The Crystal Structure of Manganotychite, Na₆Mn₂(CO₃)₄(SO₄), and Structural Relations in the Northupite Group

Abstract

The crystal structure of manganotychite has been refined using the holotype specimen from the Alluaiv Mountain, Lovozero massif, Kola peninsula, Russia. The mineral is cubic, Fd$\overline{3}$, a = 14.0015(3) Å, V = 2744.88(18) ų, Z = 8, R₁ = 0.020 for 388 independently observed reflections. Manganotychite is isotypic to tychite and ferrotychite. Its crystal structure is based upon a three-dimensional infinite framework formed by condensation of MnO₆ octahedra and CO₃ groups by sharing common O atoms. The sulfate groups and Na⁺ cations reside in the cavities of the octahedral-triangular metal-carbonate framework. In terms of symmetry and basic construction of the octahedral-triangular framework, the crystal structure of manganotychite is identical to that of northupite, Na₃Mg(CO₃)₂Cl. The transition northupite → tychite can be described as a result of the multiatomic 2Cl⁻ → (SO₄)²⁻ substitution, where both chlorine and sulfate ions are the extra-framework constituents. However, the positions occupied by sulfate groups and chlorine ions correspond to different octahedral cavities within the skeletons of Na atoms. The crystal structure of northupite can be considered as an interpenetration of two frameworks: anionic [Mg(CO₃)₂]²⁻ octahedral-triangular framework and cationic [ClNa₃]²⁻ framework with the antipyrochlore topology. Both manganotychite and northupite structure types can be described as a modification of the crystal structure of diamond (or the dia net) via the following steps: (i) replacement of a vertex of the dia net by an M₄ tetrahedron (no symmetry reduction); (ii) attachment of (CO₃) triangles to the triangular faces of the M₄ tetrahedra (accompanied by the Fd$\overline{3}$m → Fd$\overline{3}$ symmetry reduction); (iii) filling voids of the resulting framework by Na⁺ cations (no symmetry reduction); and (iv) filling voids of the Na skeleton by either sulfate groups (in tychite-type structures) or chlorine atoms (in northupite). As a result, the information-based structural complexity of manganotychite and northupite exceeds that of the dia net.

1. Introduction

The northupite group of minerals contains four mineral species: northupite, tychite, ferrotychite, and manganotychite. Historically, the first mineral of the group was northupite, Na₃Mg(CO₃)₂Cl, discovered in Borax Lake, California, by C.H. Northup and described by Foote in 1895 [1] and Pratt in 1896 [2]. Its sulfate analogue, tychite, Na₆Mg₂(CO₃)₄(SO₄), was described several years later from the same locality and the same sample [3,4]. Later, both minerals have been found in other saline lakes [5,6,7], e.g., Lake Katwe, Uganda [8]. A.P. Khomyakov and coauthors described Fe and Mn analogues of tychite, which were named ferrotychite, Na₆Fe₂(CO₃)₄(SO₄) [9], and manganotychite, Na₆Mn₂(CO₃)₄(SO₄) [10], respectively. Ferrotychite was found in the Oleniy Ruchey mineral deposit in the Eastern part of the Khibiny alkaline massif, Kola peninsula, Russia, whereas manganotychite was described from the underground workings of the Alluaiv mountain, North-Western part of the Lovozero alkaline massif, Kola peninsula, Russia. In both cases, the formation of minerals was associated with the late stages of hydrothermal crystallization from solutions enriched by alkali and fugitive components of the alkali silicate liquids [9,10]. Recently tychite [11] and northupite [12] were described from melt inclusions in olivine from mantle xenoliths of kimberlite pipes from Siberia, Russia.

The investigations of the crystal chemical and spectroscopic properties of the northupite-group minerals started in 1931 when Gossner and Koch [13] and Shiba and Watanabé [14] provided preliminary structural models for northupite. In particular, Shiba and Watanabé [14] attempted to solve the structure of tychite in the space group Fd$\overline{3}$m. The correct space group, Fd$\overline{3}$, was determined by Gossner and Koch [13]. Watanabé [15] solved the crystal structures of both tychite and northupite in the Fd$\overline{3}$ space group with the subsequent refinements for northupite and tychite provided by Dal Negro et al. [16] and Schmidt et al. [17]. The crystal structure of ferrotychite was refined by Malinovskii et al. [18]. Spectroscopic properties of the northupite-group minerals have been studied in a number of works [17,19]. In the most recent work, Sidorov et al. [20] reported Raman spectra for tychite, manganotychite, and ferrotychite.

It is worthy of note that synthetic compounds with the northupite-type structure have been actively investigated recently due to their remarkable magnetic properties [21,22,23]. When the divalent metal sites are occupied by magnetic ions (Co²⁺ or Mn²⁺), these ions form a so-called pyrochlore lattice consisting of the framework of corner-sharing M₄ tetrahedra (M = Co²⁺ or Mn²⁺) that form a cristobalite-type arrangement with the diamond topology. For the cobalt analogue of northupite, Na₃Co(CO₃)₂Cl, the coexistence of magnetic order and spin-glass state have been reported by Fu et al. [22]; whereas for Na₃Mn(CO₃)₂Cl, no signature of a spin-glass transition or magnetic order had been detected [23].

In this paper, we provide the results of a single-crystal diffraction study of manganotychite, the only natural member of the northupite group that had not been characterized structurally. In addition, we discuss structural relations in the northupite group, in particular, the structural aspects of the chlorine-for-sulfate substitution and antiperovskite (antipyrochlore) features of the northupite structure type.

2. Materials and Methods

2.1. Sample Description and Composition

The sample of manganotychite used in this study is the holotype specimen deposited by A.P. Khomyakov at the Geological and Mineralogical Museum of the Geological Institute, Kola Science Centre of the Russian Academy of Sciences, Apatity, Russia (catalogue no. GIM 6379). The sample originated from the Alluaiv Mountain, Lovozero massif, Kola peninsula, Russia (Figure 1a,b) and was found in intensely mineralized pegmatitic formations close to pegmatites [24]. They are represented by well-shaped veins up to 0.3 m thick in poikilitic nepheline-sodalite syenites. The veins are composed of coarse-grained (1–5 cm) aggregates of microcline, cancrinite, aegirine, and a wide range of salt minerals: villiaumite, cryolite, kogarkoite, trona, shortite, sidorenkite, and manganotychite. Less widespread minerals of this group are nahcolite, wegscheiderite, thermonatrite, pirssonite, burbankite, rhodochrosite, and neighborite. Other minerals in the veins are nepheline, sodalite, analcime, albite, natrolite, lorenzenite, lamprophyllite, eudialyte, catapleite, zircon, neptunite, serandite, leucosphenite, leifite, apatite, fluorite, cleiophane, galena, pyrite, and ilmenite. Manganotychite aggregates are mainly confined to the axial zones of veins, where they form nests of blows, up to 10–15 cm with glassy lustre (Figure 1c).

The composition of manganotychite was studied by Cameca MS-46 microprobe (Cameca, Gennevilliers, France) operated in a WDS mode at 22 kV and 20–40 nA and a beam diameter of 10 µm with counting times of 20 s (for a peak) and 2 × 10 s (for background before and after the peak), with 5–10 counts for every element in each point. The following standards were used: lorenzenite (Na), pyrope (Mg), barite (S), MnCO₃ (Mn), and hematite (Fe). The CO₃ content was calculated based on the crystal structure refinement.

The mean chemical composition (FeO 4.25, MnO 16.67, MgO 2.04, Na₂O 31.94, SO₃ 14.22, and CO₂ 30.65, sum 99.77 wt. %) measured from 5 spots and normalized on 16 O atoms is Na_5.92(Mn_1.35Fe_0.34Mg_0.29)_Σ1.98S_1.02C_4.00O_16.00, which is in agreement with the results of the single-crystal diffraction study (see below). The Raman spectra for the sample were reported by Sidorov et al. [20].

2.2. Single-Crystal X-ray Diffraction

A single-crystal X-ray diffraction study was done on the same sample of manganotychite that was used for the Raman spectroscopic study as reported by Sidorov et al. [20]. The X-ray diffraction data were collected by means of an XtaLAB Synergy-S diffractometer (Rigaku corp., Tokyo, Japan) equipped with a hybrid photon counting detector HyPix-6000HE at the Centre of the Collective Use of Equipment, Kola Science Centre. The data were integrated and corrected by means of the CrysAlisPro [26] program package, which was also used to apply an empirical absorption correction using spherical harmonics, as implemented in the SCALE3 ABSPACK scaling algorithm. The SHELXL program [27] was used for the crystal structure refinement. The structure of manganotychite was refined to R₁ = 0.021 for 388 independent reflections with F_obs > 4σ(F_obs). The refined occupancy of the divalent metal M site is Mn_0.87Mg_0.13, which agrees well with the occupancy of Mn_0.60Fe_0.25Mg_0.15 derived from the empirical chemical data, taking into account close proximity between the site-scattering powers of Mn and Fe. Crystal data, data collection information, and structure refinement details are given in

Table 1. Crystal data and structure refinement for manganotychite.

Temperature/K	293(2)
Crystal system	cubic
Space group	Fd$\overline{3}$
a/Å	14.0015(3)
Volume/Å3	2744.88(18)
Z	8
Dcalc, g/cm3	2.788
μ/mm−1	2.071
F(000)	2245.0
Crystal size/mm3	0.18 × 0.14 × 0.14
Radiation	MoKα (λ = 0.71073)
2Θ range for data collection/°	8.234 to 65.960
Index ranges	−15 ≤ h ≤ 21, −16 ≤ k ≤ 12, −20 ≤ l ≤ 8
Reflections collected	1340
Independent reflections	388 [Rint = 0.0182, Rsigma = 0.0166]
Data/restraints/parameters	388/0/26
Goodness-of-fit on F2	1.129
Final R indices [I ≥ 2σ (I)]	R1 = 0.0201, wR2 = 0.0532
Final R indices [all data]	R1 = 0.0214, wR2 = 0.0536
Largest diff. peak/hole/e Å−3	0.24/−0.51

, and atom coordinates and selected interatomic distances are in

Table 2. Atomic coordinates, site-occupancy factors (s.o.f.s), and equivalent isotropic displacement parameters (10⁻⁴ Ų) for manganotychite.

Site	s.o.f.	x/a	y/b	z/c	Uiso
M	Mn0.87Mg0.13	½	½	½	0.0092(2)
S	S	3/8	1/8	3/8	0.0121(2)
Na	Na	3/8	0.65892(5)	3/8	0.0193(2)
O1	O	0.31404(6)	0.81404(6)	0.43596(6)	0.0194(3)
O2	O	0.48237(6)	0.52891(5)	0.34933(5)	0.0137(2)
C	C	0.47028(7)	0.47028(7)	0.27972(7)	0.0098(3)

and

Table 3. Selected interatomic distances (Å) for the crystal structure of manganotychite.

M–O	2.1622(8) 6×	Na–O1	2.4108(8) 2×
S–O	1.4784(14) 4×	Na–O2	2.3880(10) 2×
C–O	1.2855(7) 3×	Na–O1	2.4848(7) 2×
		<Na–O>	2.4279

, respectively.

3. Results

3.1. Structure Description

The crystal structure of manganotychite is isotypic to those of tychite [17] and ferrotychite [18]. In their review on the crystal chemistry of sulfates, Hawthorne et al. [28] classified the structure of ferrotychite as based upon infinite heteropolyhedral framework, though, technically speaking, sulfate groups do not participate in the framework construction, but reside within the cavities of the framework formed by MO₆ octahedra and CO₃ triangular groups.

The crystal structure of manganotychite is shown in Figure 2a. It is based upon a three-dimensional infinite framework formed by the condensation of MnO₆ octahedra and CO₃ groups by sharing common O atoms. The fundamental building block (FBB) of the framework has a tetrahedral shape and is formed by four MnO₆ octahedra and four CO₃ triangles that are linked by common O atoms (Figure 2b,c) so that the four Mn atoms and four C atoms are arranged at the vertices of a distorted cube (a cubane arrangement). The adjacent FBBs are linked through common MnO₆ octahedra so that each octahedron belongs to two FBBs. The core of each FBB is an ‘empty’ Mn₄ tetrahedron (Figure 2d). The Mn₄ tetrahedra share corners to produce a highly symmetrical pyrochlore-type network (Figure 2e), which is a maximum-symmetry version of cristobalite topology, which, in turn, is a derivative of diamond topology. The diamond net (dia) is one of the simplest and most common topologies in inorganic and metal-organic framework materials [29].

The sulfate groups and Na⁺ cations reside in the cavities of the octahedral-triangular metal carbonate framework.

3.2. X-ray Diffraction versus Raman Spectroscopic Study

The Raman spectrum of manganotychite studied in this work was reported and characterized in detail by Sidorov et al. [20]. The Raman bands observed for the mineral are in good agreement with the results of the single crystal X-ray diffraction study. The spectrum is dominated by the bands typical for the internal vibrations of the carbonate and sulfate groups. The bands at 209 and 233 cm⁻¹ belong to lattice vibrations. The internal vibrations of sulfate groups are responsible for the appearance of bands at 967 cm⁻¹ (symmetrical v₁ vibrations), 1136 cm⁻¹ (asymmetrical v₃ vibrations), and 464, 614, 630 cm⁻¹ (deformation v₂ and v₄ vibrations). The bands at 711, 1095, and 1419 cm⁻¹ correspond to the v₄, v₁, and v₃ vibrations of carbonate groups, respectively.

3.3. Structural Relations

In terms of symmetry and basic construction of the octahedral-triangular framework, the crystal structure of manganotychite (as well as those of tychite and ferrotychite) is identical to those of northupite, Na₃Mg(CO₃)₂Cl and related inorganic compounds (see above). The transition northupite → tychite can be described as a result of the multiatomic 2Cl⁻ → (SO₄)²⁻ substitution, where both chlorine and sulfate ions are the extra-framework constituents. However, the positions occupied by sulfate groups and chlorine ions within the framework are different, which is depicted in Figure 2 in terms of the arrangement of Na atoms. In both tychite and northupite structure types, Na atoms form identical arrangements corresponding to the arrangement of O atoms of an octahedral framework in a classical cubic pyrochlore A₂B₂O₇ = [A₂O’][B₂O₆], where [A₂O’] is a pyrochlore network of oxocentered (O’A₄) tetrahedra [30] and [B₂O₆] is a framework of corner-sharing (BO₆) octahedra. In both northupite and tychite, Na atoms form a skeleton with Na₆ octahedral cavities of two types. In the cluster shown in Figure 3, the octahedral cavity of the first type is a Na₆ octahedron flattened parallel to one of its threefold axes. This octahedron is empty in manganotychite (Figure 3a) and occupied by Cl atoms in northupite (Figure 3b). The flattening character of this octahedral cavity is manifested in the difference between Na–Na distances, as it has six Na–Na edges of 3.564 Å (in both structures), and six Na-Na edges of 4.279 Å (in manganotychite) and 4.522 Å (in northupite). The second octahedral cavity is a regular Na₆ octahedron with equal twelve Na–Na edges of 4.279 Å (in manganotychite) and 4.522 Å (in northupite). The smaller size of this cavity in northupite is explained by its occupancy by a sulfate group (Figure 3a). The same cavity in northupite is empty.

The occupancy of the flattened Na₆ octahedra by Cl anions in northupite (Figure 3b) allows for the consideration of (ClNa₆) octahedra as separate coordination polyhedra and to analyse their connectivity. It is of interest that the (ClNa₆) octahedra share corners to form in northupite and its synthetic analogues an octahedral framework similar to the [B₂O₆] framework observed in cubic pyrochlores A₂B₂O₇ = [A₂O’][B₂O₆]. However, in contrast to the latter, where the framework is formed by cation-centred (BO₆) octahedra, the framework in northupite is formed by anion-centred (ClNa₆) octahedra. This allows for the consideration of the respective [Cl₂Na₆] framework in northupite as an antipyrochlore or an inverse pyrochlore, by analogy with antiperovskites or inverse perovskites [31]. Similar antipyrochlore frameworks formed by N-centered metal octahedra are known in alkaline earth metal nitrides [32,33] and by O-centered (OTi₆) octahedra in Ti₂[Ti₃O]₂(Pd)₄ [34].

Thus, the crystal structure of northupite and its synthetic analogues can be described as consisting of the antipyrochlore-type octahedral framework [Cl₂Na₆]⁴⁺ formed by (ClNa₆) octahedra (Figure 4a) and the [M₂(CO₃)₄]⁴⁻ octahedral-triangular framework (Figure 4b; M = Mg, Co, Mn). In the whole crystal structure, the anionic and cationic frameworks of the two types interpenetrate (Figure 4c).

4. Discussion

The northupite-group minerals and related synthetic northupite-type compounds belong to two structure types that correspond to the chemical formulas, Na₃M(CO₃)₂Cl or Na₆M₂(CO₃)₄Cl₂ (M = Mg, Fe, Mn) and Na₆M₂(CO₃)₄(SO₄) (M = Mg, Mn, Co). The transition between the two structure types can be described by the 2Cl⁻ → (SO₄)²⁻ substitution, which, as to our knowledge, has not been observed so far either in nature or under laboratory conditions. Among mineral species, the only mineral of the second structure type is northupite (M = Mg), and other members of the group (M = Mn, Fe) can be expected to exist in nature, by analogy with tychite, ferrotychite, and manganotychite.

The genealogy of both structure types originates from the diamond-like dia net, which is one of the five simplest nets observed in inorganic and metal–organic compounds. The complexity of the dia net considered as the Shannon information amount that takes into account vertices and edges [35,36,37] is equal to 0.918 bits per element (vertex or edge) and 5.510 bits per unit cell. The atomic structural complexities of the Na₃M(CO₃)₂Cl and Na₆M₂(CO₃)₄(SO₄) structure types are equal to 1.988 and 2.219 bits per atom, respectively, whereas their total structural complexities are 103.364 and 128.704 bits per unit cell, respectively. Taking into account that both structure types contain the same [M₂(CO₃)₄]⁴⁻ octahedral-triangular anionic framework and the same amount of Na⁺ cations, the difference is due to the 2Cl⁻ → (SO₄)²⁻ substitution, which corresponds to the larger number of atoms for tychite-type structures and the lower number of atoms for northupite-type structures. The rise of complexity from the dia net to the Na₃M(CO₃)₂Cl and Na₆M₂(CO₃)₄(SO₄) structure types is due to the various modifications of the net through the introduction of different atomic groups. The latter is accompanied by the decrease of the symmetry from the Fd$\overline{3}$m space group for the dia net to the Fd$\overline{3}$ space group.

The set of modifications corresponding to the transition from the dia net to the Na₃M(CO₃)₂Cl and Na₆M₂(CO₃)₄(SO₄) structure types can be described as follows: (i) replacement of a vertex of the dia net by an M₄ tetrahedron (no symmetry reduction); (ii) attachment of (CO₃) triangles to the triangular faces of M₄ tetrahedra (Figure 2c) (accompanied by the Fd$\overline{3}$m → Fd$\overline{3}$ symmetry reduction); (iii) filling voids of the resulting framework by Na⁺ cations (no symmetry reduction); and (iv) filling voids of the Na skeleton by either sulfate groups (in tychite-type structures) or chlorine atoms (in northupite).

It is remarkable that, at the bottom, the structural topologies of the northupite-group minerals relate to such a simple and highly symmetric structure as that of a diamond.

It is of interest to investigate the position of manganotychite, tychite, and ferrotychite among the known sulfate carbonates. There are, in total, 26 known mineral species that have crystallographically well-defined carbonate and sulfate groups. Among them, only five do not contain hydrogen in any form (manganotychite, tychite, ferrotychite, burkeite, and hanksite), whereas 13 are hydrates, i.e., contain symmetrically independent H₂O molecules in their crystal structures. The latter minerals include such remarkable rarities as ramazzoite, [Mg₈Cu²⁺₁₂(PO₄)(CO₃)₄(OH)₂₄(H₂O)₂₀][(H_0.33S⁶⁺O₄)₃(H₂O)₃₆] [38], and putnisite, SrCa₄Cr³⁺₈(CO₃)₈(SO₄)(OH)₁₆·25H₂O [39], both containing polyoxometalate units in their crystal structures [40]. It is worthy of note that, in ramazzoite, the polyoxometalate clusters contain phosphate and carbonate groups, whereas sulfate groups provide the linkage between polycationic units. The same situation is observed in putnisite as well, where the polyoxometalate cluster has the composition [Cr³⁺₈(OH)₁₆(CO₃)₈]⁸⁻. In this cluster, eight Cr³⁺-centered octahedra share edges to form a ring, which is decorated by eight (CO₃) triangles. This wheel-shaped cluster has no analogues among minerals and synthetic compounds. Putnisite is a secondary mineral formed during the oxidation of the primary sulfide-bearing rocks, whereas ramazzoite crystallized from low-temperature (<50 °C) aqueous solutions at pH from 6 to 12 and low fugacity of CO₂ [38]. Both ramazzoite and putnisite possess a high structural complexity with 1848.162 and 2932.730 bits of information per reduced unit cell, respectively.

According to their paragenetic modes [41], most of the known sulfate carbonate minerals are secondary in origin and formed as a result of low-temperature subaerial oxidative hydration and weathering. Only five minerals (tychite, ferrotychite, manganotychite, burkeite, and hanksite) are considered as primary, occurring either in evaporites and saline lakes (tychite, burkeite, Na₄(SO₄)(CO₃), and hanksite, KNa₂₂(SO₄)₉(CO₃)₂Cl) or as late-stage minerals in the ultra-alkali and agpaitic igneous rocks (ferrotychite and manganotychite). All five primary sulfate carbonate minerals contain Na as an essential mineral-forming component that indicates the importance of high Na potential for their formation.