрасный спектр поглощения порошка ниобоиксиолита-(Mn2+).

3.2. Chemical composition. Analytical data for nioboixiolite-(Mn2+) based on 14 spot analyses of a polished section are given in Table 3. Contents of other elements with atomic numbers higher than that of carbon are below detection limits. H2O was not measured because no bands corresponding to H-bearing groups are observed in the IR spectrum.

Table 3.  

Chemical composition of nioboixiolite-(Mn2+) Таблица 3.  Химический состав ниобоиксиолита-(Mn2+)

Constituent Average content, wt % Range of contents, wt % Standard deviation Probe Standard
MnO 14.94 13.28–16.30 0.91 Mn
Sc2O3 1.80 1.31–2.06 0.26 Sc
Fe2O3 0.20 0.09–0.26 0.05 Pyroxene Hyp-746
Y2O3 1.34 0.93–2.59 0.59 Y
TiO2 7.66 5.43–10.16 1.56 Ti
ZrO2 1.74 0.51–2.66 0.73 Zr
SnO2 1.01 0.43–1.99 0.40 Sn
ThO2 0.26 0.00–0.98 0.37 ThO2
UO2 1.44 0.62–3.75 1.05 UO2
Nb2O5 42.80 38.54–46.33 2.38 Nb
Ta2O5 26.77 24.71–28.48 1.15 Ta
Total 99.96    

The empirical formula of nioboixiolite-(Mn2+) calculated on the basis of 8 oxygen atoms per formula unit (apfu) is (Nb1.59${\text{Mn}}_{{1.04}}^{{{\text{2 + }}}}$Ta0.59Ti0.47Sc0.13Zr0.07Y0.06Sn0.03U0.03${\text{Fe}}_{{1.01}}^{{{\text{3 + }}}}$)Σ4.02O8 (Z = 1). The total number of cations in this charge-balanced formula calculated on the anionic basis is very close to 4.00 apfu which confirms bivalent state of Mn. The number of electrons per cation site calculated from this formula is equal to 39.2 which is close to the value of 40.5 obtained as a result of the crystal structure refinement. The relative difference between these values is in the frame of the standard deviations in Table 3. The simplified formula is (Nb,Mn2+,Ta,Ti)O2 and the formal, idealized end-member formula is $\left( {{\text{N}}{{{\text{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-0em} 3}}}}{\text{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0em} 3}}}^{{{\text{2 + }}}}} \right){{{\text{O}}}_{{\text{2}}}}{\text{.}}$

3.3. X-ray diffraction data and crystal structure. Powder X-ray diffraction data of nioboixiolite-(Mn2+) are given in Table 4. The orthorhombic unit-cell parameters refined from the powder data are: a = 4.762(2) Å, b = 5.739(1) Å, c = 5.149(1) Å, V = 140.7(1) Å3.

Table 4.  

Powder X-ray diffraction data of nioboixiolite-(Mn2+) Таблица 4. Порошковая рентгенограмма ниобоиксиолита-(Mn2+)

Iobs, % dobs, Å Icalc*, % dcalc**, Å h k l
29 3.662 31 3.660 110
100 2.984 100 2.980 111
14 2.868 7 2.866 020
14 2.573 15 2.567 002
21 2.505 17 2.502 021
8 2.380 8 2.378 200
5 2.262 2 2.259 102
9 2.217 4 2.215 121
11 2.105 9 2.102 112
7 1.916 5 1.912 022
7 1.832 5 1.830 220
21 1.775 13 1.773 130
28 1.748 15 1.745 202
35 1.726 21 1.724 221
20 1.553 13 1.550 113
7 1.492 4 1.490 222
19 1.473 8 1.469 023
30 1.463 10, 12 1.464, 1.459 311, 132
4 1.435 1 1.433 040
7 1.382 5 1.380 041
3 1.316 2 1.313 312
2 1.288 2 1.284 004
5 1.252 3 1.250 223

 * For the calculated pattern, only reflections with intensities ≥1 are given. ** For the unit-cell parameters calculated from single-crystal data; the strongest reflections are marked in boldtype.

The crystal structure of nioboixiolite-(Mn2+) was refined using the SHELX software package (Sheldrick, 2015) to R = 0.0474 using 151 unique reflections with I > 2σ(I) in the frame of space group Pbcn. The crystal data and the experimental details are presented in Table 1, atom coordinates, displacement parameters and site occupancies in Table 5. Selected interatomic distances are given in Table 6.

Table 5.  

Coordinates and equivalent displacement parameters (Ueq, in Å2) of atoms in the structure of nioboixiolite-(Mn2+) Таблица 5.  Координаты позиций и параметры атомных смещений (Ueq, Å2) в структуре ниобоиксиолита-(Mn2+)

Site x y z Ueq
M 0.0 0.32861(19) 0.25 0.0191(5)
O 0.7270(14) 0.1132(11) 0.4170(14) 0.021(2)

The refined number of electrons at the M site is eref = 40.52.

Table 6.  

Cation-oxygen distances (Å) in the structure of nioboixiolite-(Mn2+) Таблица 6.  Расстояния катион–кислород в структуре ниобоиксиолита-(Mn2+)

M–O 1.984(7) × 2
–O 2.052(8) × 2
–O 2.137(7) × 2
Mean 2.058

Nioboixiolite-(Mn2+) is a representative of the α-PbO2 structure type. The structure (Fig. 3) is based on the zig-zag chains of edge-sharing octahedra MO6 (the major M cations are Nb, Mn, Ta, and Ti), running along the c axis. Adjacent chains are linked in a direction via common oxygen vertices. All cations are disordered and fill a single M site.

Fig. 3.

The crystal structure of nioboixiolite-(Mn2+). The unit cell is outlined. Рис. 3. Кристаллическая структура ниобоиксиолита-(Mn2+). Показана элементарная ячейка.

4. DISCUSSION

The Nb-dominant structural analogue of ixiolite was described in several publications (von Knorring and Sahama, 1969; Wise et al., 1998; Badanina et al., 2008; Alekseev et al., 2010; Zubkova et al., 2020), usually as ixiolite (or its varieties, scandian ixiolite, wolframo-ixiolite, etc.), despite the prevailing of Nb over Ta. So-called “ashanite” initially described as a Nb-dominant analogue of ixiolite (Zhan Rubo et al., 1980) has been discredited by the IMA CNMMN because of unfair chemical data, corresponding to a mixture of several minerals, presumably ixiolite, samarskite-(Y), and uranmicrolite (Ganfu Shen, 1998; Jambor et al., 1999).

An overwhelming majority of finds of ixiolite and its Nb-dominant analogue, forming a continuous isomorphous series, is related to Li-F granites and, especially, rare-element granitic pegmatites. All samples of ixiolite series minerals from these formations contain significant amounts of Ta. Some of them contain U and/or Th admixtures and are partly or completely metamict.

The crystal structure of a natural niobium analogue of ixiolite was first published only recently, for an unusual Ta- and Sn-free and Ti- and Fe-rich sample from the Eifel paleovolcanic region, Germany (Zubkova et al., 2020), but a detailed investigation of this sample was not carried out because of scarcity of available material.

Synthetic Nb-dominant oxides isostructural with ixiolite are described in a number of works. In particular, the crystal structures of the compounds Fe3+NbO4–II (Harrison and Cheetham, 1989), $\left( {{\text{N}}{{{\text{b}}}_{x}}{\text{Fe}}_{x}^{{{\text{3 + }}}}{\text{Z}}{{{\text{n}}}_{{{\text{1}} - x}}}} \right)$(O4xF2 – 2x) (with х from 0.75 to 1.00: Pourroy et al., 1990), and Nb2TiZnO8 (Baumgarte and Blachnik, 1992) have been investigated. All of them belong to the α-PbO2 structure type.

Comparative data for nioboixiolite-(Mn2+) and closely related minerals are given in Table 7. Nioboixiolite-(Mn2+) is chemically related to columbite-(Mn), Mn2+Nb2O6, and can be considered as a cation-disordered analogue of this mineral.

Table 7.  

Comparative data of nioboixiolite-(Mn2+) and closely related minerals Таблица 7.  Сравнительные данные для ниобоиксиолита-(Mn2+) и родственных ему минералов

Mineral Nioboixiolite-(Mn2+) Ixiolite-(Mn2+) Columbite-(Mn)
Idealized formula (Nb2/3Mn2+1/3)O2 (Ta2/3Mn2+1/3)O2 Mn2+Nb2O6
Crystal system Space group Orthorhombic
Pbcn
Orthorhombic
Pbcn
Orthorhombic
Pbcn
a, Å
b, Å
c, Å
V, Å3
Z
4.7559
5.7318
5.1344
139.97
4
4.74–4.76
5.70–5.74
5.10–5.16
138.4–140.6
4
14.433
5.7637
5.0832
422.86
4
Strongest reflections of the powder X-ray diffraction pattern:
d, Å (I, %)
3.662 (29)
2.984 (100)
2.505 (21)
1.775 (21)
1.748 (28)
1.726 (35)
1.463 (30)
3.65 (32)
2.98 (100)
2.57 (13)
2.51 (20)
1.746 (17)
1.722 (24)
1.459 (29)
3.678 (90)
2.985 (100)
2.880 (50)
2.505 (50)
1.905 (60)
1.840 (60)
1.776 (60)
Density, g cm–3 5.803 (calc.) 6.94–7.23 (meas.)
7.392 (calc.)
5.20–6.65 (meas.)
5.30 (calc.)
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