Brattforsite, Mn19(AsO3)12Cl2, a new arsenite mineral related to magnussonite, from Brattforsgruvan, Nordmark, Värmland, Sweden

Brattforsite is an approved mineral (IMA2019-127), with ideal formula Mn19(AsO3)12Cl2. Associated minerals in the type specimen from the Brattfors mine, Nordmark (Värmland, Sweden) include jacobsite, alleghanyite, phlogopite, calcite and dolomite. Brattforsite, forming subhedral, mostly equant crystals up to 0.5 mm across, is orange to reddish-brown with a white streak, and translucent with a resinous to vitreous lustre. The fracture is uneven to subconchoidal, and no cleavage is observed. It is very weakly pleochroic in yellow, optically biaxial (–) with 2V = 44(5)° and has calculated mean refractive index of 1.981. Measured and calculated density values are 4.49(1) and 4.54(1) g·cm− 3, respectively. Chemical analyses yields (in wt%): MgO 0.62, CaO 1.26, MnO 48.66, FeO 0.13, As2O3 46.72, Cl 2.61, H2Ocalc 0.07, O ≡ Cl –0.59, sum 99.49, corresponding to the empirical formula (Mn17.67Ca0.58Mg0.40Fe0.05)∑18.70As12.17O35.90Cl1.90(OH)0.20, based on 38 (O + Cl + OH) atoms per formula unit. The five strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 2.843,100, (4-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{4} $$\end{document}44); 2.828, 99, (444); 1.731, 32, (880); 2.448, 28, (800); 1.739, 25, (088). Brattforsite is monoclinic and pseudotetragonal, space group I2/a, with unit-cell parameters a = 19.5806(7), b = 19.5763(7), c = 19.7595(7) Å, β = 90.393(3)°, V = 7573.9(5) Å3 and Z = 8. The crystal structure was solved and refined to an R1 index of 3.4 % for 7445 reflections [Fo > 4σ(Fo)]. Brattforsite has the same overall structural topology as magnussonite (i.e., the species can be considered as homeotypic), but with 12 independent tetrahedrally coordinated As sites and 21 Mn sites with varying (4–8) coordination. The Mn-centered polyhedra, bonded through edge- and face-sharing, give rise to a three-dimensional framework. The (AsO3)3− groups are bonded to this framework through corner- and edge-sharing. Spectroscopic measurements (optical absorption, Raman, FTIR) carried out support the interpretation of the compositional and structural data.


Introduction
Arsenites form a subclass of rare, but significantly diverse, minerals with about 20 known species. They are most commonly characterized by the isolated (AsO 3 ) 3− anion in their crystal structures, and typically formed during the late-stage The mineral name is given for the type locality, the mine Brattforsgruvan. This name was previously used informally, on a label to the type specimen, and mentioned by Paul B. Moore in a letter from 1967 (F.E. Wickman file, Archives of the Royal Swedish Academy of Sciences, Stockholm). "Brattfors" is the name of a neighbouring parish, and of an old iron smelter. Brattforsite corresponds to UM1984-09 in the list of valid unnamed mineral species (Smith and Nickel 2007). The holotype specimen is deposited in the type mineral collection of the Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden, under collection number GEO-NRM 19100303. The single crystal used for X-ray diffraction intensity data measurement (a part of the holotype) is kept in the mineralogical collection of the Natural History Museum of Pisa University, under catalogue number 19912.
The original occurrence of the present sample in the mine is unknown; it was purchased by the Swedish Museum of Natural History from Olof Backelin, a mining foreman and local mineral collector, in 1910. Backelin is noted as the source for two other type mineral specimens from this mine, katoptrite (Flink 1917) and manganhumite (Moore 1978). The mines of the Nordmark field closed in 1962 and are now inaccessible.
Arsenite minerals are typically confined to late-stage mineralization in fissures and druses in Långban-type deposits (Nysten et al. 1999); in the present case, it is probable that the new mineral has replaced a Mn-rich precursor mineral in the skarn mass, from alteration by a late-or post-metamorphic, Asand Cl-bearing fluid. Slightly reducing conditions in these rocks are indicated by the presence of manganosite and alabandite, observed in minor amounts in similar skarn matrices. Brattforsite occurs as crystals forming aggregates up to 8 mm across, in a granular matrix of ( Fig. 1) jacobsite, calcite, dolomite, phlogopite and alleghanyite. Minor associated minerals identified by energy-dispersion microanalysis are rhodochrosite and johnbaumite. Subrounded grains of jacobsite commonly appear as inclusions in brattforsite. A Mn-As-(OH) mineral, tentatively identified as allactite, occurs filling fine fractures of brattforsite. Allactite is a late-stage vein mineral in the Mn mineralizations at Nordmark (Sjögren 1884).

Physical and optical properties
Individual crystals of brattforsite are somewhat irregular in shape but essentially equant, and up to 0.5 mm in size. The macroscopic colour is orange to reddish brown, with a white streak. It has a vitreous to resinous lustre. Brattforsite is translucent and transparent in thin section. No fluorescence effects were detected in ultra-violet light. The hardness (Mohs) is estimated to 3-4. Neither an obvious cleavage nor parting is observed. Brattforsite is brittle and shows an uneven to subconchoidal fracture. Moore (1970a) reported 4.49 g·cm − 3 for the density; a calculated value of 4.54(1) g·cm − 3 is obtained for the ideal formula and unit-cell volume from singlecrystal X-ray diffraction data. The mineral dissolves in 30 % hydrochloric acid at room temperature.
The refractive indices were not measured conventionally, because found higher than available reference liquids (> 1.8); the average calculated n is 1.981 from Gladstone-Dale constants (Mandarino 1981). The mineral is optically biaxial (-), Fig. 1 Image of the brattforsite (arrow points to the largest aggregate) type specimen with jacobsite (black) and calcite (whitish), GEO-NRM #19100303. Inset image shows a detail on the specimen (red frame). Photos: Torbjörn Lorin with 2V meas = 44(5)°. The estimated birefringence is small, 0.002. The dispersion is weak, with v > r. Brattforsite is very weakly pleochroic, in yellow hues. Polysynthetic twinning has been observed in a couple of grains. The angle of extinction between twin lamellae, up to 80 μm wide, is 27(1)°.

Mineral chemistry
The mineral was mounted in epoxy resin and polished prior to analysis. Electron probe microanalysis (EPMA) was performed with a Cameca Camebax SX-50 instrument, running at 20 kV, with the sample current 12 nA, spot size = 2 μm and a take-off angle of 40°. Corrections of the raw data were executed with Cameca's PAP-routine, a modified ZAF procedure (Pouchou and Pichoir 1984). Used reference materials and the results from four spot analyses are given in Table 1. The concentration values for CuO and ZnO were below or close to the detection limit. Minor (OH) − in the crystal structure is indicated by infrared spectra (see the following). The empirical formula for brattforsite, calculated on the basis of 38 (O + Cl + OH) atoms per formula unit (apfu), with As as a trivalent cation, is (Mn 17.67

Infrared spectroscopy
Fourier-transform infrared (FTIR) spectra were measured on a double-side polished single crystal (thickness = 116 μm) using a Bruker Vertex spectrometer equipped with a Hyperion II microscope, a Globar source, a CaF 2 beam-splitter, and an InSb detector. Data were acquired during 64 scans in the wavenumber range 2000-12,000 cm − 1 at a spectral resolution of 4 cm − 1 . No absorption bands apart from very weak ones in the OH stretching region (Fig. 2) were observed. From comparison with the intensity of OH stretching bands in the infrared spectrum of a magnussonite single crystal (Fig. 2) from the holotype specimen GEO-NRM #g32215, the H 2 Oconcentration in the brattforsite crystal is estimated to be ≤ 0.07 wt %, corresponding to ≤ 0.2 OH apfu. A relative low energy, with initial absorption at ca. 2700 cm − 1 , and a broadness of the envelope of the bands marking OH stretching modes may be due to relatively strong hydrogen bonding in magnussonite and in brattforsite. Alternatively, these features are due to OH stretching bands superimposed on an absorption band caused by a low-energy d-d transition in a transition metal cation as e.g. Fe 2+ .

Micro-Raman spectroscopy
A Raman spectrum of brattforsite ( Fig. 3) was collected from a randomly oriented, polished crystal fragment on a LabRAM HR 800 micro-spectrometer, using a 514 nm Ar-ion laser source at < 1 mW power, a Peltier-cooled (-70°C) 1024 × 256 pixel CCD detector (Synapse), an Olympus M Plan N 100×/0.9 NA objective and laser spot of ca. 3 μm. A 600 grooves/cm grating was used, and the resolution is about 1 cm − 1 . Spectral positions were corrected against the Raman band at 789 cm − 1 of a SiC-6H crystal measured on {0001}. Instrument control and data acquisition (range 50-4000 cm − 1 , 10 s exposure time in 20 cycles) were made with the LabSpec 5 software. No laser-induced degradation of the sample was observed. A sample of magnussonite (type specimen) was measured for a comparison (Fig. 3).
The spectra were found to be featureless in the range 4000-1000 cm − 1 ; prominent Raman peaks are identified for brattforsite at 786, 760 (shoulder), 708, 660, 510, 203 and 117 cm − 1 (Fig. 3). The most intense band at 786 cm − 1 is ascribed to symmetric stretching of the (AsO 3 ) 3− groups and the shoulder at 760 cm − 1 to antisymmetric stretching (cf. Frost and Bahfenne 2010). The corresponding bands in magnussonite are found at 811 and 794 cm − 1 . The peaks between 700 and 500 cm − 1 , including minor ones, for both minerals are resulting either from stretching vibrations of Mn 2+ -O(Cl,OH) bonds or from bending vibrations of (AsO 3 ) 3− . Specifically, the band at 510 cm − 1 is consistent with octahedrally coordinated Mn 2+ , whereas the one at 660 cm − 1 would be compatible with lower coordination numbers for this cation (Bernardini et al. 2021). Peaks in the low-frequency region from ca. 200 cm − 1 are probably related to lattice modes.

Optical absorption spectroscopy
Polarized optical absorption spectra ( Fig. 4) were measured on two double-side polished oriented crystal sections (one XZ and one YZ section), both 70 μm thick, of brattforsite. The crystals were oriented by means of optical microscopy. The spectra were recorded in the range 10,000-32,000 cm − 1 with an AVASPEC-ULS2048 × 16 spectrometer attached via a 400-µm UV fibre cable to a Zeiss Axiotron UV-microscope. A 75 W Xenon arc lamp served as a light source and Zeiss Ultrafluar 10× lenses were used as objective and condenser. The size of the circular measure aperture was 64 μm in diameter. An UV-quality Glan-Thompson prism with a working range from 250 to 2700 nm (40,000 to 3704 cm − 1 ) was used as polarizer. The recorded spectra show a set of absorption bands caused by spin-forbidden electronic d-d transitions in Mn 2+ superimposed on an UV absorption edge. The most prominent of these bands, at 23,650 cm − 1 , marks the field independent 6 A 1 (S) → 4 A 1 4 E(G) transition in Mn 2+ . The calculated molar absorption coefficient (ε) of the band is approximately 1 l·mole − 1 ·cm − 1 , which is comparable to the ε-value recorded for the corresponding band in spectra of magnussonite (Hålenius and Lindqvist 1996). An additional, broad and strongly polarized band at~13,700 cm − 1 is tentatively assigned to a spin-allowed d-d transition in Fe 2+ in fourfold coordination or in a strongly axial distorted octahedral coordination. Based on the molar absorption coefficient (1 65 l·mole − 1 ·cm − 1 ) of a corresponding absorption band at1 5,300 cm − 1 in spectra of magnussonite (Hålenius and Lindqvist 1996), the FeO concentration in brattforsite is calculated at 0.15 wt%, which compares well with a concentration of 0.13 wt% determined by EPMA (Table 1).
Single-crystal X-ray intensity data were collected using a Bruker Smart Breeze diffractometer equipped with a Photon II CCD detector and graphite monochromatized MoKα radiation. The detector-to-crystal working distance was set at 50 mm. Data were collected using ω scan mode in 0.5°slices, with an exposure time of 20 s per frame. Correction for Lorentz and polarization factors, absorption, and background were applied using the package of software Apex3 (Bruker AXS Inc. 2016). Brattforsite is monoclinic, with unit-cell parameters a = 27.7223(9), b = 19.5763 (7), c = 19.5806(7) Å, β = 134.5410(10)°, V = 7573.9(5) Å 3 . The statistical tests on the distribution of |E| values (|E 2 -1| = 1.006) indicated the occurrence of an inversion center. The examination of systematic absences suggested the space group symmetry C2/c. The crystal structure of brattforsite was solved in this space group through direct methods using Shelxs-97 and refined using Shelxl-2018 (Sheldrick 2015). Following the recommendation of Mighell (2003), the C-centered monoclinic cell was transformed in a conventional I-centered cell, through the matrix [0 0-1 | 0 1 0 | 1 0 1], obtaining the pseudotetragonal unit-cell parameters a = 19.5806 (7), b = 19.5763 (7), c = 19.7595(7) Å, β = 90.393(3)°, V = 7573.9(5) Å 3 , space group I2/a. These unit-cell parameters can be compared with those reported by Moore (1970a), a = 19.58(2), c = 19.72(2) Å, space group I4 1 / amd. The following neutral scattering curves, taken from the International Tables for Crystallography (Wilson 1992), were used: Mn vs. Ca at Mn sites, As at As sites, O at O sites, and Cl vs. O at Cl sites. Several Mn sites were found fully occupied by Mn and their site occupancy factors (s.o.f.) were fixed to the full occupancy by Mn. The anisotropic structural model of brattforsite converged to R 1 = 0.0343 for 7445 reflections with F o > 4σ(F o ) and 640 refined parameters. A crystallographic information file (CIF) is available as electronic supplementary material. Details of data collection and refinement are given in Table 3. Fractional atom coordinates and displacement parameters are reported in Table 4, whereas Table 5 contains interatomic distances and bond-valence sums (BVS) for cation sites, and Table 6 the BVS sums for the anion sites. BVS were calculated using the bond parameters of Gagné and Hawthorne (2015) for As-O, Ca-O, and Mn-O bonds, and those of Brese and O'Keeffe (1991) for Mn-Cl and Ca-Cl bonds.

General features
The crystal structure of brattforsite (Fig. 5) shows the occurrence of thirty-three independent cation sites (twelve As sites and twenty-one Mn-dominant sites) and thirty-eight anion positions.
The crystal structure can be described as formed by polyhedral layers stacked along c. Within a unit-cell, eight layers occur; three symmetrically distinct layers, labelled L 0 , L 1 , and L 2 , alternate in the unit-cell according to the sequence shown in Fig. 5. Figure 6 shows each single layer as seen down c. The layer L 0 is formed by rows of Mn-centered polyhedra running along [110] and [-110]. These rows show the sequence ···Mn(2)-Mn (6) Mn(13) occurring at the intersection between perpendicular rows. The intersection of the rows form a square cavity where a trimer composed of Mn (7)-Mn(20)-Mn (5) occurs, along with (AsO 3 ) groups. Two As sites, i.e., As(1) and As (5), point in the + c direction, whereas As (4) and As (8) point in the -c direction. The layer L 1 shows clusters formed by four independent Mn sites, i.e., Mn(10), Mn(11), Mn(15), and Mn(16). This cluster forms chains along a, obtained through corner-sharing between Mn(15) and Mn(16) belonging to consecutive tetramers, and through (AsO 3 ) groups [As (2) and As (7)]. Along b, the connection between these chains is due to (AsO 3 ) groups [As(9) and As (12)]. The layer L 2 has two independent tetramers, formed by Mn (1), Mn(21), and two Mn (7) and by Mn (14), Mn(18), and two Mn (12), respectively. These clusters are bonded along a through As(10) and As (11), whereas along b the connection is due to the occurrence of As(3) and As (6).
Notwithstanding such a description, the large number of connections between Mn-centered polyhedra, bonded through edge-and face-sharing, give rise to a three-dimensional framework. The (AsO 3 ) 3− groups are bonded to this three-dimensional framework through corner-and edge-sharing. They are located on the walls of the structural cavities, with their 4s 2 lone-electron pairs pointing towards the center of the cavities.

Atom coordination
Arsenic displays the typical asymmetric trigonal pyramidal coordination, showing < As-O > distances ranging between 1.764 and 1.793 Å, in agreement with the < As-ϕ > value of 1.782 Å reported by Majzlan et al. (2014). The BVS at these sites range between 2.88 and 3.08 valence units (v.u.), in agreement with the occurrence of As 3+ .

Brattforsite and its relationships with magnussonite
Brattforsite is related to the manganese arsenite mineral magnussonite. The definition of this latter mineral has yet not been fully clarified. Its crystal structure was solved by Moore and Araki (1979a) in the cubic space group Ia3d, with unit-cell parameter a = 19.680(4) Å. The authors proposed the ideal formula Mn 2 + 18 [As 3+ 6 Mn + O 18 ] 2 Cl 2 (Z = 8) or Mn 20 As 12 O 32 Cl 2 . This was questioned by Dunn and Ramik (1984), who highlighted that their analyses of magnussonite, as well as those given by Gabrielson (1956), showed (OH) > Cl. Similar results were obtained by Hålenius and Lindqvist (1996). Despite the apparent cubic character of magnussonite, some details suggest that further studies are needed to fully characterize this mineral. For example, crystals are optically anisotropic and biaxial (-) in sufficiently thick sections (~0.1 mm), with a very small 2V and birefringence. In addition, the stability of the Mn + ion in Nature is questioned, and no features ascribed to this rare cationic species have observed in optical spectra of magnussonite (Hålenius and Lindqvist 1996). Finally, the role of minor elements (e.g., Cu) has to be clarified. Figure 7 compares the crystal structure of magnussonite with that of brattforsite. One feature common to both structures is an As-octahedron, with As-As distances of 3.64 Å in magnussonite (actually ranging between 3.56 and 3.71 Å) and 3.66 Å in brattforsite (ranging between 3.49 and 3.78 Å). This octahedron is occupied by Mn 1+ (at 0, 0, 0) in magnussonite (according to Moore and Araki 1979a), with <Mn-As> distance of 2.65 Å. In brattforsite, this position is empty, and the shortest Mn-As distance was observed Fig. 6 The polyhedral layers occurring in the crystal structure of brattforsite: L 0 (a), L 1 (b), and L 2 (c), as seen down c (a horizontal) between As(12) and Mn(19), with a distance of 2.9482(10) Å. No residuals were found in the difference-Fourier maps at coordinates (0, 0, 0). Neglecting minor differences in the cation coordinations, this is the main difference between brattforsite and magnussonite, which can be considered as having a homeotypic relationship. The spectroscopic data strengthen the picture of a close structural relationship between these two minerals, with, e.g., only small shifts in Raman bands (ca. 25 cm − 1 ) mainly related to the variation in overall < As-O > bond distances, 1.778 Å for brattforsite and 1.760 Å for magnussonite.
Consequently, based on structural details, the overall crystal symmetry, the chemical composition and the optical characteristics, brattforsite is distinct from magnussonite, and a bona fide Cl-analogue. At present there is no suggestion of the upper limit of Cl incorporation in magnussonite. Nysten (2003) identified a Cl-rich specimen (1.9 wt% Cl) from the Garpenberg Norra deposit, Dalecarlia, Sweden, with no indication of lower symmetry than cubic from single-crystal X-ray diffraction data.

Brattforsite in the domain of Mn arsenite minerals
Currently, ca. 70 mineral species have Mn and As as essential chemical constituents. However, the large majority of these minerals has As in its pentavalent state, forming (As 5+ ϕ 4 ) oxyanions. Other phases have mixed (As 5+ ϕ 4 ) and (As 3+ ϕ 3 ) arrangements. Ten mineral species containing Mn and As 3+ are currently known; brattforsite is the eleventh mineral having species-defining Mn and As 3+ . Table 7 lists these arsenite minerals.
Armangite is another example of fluorite-derivative structure (Moore and Araki 1979b), along with brattforsite and magnussonite. Its crystal structure is formed by a framework of Mn-centered polyhedra, with cavities lined by six (AsO 3 ) groups, which point their lone-electron-pairs into the center of Fig. 7 Projection of the crystal structure of brattforsite (a) and magnussonite (b) down b (a horizontal). Circles represent As (violet), Cl (green), and O (red) sites. In magnussonite, the Mn (5) site is shown as yellow circles. As-O bonds are shown as red thick lines, whereas As-As contacts are shown by thick black lines. In the center of the Fig., the As6-octahedra occurring in brattforsite (above) and magnussonite (below) are shown. In this latter image, Mn-As contacts are shown as dotted grey lines  17 . Whereas the crystal structure of the latter was solved by Kato and Watanabe (1992), the crystal structure of nelenite is still unknown, although Dunn and Peacor (1984) proposed a relation with friedelite. Both are T-O phyllosilicates, with (AsO 3 ) groups hosting at partially occupied positions, and residing within large twelve-membered rings.
Manganarsite is another mineral related to schallerite. Also in this case the crystal structure is only hypothetical, and Peacor et al. (1986) proposed the occurrence of rings or chains of (AsO 3 ) groups. Trigonite displays an open heteropolyhedral network, with (010) heteropolyhedral sheets cross-linked by additional (Asϕ 3 ) groups; the large cavities of this framework host Pb 2+ cations (Pertlik 1978). Dunn et al. (1986) proposed that trigonite is structurally related to rouseite, whose crystal structure is currently unknown. It is worth noting that all these minerals have their type localities at Långban-type ore deposits (i.e., Långban and Nordmark, in Sweden), or at Franklin-Sterling Hill-type deposits (in USA). Another mineral found in the same kind of occurrence (Långban) is wiklundite, a mineral representing a transition towards the mixed As 3+ -As 5+ species (Cooper et al. 2017). Indeed, As 5+ occurs only as a minor substituent of Si 4+ and consequently only As 3+ can be considered as species-defining. Its complex layered crystal structure is characterized by the presence of isolated (AsO 3 ) groups. Like brattforsite, wiklundite contains Mn and Cl as species-defining elements. An additional example is the recently described mineral cuyaite (Kampf et al. 2020), although with a different Mn valency (3+ instead of 2+). Its crystal structure can be described as an arsenite framework, with (AsO 3 ) groups sharing corners. In contrast, brattforsite can be considered as a Mn-polyhedral framework. It is worth noting that Cl in cuyaite is severely underbonded (only 0.26 v.u.), and Kampf et al. (2020) attributed this discrepancy to the inability of the bond-valence theory to take the effect of the lone-electron-pair in account. In fact, Cl is hosted at the centre of channels accommodating the loneelectron-pairs of As sites. The underbonding of the Cl sites in brattforsite cannot be explained in this a way, since the loneelectron-pairs of the As 3+ sites do not point towards Cl atoms. Cuyaite is one of the two manganese arsenites not found in Långban-type occurrences, with the type locality in the Camarones Valley, Chile. The other one is lepageite, discovered in the Szklary pegmatite, Poland (Pieczka et al. 2019). Its crystal structure is characterized by a finite [Sb 4 As 4 O 19 ] cluster and by isolated (AsO 3 ) groups.
Manganese arsenites are apparently very rare in Nature, with reduced mineral assemblages of Långban or Franklin-Sterling Hill types of deposits representing their preferred kind of occurrence. Brattforsite, showing a close relationship with magnussonite and some structural similarity with armangite, is thus an interesting new addition to this category of minerals. In terms of mineral classification, it belongs to the Strunz group 4.JB, i.e., Arsenites, antimonites, bismuthites, with additional anions, without H 2 O (Strunz and Nickel 2001).
Funding Open access funding provided by Swedish Museum of Natural History.
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