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Article

Rüdlingerite, Mn2+2V5+As5+O7·2H2O, a New Species Isostructural with Fianelite

1
Swiss Seismological Service, ETH Zürich, 8092 Zurich, Switzerland
2
Musée Cantonal de Géologie, Université de Lausanne, 1015 Lausanne, Switzerland
3
Dipartimento di Geoscienze, Università degli studi di Padova, 35131 Padova, Italy
4
Department of Geological Sciences, Masaryk University, 611 37 Brno, Czech Republic
5
Dipartimento di Scienze della Terra, Università degli Studi di Milano, 20133 Milano, Italy
6
Dipartimento di Scienze della Terra, Sapienza Università di Roma, 00185 Rome, Italy
7
CNR–Istituto di Geoscienze e Georisorse, UOS Roma, 00185 Roma, Italy
8
Dipartimento di Scienze della Terra, Università degli Studi di Torino, 10125 Torino, Italy
9
Associazione Micromineralogica Italiana, 10073 Devesi-Cirié, Italy
10
Department of Geosciences, Swedish Museum of Natural History, 10405 Stockholm, Sweden
11
Secteur des Sciences de la Terre, Muséum d’histoire naturelle, 1208 Genève, Switzerland
12
Associazione Micromineralogica Italiana, 17100 Savona, Italy
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(11), 960; https://doi.org/10.3390/min10110960
Received: 17 September 2020 / Revised: 12 October 2020 / Accepted: 22 October 2020 / Published: 27 October 2020
(This article belongs to the Collection New Minerals)

Abstract

:
The new mineral species rüdlingerite, ideally Mn2+2V5+As5+O7·2H2O, occurs in the Fianel mine, in Val Ferrera, Grisons, Switzerland, a small Alpine metamorphic Mn deposit. It is associated with ansermetite and Fe oxyhydroxide in thin fractures in Triassic dolomitic marbles. Rüdlingerite was also found in specimens recovered from the dump of the Valletta mine, Canosio, Cuneo, Piedmont, Italy, where it occurs together with massive braccoite and several other As- and V-rich phases in richly mineralized veins crossing the quartz-hematite ore. The new mineral displays at both localities yellow to orange, flattened elongated prismatic, euhedral crystals measuring up to 300 μm in length. Electron-microprobe analysis of rüdlingerite from Fianel gave (in wt%): MnO 36.84, FeO 0.06, As2O5, 25.32, V2O5 28.05, SiO2 0.13, H2Ocalc 9.51, total 99.91. On the basis of 9 O anions per formula unit, the chemical formula of rüdlingerite is Mn1.97(V5+1.17 As0.83Si0.01)Σ2.01O7·2H2O. The main diffraction lines are [dobs in Å (Iobs) hkl]: 3.048 (100) 022, 5.34 (80) 120, 2.730 (60) 231, 2.206 (60) 16-1, 7.28 (50) 020, 2.344 (50) 250, 6.88 (40) 110, and 2.452 (40) 320. Study of the crystal structure showcases a monoclinic unit cell, space group P21/n, with a = 7.8289(2) Å, b = 14.5673(4) Å, c = 6.7011(2) Å, β = 93.773(2)°, V = 762.58(4) Å3, Z = 4. The crystal structure has been solved and refined to R1 = 0.041 on the basis of 3784 reflections with Fo > 4σ(F). It shows Mn2+ hosted in chains of octahedra that are subparallel to [-101] and bound together by pairs of tetrahedra hosted by V5+ and As5+, building up a framework. Additional linkage is provided by hydrogen-bonding through H2O coordinating Mn2+ at the octahedra. One tetrahedrally coordinated site is dominated by V5+, T(1)(V0.88As0.12), corresponding to an observed site scattering of 24.20 electrons per site (eps), whereas the second site is strongly dominated by As5+,T(2)(As0.74V0.26), with, accordingly, a higher observed site scattering of 30.40 eps. The new mineral has been approved by the IMA-CNMNC and named for Gottfried Rüdlinger (born 1919), a pioneer in the 1960–1980s, in the search and study of the small minerals from the Alpine manganese mineral deposits of Grisons.

1. Introduction

In 1996, Brugger and Berlepsch (1996) [1] described fianelite, the then only second Mn vanadate and the first new mineral species for the Fianel mine, where the exotic mineral association was summarily sensed by Stucky (1960) [2]. Later, two new Mn species, ansermetite [3] and scheuchzerite [4], followed. In their description of fianelite, Mn2V2O7·2H2O, Brugger and Berlepsch (1996) [1] noticed that substitution of As for V occurs on the second tetrahedrally coordinated V site and that As could potentially even dominate in this site. The IMA-CNMNC dominant-constituent rule [5,6,7] generally states that in order for a mineral to be a distinct species: “At least one structural site in the potential new mineral should be predominantly occupied by a different chemical component than that which occurs in the equivalent site in an existing mineral species”. The dominant-constituent rule is applied in most approved new-mineral proposals but in complex systems, like e.g., the epidote group, “the dominant-constituent rule has been extended by considering “a group of atoms with the same valency state” as a single constituent” [6,7]. During a systematic energy-dispersive X-ray spectroscopy (EDXS) screening of fianelite material from the Fianel mine, a specimen turned out to be abnormally As-rich, pointing to the possible existence of a new mineral species.
During the work related to the project “Characterization of new and rare mineral species” started in 2010 by the Italian Micromineralogical Association (AMI) and carried out through EDXS and microRaman complementary spectroscopies, a particular sample from Fianel (R.B. collection) was identified in which bright yellow crystals were associated with bright red crystals. The Raman spectrum of the red mineral (fianelite) turned out to be rather different from the analogous spectrum of the yellow mineral (now rüdlingerite). At the same period, studies were underway on a lot of specimens from the recently discovered Valletta mine. In some of them, the mineralogical association appeared to be the same and consequently the yellow and red Valletta crystals were also analysed (co-type specimen), leading to the discovery that the EDXS and Raman analytical responses were the same as their Fianel counterparts.
The As-rich mineral was shown to have an As dominance on the second tetrahedrally coordinated site. Sticking to the general dominant-constituent rule and thereby considering that the homovalent substitution encountered at the two tetrahedrally coordinated sites in this phase is not a complex system, the new mineral and its name, rüdlingerite, were submitted to and accepted by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) (proposal # 2016-054a). The description is based primarily upon one holotype and one cotype specimen. The holotype from the Fianel mine is deposited in the Mineralogical Collection of the Musée cantonal de géologie, University of Lausanne, Anthropole-Dorigny, CH-1015 Lausanne, Switzerland, catalogue number MGL N° 080116. A cotype from the Valletta mine is deposited in the Mineralogical Collection of the Dipartimento di Scienze della Terra of the Università di Torino stored in the Museo Regionale di Scienze Naturali di Torino, Sezione di Mineralogia, Petrografia e Geologia, via Giovanni Giolitti 36, I-10123 Torino, Italy, catalogue number M/U 17121.
The name rüdlingerite honors Gottfried Rüdlinger (born 1919), a private collector and pioneer in the 1960–1980s who engaged in the search and study of the small minerals from the Alpine manganese mineral deposits of Grisons. G. Rüdlinger was long-time president of the local section of the Swiss association of ‘strahlers’ and mineral collectors and designed the mineral exhibit at the Nature Museum in Chur [8].

2. Occurrence

2.1. Holotype

Rüdlingerite was found in a specimen labeled as “fianelite”, originally from the Fianel mine in Val Ferrera, Grisons, Switzerland (46°32’47’’ N, 9°27’43’’ E). The Fianel deposit is a lenticular ore body within the Triassic dolomitic marbles that are part of the Mesozoic cover of the Suretta nappe, a member of the paleogeographic “Briançonnais” domain. The deposit underwent several phases of Alpine metamorphism with a climax at conditions of the blueschist to greenschist-facies [9]. Fianel is remarkable for the diversity of its ore types and consequently of its mineralogy. One type of ore consists of stratabound, medaite-rich, up to 20 cm thick and 2 m long lenses, crosscut by several generations of cm-wide veinlets [1]. These veins are themselves crosscut by thin fractures containing fianelite, rüdlingerite, ansermetite, and Fe oxyhydroxide. Fianelite, rüdlingerite, and ansermetite represent the last stage in the remobilization of V and As [1]. On the specimens investigated, rüdlingerite occurs as light orange, slightly flattened, prismatic crystals up to 300 μm in length. They are associated with anhedral ansermetite.

2.2. Cotype

Rüdlingerite was also found on specimens recovered from the dump of the Valletta mine, Canosio, Piedmont, Italy (44°23’42’’ N, 7°5’42’’ E). The Valletta mine is a small Fe-Mn deposit that has not been worked in modern times and has never been studied geologically or petrologically. The geological setting is rather similar to that of the Fianel mine and has been summarized by Cámara et al. (2014) [10] in their description of the new mineral grandaite, for which, with braccoite [11], canosioite [12], castellaroite [13], lombardoite [14], and piccoliite [15], the Valletta mine is also the type locality.
At the Valletta mine, rüdlingerite is a secondary mineral crystallized from hydrothermal fluids in an oxidizing environment. It was found in richly mineralized veins crossing the quartz-hematite ore. The main mineralization is made up of extended, massive braccoite veins and millimeter-sized grains having manganberzeliite/palenzonaite composition at the core and berzeliite on the rim. Other As- and V- rich phases (adelite, arseniopleite, canosioite, castellaroite, coralloite, bariopharmacosiderite, berzeliite, gamagarite, grandaite, palenzonaite, pyrobelonite, saneroite, tilasite, tiragalloite, tokyoite, wallkilldellite, some brackebuschite group arsenates currently under study, and other not yet well characterized As- and V-bearing minerals) are also present at the locality.
Like at the Fianel mine, the very rare open fractures host late phases that have developed euhedral crystals: rüdlingerite as yellow to light yellow-orange, flattened, elongated prismatic crystals measuring up to 200 μm in length, and fianelite as smaller, less elongated, deep red crystals closely resembling those from the Swiss type locality. Depending on local conditions, either species can be dominant in a single specimen. Rare radiating aggregates of bronze-brown wallkilldellite have also been observed.

3. Appearance and Physical Properties

At the Fianel mine, rüdlingerite occurs as light orange, prismatic crystals that reach 300 μm in length (Figure 1). At the Valletta mine, the new mineral occurs as yellow to light yellow-orange, flattened, elongated prismatic crystals measuring up to 200 μm length, associated with fianelite as smaller, less elongated, deep red crystals (Figure 2) closely resembling those from the Swiss type locality. Crystal forms were not determined but on the basis of SEM observations and photos, the dominant forms can be assigned to the pinacoid {010} and the prisms {110}, {041} and {011}, in agreement with the crystal habit and forms of fianelite, goniometrically determined by Brugger and Berlepsch (1996) [1]. Accordingly, crystal elongation is parallel [001]. Twinning has not been observed.
Rüdlingerite is transparent to translucent, has a vitreous luster, and a yellowish-white streak. It is non-fluorescent. Mohs hardness has been determined through scratch tests and found to be 3½. Rüdlingerite is brittle, with a perfect cleavage on {001} and a curved fracture. Parting was not observed. The density could be determined by floatation of a Fianel sample in diiodomethane–1–chloronaphtalene system (23° C) and was found to be 3.28(2) g·cm−3. Based on the empirical formulae, the calculated densities are 3.298 g·cm−3 for the Fianel sample and 3.322 g·cm−3 for the Valletta sample.
Optically, rüdlingerite is biaxial (sign unknown) and 2Vmeas. is smaller than 10°. The average refractive index calculated using the Gladstone-Dale relationship is 1.791 < nav < 1.799 for the average composition of the Fianel material. The refractive index nav, measured using immersion method in refractive index liquid in the diodomethane–tin tetrabromide–sulphur system (CH2I2–SnI4–S; 1.78 < n < 1.82) [16] and verified using a Leitz-Jelley microrefractometer [17] is nav ≥ 1.80 (589 nm). The high chemical reactivity of index liquid, due to the hydrolysis toward hydrated minerals, precludes complete data measurements. After several minutes of immersion, rüdlingerite crystals are destroyed. The flattened prismatic habitus of rudlingerite, with resulting small particle dimensions perpendicular to Z, precluded observations of bisectrix-centered interference figures of required quality. Rüdlingerite shows a strong pleochroism from yellow to orange; it displays positive elongation; Z ˄ c ~ 14°. For the Valletta specimen, X = yellow, Y = orange yellow, Z = brownish orange; X < Y << Z.

4. Chemical Data

The results of the chemical analyses are summarized in Table 1. Chemical compositions of the Fianel and Valletta specimens were obtained by electron microprobe techniques (WDS mode) using 16 point analyses performed with the use of a Cameca SX100 instrument (located at the Department of Geological Sciences in Brno, Czech Republic, and operated at 15 kV, 10 nA at a beam diameter of 8 μm) and 4-point analyses performed with a Cameca SX50 instrument (located at the Department of Earth Sciences, Sapienza University, Rome, Italy, and operated at 15 kV, 15 nA at a beam diameter of 1 μm), respectively. The standards used are given in Table 1. Other possible or likely elements were sought by WDS, but found to be below the detection limits.
Raw X-ray intensities were corrected for matrix effects with a φ(ρz) algorithm of X-PHI routine [18]. Because insufficient material was available for a direct determination of H2O, the amount of water was calculated on the basis of charge balance and O = 9 apfu, as determined by the crystal structure analysis.
The empirical formulae calculated on the basis of 9 O apfu are:
  • For Fianel: Mn1.97T(1)+T(2)(V5+1.17As0.83Si0.01)Σ2.01O7.00∙2H2O
  • For Valletta: (Mn1.93V3+0.07)Σ2.00T(1)+T(2)(V5+1.06As0.93)Σ1.99[O6.98(OH)0.02]Σ7.00∙1.98H2O
The ideal formula is Mn2T(1)V5+ T(2)As5+O7 ∙ 2H2O, which requires As2O5 31.25, V2O5 20.38, MnO 38.58, H2O 9.79, total 100 wt%.
Chemical tests have shown rüdlingerite to be insoluble in H2O and slightly soluble in dilute (10%) HCl at room temperature.

5. Raman Spectroscopy

A Raman spectrum of rüdlingerite from Fianel mine was obtained with a Leica DM 2500 microscope, equipped with 5×, 10×, 20×, 50×, and 100× objectives and part of a Renishaw inVia spectrometer, which includes a monochromator, a filter system, and a Charge Coupled Device (CCD), located in the Natural History Museum of Geneva, Geneva, Switzerland. The Raman spectrum was obtained by a He-Ne Laser (633 nm) at a nominal resolution of 2 cm−1 in the range 100–4000 cm−1. A power of 1.7 mW on the sample was set, during a time of 10 s for a single analysis. The analysis was conducted on a single prismatic crystal, with the laser beam orthogonal to the elongation direction of the crystal. The representative spectrum is shown in Figure 3.
Raman spectra of rüdlingerite from the Valletta mine and of fianelite from the Fianel mine (see Figure 3) were acquired using a micro/macro Jobin Yvon Mod. LabRam HRVIS, equipped with a motorized x-y stage and an Olympus microscope and located at the Department of Earth Sciences, Torino University, Torino, Italy. The backscattered Raman signal was collected with 20× objective and the Raman spectrum was obtained for a non-oriented single crystal. The 532 nm line of a Nd:YAG laser was used as excitation; laser power (80 mW) was controlled by means of a series of density filters. The minimum lateral and depth resolution was set to few μm. The system was calibrated using the 520.6 cm−1 Raman band of silicon before each experimental session. The spectra were collected with multiple acquisitions (2 to 4) with single counting times ranging between 50 and 100 s. Spectral manipulation such as baseline adjustment, smoothing and normalization were performed using the Labspec 5 software package [19]. After background removal, band component analysis was undertaken using the Fityk software package [20], assuming combined Lorentzian-Gaussian band shapes and applying Voigt function.
The presence of (AsO4)3− is confirmed in both samples, but it is very difficult to separate the contributions of arsenate from those of vanadate units as the bands of the two units partly overlap each other (e.g., [21]). In the region 100–600 cm−1 multiple Raman bands are observed at 122 (Fianel), 245 (Valletta), 280 (both) (all corresponding to lattice modes, according to Frost et al., 2009 [22]), 344 (Fianel), and 374 (Valletta) cm−1 (As5+–O symmetric bending vibrations [23]). A weak and sharp band centred around 466 cm−1 is also present in the Valletta spectrum. In the region of 600–800 cm−1, weak, multiple overlapping bands are observed at 605, 672, and 737 cm−1 in the spectra of both localities. The former two bands are assigned to the assumed OH vibration modes, while the latter is thought to be related to the As–metal-oxygen bonds. The most intense bands are centred at 867 and 909 cm−1, corresponding to (AsO4)3− and (VO4)3− symmetric stretching nodes [22,24,25]. The splitting of this main peak may be attributable to As occupying both T sites in rüdlingerite, but only T(2) in fianelite. The flattened band at ~1710 cm−1 in the Valletta spectrum could be assigned to δ H2O bending mode of hydrogen bonded water molecules. Two weak and broad bands in the region 3300–3600 cm−1 of the Fianel spectrum are attributable to O–H stretching modes of the H2O groups [22].

6. Crystallography and Crystal Structure

Rüdlingerite is monoclinic, space group P21/n. Powder X-ray diffraction (PXRD) data for rüdlingerite from Fianel were collected using a 114.6 mm diameter Gandolfi Camera (Ni-filtered CuKα radiation and Si as internal standard) located at the Musée cantonal de géologie, Lausanne. The LATCON program [26] was used to refine the PXRD data after light correction using Si as internal standard. The results are summarized in Table 2. The unit cell parameters calculated from the powder data are as follows: a = 7.814(7) Å, b = 14.63(1) Å, c = 6.690(6) Å, β = 93.63(6)°, V = 764(2) Å3 and Z = 4.
Single-crystal X-ray studies were carried out on the Fianel holotype using a Rigaku-Oxford Diffraction Supernova diffractometer installed at the Department of Geosciences, University of Padova, Italy, equipped with a 200K Pilatus Dectris detector and with a micro-source with MoKα radiation (λ = 0.71073 Å); the detector-to-crystal distance was 68 mm. They confirmed the space group and delivered the following data: a = 7.8289(2) Å, b = 14.5673(4) Å, c = 6.7011(2) Å, β = 93.773(2)°, V = 762.58(4) Å3 and Z = 4.
Single crystal data were also obtained for a cotype specimen from Valletta using an Oxford Gemini R Ultra diffractometer equipped with a CCD area detector and graphite-monochromatized MoKα radiation (λ = 0.71073 Å) installed at Centro Interdipartimentale di Ricerca per lo Sviluppo della Cristallografia Diffrattometrica-CRISDI, Torino University, Torino, Italy. The space group was confirmed, and the following unit cell parameters were obtained: a = 7.8278(3) Å, b = 14.5739(7) Å, c = 6.6962(3) Å, β = 93.820(4)°, V = 762.22(6) Å3, Z = 4.
The a:b:c ratios calculated from the unit-cell parameters are (single-crystal data) 0.5374:1:0.4600 for Fianel and 0.5371:1:0.4595 for Valletta.
Complete diffraction intensity data collection allowed refining the structure of the Fianel holotype and Valletta cotype starting from the atom coordinates of Brugger and Berlepsch (1996) [1]. The crystal structure was refined to R1 = 0.041–0.046 by means of the SHELX-2018 package [27] on the basis of 3784–2027 independent reflections with Fo > 4σ(F). Experimental results are reported in Table 3. The crystal structure refinement of rüdlingerite provides results very close to those of fianelite. The coordinates of H atoms were located in the Fourier difference map and added to the refinement soft restraints of 0.96(2) Å on the O–H distances, and with the Uiso of each H set to ×1.2 that of its donor O atom.
Table 4 summarizes site occupancies, fractional atom coordinates and equivalent and isotropic displacement parameters. Crystallographic Information Files (CIF) are on deposit (see Supplementary Materials). From the interatomic distances listed in Table 5 and from a comparison of the two average Mn-O distances, 2.181 and 2.186 Å, with the mean octahedral bond length reported in the literature for Mn2+, one can preclude the presence of Mn in the trivalent oxidation state. The presence of V and As hosted in tetrahedra and the tetrahedrally coordinated site dimensions are in agreement with the pentavalent oxidation state for both cations, with T(1) slightly larger than T(2). The empirical bond valences estimated from the final refined structure (Table 6) also confirm the inferred cation oxidation states. The observed site scattering values obtained by structure refinement revealed that As5+ substitutes V5+ both at T(1) and T(2), but predominantly only at T(2) (Table 4), in agreement with a larger size for T(1).
The polyhedral framework consists of octahedra and tetrahedra. The structural arrangement can be described in terms of edge-sharing octahedra forming zigzag chains subparallel to the [-101] direction. The chains of octahedra are decorated with T2O7 corner-connected dimmers (Figure 4a). The polyhedral framework contains cavities along [001] (Figure 4b). The cavities are elongated along [130] and [1-30]. The H2O molecules are located on the unshared apexes of the octahedra that point towards these cavities (OW(1) and OW(2) sites), which form weak hydrogen bonds with neighboring O(5) atoms.
The crucial difference of the crystal structure of rüdlingerite with that of fianelite relates to the scattering at the T(2) site. Whereas the T(1) site has a scattering providing 24.20 electrons per site (eps), for the T(2) site, we refined 30.40 eps, which is consistent with a cation site occupancy strongly dominated by As5+: T(2)(As0.74V0.26) (Table 4). On the other hand, the T(1) site scattering, 24.20 eps, is consistent with a cation site occupancy dominated by V5+: T(1)(V0.88As0.12).
The refinement of intensity data collected on the cotype sample also yields a larger amount of scattering at the T(2) site (29.4(1) eps versus 24.7(1) eps at the T(1) site), also indicating the ordering of As at this site.
The mineral formula derived from the SREF information for the holotype and cotype samples are Mn1.94T(1)(V5+0.88As0.12)T(2)(As0.74V5+0.26)O7.00∙2H2O and Mn2T(1)(V5+0.83As0.17)T(2)(As0.64V5+0.36)O7.00∙2H2O, respectively. These formulae show the dominance of V5+ at the T(1) site and As5+ at the T(2) site, and are consistent with the formulae derived from the chemical data (see above).

7. Discussion

Rüdlingerite is the T(2) site As-dominant analogue of fianelite [1], with which it is isostructural. In their description of fianelite, Brugger and Berlepsch (1996) [1] reported an extended solid solution of As in fianelite (see Figure 5), which in fact reached more than 0.5 apfu of As in a couple of data points. However, the average of the chemical analyses yielded an amount of As which was lower than 0.5 apfu As. In fact, these authors described fianelite as Mn1.98(V1.57As0.44Si0.01)O7∙2H2O. Figure 5 shows the V-As homovalent substitution in rüdlingerite and fianelite, as derived from the available chemical analyses. However, all the structural data in fianelite and rüdlingerite show that As orders preferentially at the T(2) site (0.38 apfu in the model of Brugger and Berlepsch, 1996 [1]). Being T(1) and T(2) two crystallographically different sites, the dominance of an element on one of these sites establishes a new mineral, in accord with the IMA-CNMNC rules. If As was ever found dominant in both the tetrahedral sites, that would represent a further new mineral species.
There is a geometrical vantage to host a smaller cation at the T(2) sites: as can be seen in Figure 4, the chains of octahedra are bent, thus making it more propitious to have a smaller cation hosted at the T(2) tetrahedra at the concave hinge of the bent chain, while the T(1) tetrahedra acts as linkage of adjacent chains and therefore a larger size favours linkage. In fact, it is also more distorted (Table 5). This can explain the ability of fianelite/rüdlingerite structure to host both As and V, if available. The V5+, on the other hand, is a d0 transition metal and consequently prefers a more distorted bonding environment as the one observed in T(1) (Table 5).
Regarding hydrogen bonding, from Table 6, it is evident that the O(1) and O(4) have bond valence values clearly lower than 2, whereas the rest have values closer to 2 valence units (v.u.). This, along with the favourable geometries reported in Table 5, allow us to propose that the missing H atoms not located in the difference Fourier maps can form weak bonding to these oxygen atoms. Assuming a contribution equivalent to the one calculated for O(5) (Table 6) would be enough to reach a bond valence sum of 2 v.u. The bond valence sum values lower than 2 at OW(1) and OW(2) indicate the respective bonds to the two missing H atoms, probably longer than the ones observed during refinement for H(1) and H(2). It is worth noting that the O(3) atom, proposed by Brugger and Berlepsch (1996) as acceptor of H-bonding, does not seem a probable acceptor as the bond valence incidence is about 2 v.u. Accounting for the above scheme, it is possible to use the empirical correlation of Libowitzky [31] to calculate the frequency of the corresponding O–H using the observed OD···OA lengths (Table 5) for the four proposed hydrogen bonds. We obtain 3400, 3542, 3581, and 3587 cm−1, which are in line with the bands observed in the Raman of the sample from Fianel (Figure 3).
Mesaite [32], ideally CaMn2+5(V2O7)3·12H2O, is structurally related to the above two mineral species, having the same zig-zag chains of Mn2+ centred octahedra and divanadate tetrahedra groups as fianelite and rüdlingerite. However, it has a layer structure, with interlayers regions having solvated Ca cations, whereas fianelite and rüdlingerite have a framework structure. Comparisons of physical data of rüdlingerite, fianelite, and mesaite are summarized in Table 7.
In the Strunz system [33], rüdlingerite belongs to 8.FC: polyphosphates, polyarsenates, and polyvanadates with H2O.
Arsenates and vanadates are known to occur in numerous, metamorphosed Fe-Mn deposits that underwent multiphase Alpine metamorphism all across the Alps and the Apennines. The paleogeographic environment of these mineralizations are diverse, with some deposits being hosted in Jurassic-aged radiolarites like Falotta, Grisons, Switzerland [34] or Val Graveglia, Liguria, Italy [35], another occurring in platform-carbonates sedimented over a continental basement like Fianel or in Middle Jurassic-aged paleokarsts filling with hydrothermal input like Pipjigletscher, Valais, Switzerland [36], and yet other being hosted in Permian quartzites like Valletta. These vanadium-arsenic-manganese-minerals Alpine parageneses have formed partly during prograde metamorphism, up to upper greenschist-facies conditions culminating at about 450° C at pressures of 4 to 6 kbar in Pipjigletscher [36] and—like rüdlingerite—partly though hydrothermal processes during retrograde metamorphism [37]. Strangely, the Prabornaz Mn-deposit, Aosta, Italy, also of sedimentary-hydrothermal origin but affected by a high metamorphism at about 550° C at pressure of 21 kbar, is almost devoid of V and weakly enriched in As [38,39].
Rüdlingerite is the twenty-fifth new Mn vanadate or arsenate to be described from such Fe-Mn Alpine deposits. While their mineralogy is highly contrasted and fascinating, their economic significance was very modest, due to their small volumes, pronounced compartmentalization and/or difficult, high-altitude access. This strongly contrasts with the other large providers of Mn arsenates and vanadates deposits that are not linked to an Alpine event: Långban, Sweden, Franklin, and Sterling Hill, USA as well as Kombat, Namibia.

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/10/11/960/s1, Crystallographic Information Files: ruedlingerite_Fianel.cif and ruedlingerite_Valletta.cif.

Author Contributions

Recognition of first rüdlingerite samples, P.R., R.B., and M.E.C.; preliminary analysis, P.R., N.M., and M.E.C.; physical measurements, R.Š., F.C., and U.H.; Raman spectroscopic measurements, C.S. and M.E.C.; crystal-structure solution and refinement, F.N., F.C., and F.B.; chemical analyses, R.Š., F.N., and F.B.; writing—proposal for IMA; N.M. with contributions from all authors; writing—original draft preparation, P.R. with contributions from all authors; writing—review and editing, P.R., F.C. with contributions from all authors. All authors have read and agreed to the published version of the manuscript.

Funding

The Article Processing Charge for this article was supported by Etat de Vaud through Musée cantonal de géologie in Lausanne, Switzerland.

Acknowledgments

This work would not have been possible without the enthusiasm of M. Francesco Vanini (Varese, I), who brought his “fianelite” discovery to the attention of one of us (P.R.), allowed us to link the two teams, and subsequently supported our study with his field experience. We thank Cristian Biagioni for the constructive and fruitful discussions. The review by Mark Cooper and other three anonymous referees greatly enhanced the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Scanning Electronic Microscope (SEM) image of rüdlingerite crystals from Fianel.
Figure 1. Scanning Electronic Microscope (SEM) image of rüdlingerite crystals from Fianel.
Minerals 10 00960 g001
Figure 2. Yellow to orange platy crystals of rüdlingerite (0.1 mm long) with dark orange to red fianelite crystals from Valletta (R. Bracco photo).
Figure 2. Yellow to orange platy crystals of rüdlingerite (0.1 mm long) with dark orange to red fianelite crystals from Valletta (R. Bracco photo).
Minerals 10 00960 g002
Figure 3. Comparison of the microRaman spectra of rüdlingerite and of fianelite from Fianel and of rüdlingerite from Valletta. The differences between the rüdlingerite spectra and the fianelite spectrum are evident in the range 600–1000 cm−1.
Figure 3. Comparison of the microRaman spectra of rüdlingerite and of fianelite from Fianel and of rüdlingerite from Valletta. The differences between the rüdlingerite spectra and the fianelite spectrum are evident in the range 600–1000 cm−1.
Minerals 10 00960 g003
Figure 4. Orthographic projections of the crystal structure of rüdlingerite on the plain (the orientation is shown inside the figure), showing details of the chains of Mn-centred octahedra and the linking through the T2O7 groups (a), the cavities along [001] hosting the hydrogen bonds (b) and the alternance of bent chains along [010] (c). Oxygen atoms are reported in red as anisotropic displacement parameters. Hydrogen atoms are in white. Diagrams obtained using VESTA 3.0 [30].
Figure 4. Orthographic projections of the crystal structure of rüdlingerite on the plain (the orientation is shown inside the figure), showing details of the chains of Mn-centred octahedra and the linking through the T2O7 groups (a), the cavities along [001] hosting the hydrogen bonds (b) and the alternance of bent chains along [010] (c). Oxygen atoms are reported in red as anisotropic displacement parameters. Hydrogen atoms are in white. Diagrams obtained using VESTA 3.0 [30].
Minerals 10 00960 g004
Figure 5. Distribution of As and V in rüdlingerite from the Fianel and Valletta mines and fianelite from the Fianel mine. Data of fianelite was redrawn from Brugger and Berlepsch (1996) [1].
Figure 5. Distribution of As and V in rüdlingerite from the Fianel and Valletta mines and fianelite from the Fianel mine. Data of fianelite was redrawn from Brugger and Berlepsch (1996) [1].
Minerals 10 00960 g005
Table 1. Chemical composition (wt%) of rüdlingerite from Fianel and Valletta Mines.
Table 1. Chemical composition (wt%) of rüdlingerite from Fianel and Valletta Mines.
ConstituentFianel Specimen
(16 Point Analyses)
Valletta Specimen
(4 Point Analyses)
Standard
Fianel/Valletta
MeanRangeSDMeanRangeSD
CaOb.d.l. 0.040.01−0.060.02Wollastonite
MnO36.8436.41−37.210.7235.2534.79−35.660.40Rhodonite
FeO0.060.00−0.120.07b.d.l. Almandine
ZnOb.d.l. 0.030.00−0.050.02ZnO/Zn
As2O525.3223.23−28.590.6427.5926.65−29.231.13Lammerite/
synth. GaAs
V2O528.0525.34−30.250.2026.47*24.06−28.361.80Synth. ScVO4/
vanadinite
SiO20.130.00−0.210.04b.d.l. Sanidine
H2O**9.51 9.25
Total99.91 98.34*
b.d.l. = below detection limit. *An amount of V2O3 = 1.35 wt% has been calculated on the basis of V3+ at the Mn(1,2) sites as suggested by an initial, tentative structure refinement of the Valletta sample; therefore the amount of V2O5 has to be reduced to 24.83 wt%. **Calculated on the basis of charge balance, required (OH) pfu to charge compensate for V3+ at the Mn(1,2) sites and O = 9 apfu.
Table 2. X-ray powder diffraction data for rüdlingerite from Fianel. The strongest diffraction lines are given in bold.
Table 2. X-ray powder diffraction data for rüdlingerite from Fianel. The strongest diffraction lines are given in bold.
dobs[Å]dcalc[Å]Iobs *hkl
7.287.3150020
6.886.8940110
6.076.0830011
5.345.3480120
4.934.9710021
4.684.7020111
4.124.1420130
3.9433.93920031
3.4533.44330220
3.2493.25830012
3.0483.039100022
2.7763.79330122
2.7302.73660231
2.6182.6314013-2
2.5572.5583021-2
2.4522.45140320
2.3442.34150250
2.2062.2076016-1
1.9691.99910341
1.8361.85040351
1.8331.83240143
* Intensities may be affected by preferred orientation.
Table 3. Crystal Data and Structure Refinement for Rüdlingerite Holotype from the Fianel Mine and Cotype from the Valletta Mine.
Table 3. Crystal Data and Structure Refinement for Rüdlingerite Holotype from the Fianel Mine and Cotype from the Valletta Mine.
Identification Code FianelValletta
Formula weight 378.05373.67
Wavelength (MoKα)0.71073 Å0.71073 Å
Crystal system, space groupMonoclinic, P21/nMonoclinic, P21/n
a, b, c (Å)7.8289(2), 14.5673(4), 6.7011(2)7.8278(3), 14.5739(7), 6.6962(3)
β (°)93.773(2)93.820(4)
V (Å3)762.58(4)762.22(6)
Z44
μ (mm-1)8.2048.004
Crystal size (mm)0.10 × 0.05 × 0.050.08 × 0.05 × 0.05
Θ-range for data collection (°)2.80 to 40.873.35 to 32.15
Index ranges−14 ≤ h ≤ 14,−26 ≤ k ≤ 26, −12 ≤ l ≤ 12−11 ≤ h ≤ 10, −21 ≤ k ≤ 21, −9≤ l ≤ 9
Collected, independent and
observed [I > 2σ(I)] reflections
48224, 4928, 37845213, 2452, 2027
Rint0.0680.036
Completeness to theta = 25.242°100.0% 99.9%
Refinement
Data / restraints / parameters4928/2/1292452/2/127
R[F2 > 2σ(F2)], wR(F2), S0.041, 0.092, 1.0460.046, 0.078, 1.106
Extinction coefficient0.0022(6)0.0018(4)
Δρmax, Δρmin (e Å3)1.390, −1.6160.757, −0.896
Table 4. Site Occupancy Factor (s.o.f.), Atom Coordinates and Equivalent and Isotropic Displacement Parameters (Å2) for Rüdlingerite Holotype from the Fianel Mine and Cotype from the Valletta Mine.
Table 4. Site Occupancy Factor (s.o.f.), Atom Coordinates and Equivalent and Isotropic Displacement Parameters (Å2) for Rüdlingerite Holotype from the Fianel Mine and Cotype from the Valletta Mine.
Site s.o.f.x/ay/bz/cUeq/Uiso *
Holotype
Mn(1)Mn0.969(3)0.4007(1)0.3853(1)0.2372(1)0.014(1)
Mn(2)Mn0.969(3)0.6240(1)0.2483(1)0.9594(1)0.015(1)
T(1)V0.88(2)As0.12(2)0.3312(1)0.4205(1)0.7442(1)0.012(1)
T(2)V0.26(3)As0.74(3)0.0426(1)0.3170(1)0.9482(1)0.012(1)
O(1)O1.000.1693(2)0.2927(1)0.1566(3)0.019(1)
O(2)O1.000.0609(2)0.2350(1)0.7722(3)0.017(1)
O(3)O1.000.1208(2)0.4185(1)0.8441(3)0.020(1)
O(4)O1.000.3859(3)0.5277(1)0.7039(3)0.022(1)
O(5)O1.00−0.1591(2)0.3392(1)-0.0024(3)0.020(1)
O(6)O1.000.4627(3)0.3688(1)0.9229(3)0.019(1)
O(7)O1.000.3177(3)0.3586(1)0.5294(3)0.019(1)
OW(1)O1.000.2359(3)0.5056(2)0.2375(4)0.030(1)
OW(2)O1.000.4048(3)0.1509(2)0.9400(4)0.040(1)
H(1)H1.000.234(6)0.554(2)0.148(6)0.036*
H(2)H1.000.354(6)0.138(4)0.811(4)0.047*
Cotype
Mn(1)Mn0.965V3+0.0350.40146(8)0.38561(5)0.23771(10)0.01235(18)
Mn(2)Mn0.965V3+0.0350.62483(8)0.24879(5)0.95921(10)0.01137(17)
T(1)V0.834(4)As0.166(4)0.33207(8)0.42039(4)0.74401(9)0.00762(19)
T(2)V0.360(4)As0.640(4)0.04339(6)0.31707(4)0.94792(7)0.00585(15)
O(1)O1.000.1697(4)0.2924(2)0.1562(4)0.0158(6)
O(2)O1.000.0616(4)0.2346(2)0.7722(4)0.0143(6)
O(3)O1.000.1224(4)0.4184(2)0.8441(5)0.0170(6)
O(4)O1.000.3854(4)0.5274(2)0.7025(5)0.0186(7)
O(5)O1.00−0.1579(4)0.3396(2)−0.0038(5)0.0177(7)
O(6)O1.000.4633(4)0.3690(2)0.9235(4)0.0169(6)
O(7)O1.000.3184(4)0.3584(2)0.5304(4)0.0165(6)
OW(1)O1.000.2351(4)0.5051(2)0.2393(6)0.0265(8)
OW(2)O1.000.4060(5)0.1513(3)0.9401(6)0.0354(10)
H(1)H1.000.201(7)0.544(3)0.134(6)0.032*
H(2)H1.000.369(7)0.093(2)0.897(9)0.042*
* Hydrogen atoms were refined using soft constraints and starting from the coordinates observed in the Fourier-difference maps; two further hydrogen atoms could not be located in the Fourier-difference map.
Table 5. Selected Interatomic Distances (Å) and Geometrical Parameters for the Rüdlingerite Fianel (Holotype) and Valletta (Cotype) Specimens.
Table 5. Selected Interatomic Distances (Å) and Geometrical Parameters for the Rüdlingerite Fianel (Holotype) and Valletta (Cotype) Specimens.
HolotypeCotype HolotypeCotype
T(1)–O(4)1.647(2)1.643(3)Mn(1)–O(4)2.113(2)2.113(3)
–O(7)1.696(2)1.689(3) –O(7) 2.139(2)2.142(3)
–O(6)1.703(2)1.702(3) –O(2) 2.159(2)2.158(3)
–O(3)1.818(2)1.814(3) –OW(1)2.176(2)2.175(3)
<T(1)–O>1.7161.712 –O(6) 2.205(2)2.204(3)
V3)2.5852.567 –O(1) 2.295(2)2.303(3)
Δ 0.029870.02978<Mn(1)–O>2.1812.182
T(2)–O(5)1.666(2)1.662(3)Mn(2)–O(5) 2.155(2)2.156(3)
–O(1)1.691(2)1.694(3) –O(6)2.166(2)2.164(3)
–O(2)1.696(2)1.694(3) –O(1)2.167(2)2.167(3)
–O(3)1.762(2)1.761(3) –O(2)2.198(2)2.199(3)
<T(2)–O>1.7041.703 –O(7)2.203(2)2.206(3)
V3)2.5292.525 –OW(2)2.224(3)2.223(4)
Δ 0.016960.01725<Mn(2)–O>2.1862.186
OW(1)H(1)0.92(2)0.93(2)OW(2)H(2)0.94(2)0.94(2)
OW(1)…O(5)2.798(4)2.801(5)OW(2)…O(5)2.977(4)2.984(6)
OW(1)…O(4)3.281(4)3.259(6)OW(2)…O(1)3.182(4)3.178(6)
O(5) OW(1)–O(4)120.5(1)120.4(1)O(5)–OW(2)–O(1)111.0(1)111.2(2)
Δ–distortion index defined after [28].
Table 6. Bond Valences (in Valence Units) for Rüdlingerite (Fianel Specimen) using [29].
Table 6. Bond Valences (in Valence Units) for Rüdlingerite (Fianel Specimen) using [29].
Mn(1)Mn(2)T(1)T(2)H(1)H(2)Σ b.v.
O(1)0.260.36 1.26 1.88
O(2)0.370.33 1.24 1.94
O(3) 0.941.03 1.97
O(4)0.41 1.47 1.88
O(5) 0.37 1.350.060.101.88
O(6)0.330.361.27 1.96
O(7)0.380.331.29 2.00
OW(1)0.35 1.00 1.35
OW(2) 0.31 0.951.26
SUM2.102.064.974.881.061.058
Table 7. Comparison of Rüdlingerite, Fianelite, and Mesaite.
Table 7. Comparison of Rüdlingerite, Fianelite, and Mesaite.
MineralRüdlingerite [This Work]Fianelite [1]Mesaite [32]
(Fianel/Valletta)
Simplified formulaMn2+2V5+As5+O7·2H2OMn2+2V5+2O7·2H2OCaMn2+5(V2O7)3·12H2O
Space group P21/nP21/nP2/n
a (Å)7.8289(2)/7.8278(3)7.809(2)9.146(2)
b (Å)14.5673(4)/14.5739(7)14.554(4)10.424(3)
c (Å)6.7011(2)/6.6962(3)6.705(4)15.532(4)
β (O)93.773(2)/93.820(4)93.27(3)102.653(7)
V3)762.58(4)/762.22(6)760.8(5)1444.8(6)
Z44 2
Dcalc. (g/cm3)3.298/3.3223.217 2.744
Dmeas. (g/cm3) 3.21(1)2.74(1)
Strongest reflections in the
X-ray powder diffraction data,
d/n (Å) (I)
3.048(100)3.039(100)10.47(100)
5.34(80)5.32(80)2.881(25)
2.730(60)2.721(60)3.568(24)
2.206(60)1.593(60)3.067(17)
2.344(50)3.436(50)4.30(11)
7.28(50)3.259(50)2.615(11)
6.88(30)2.573(50)8.60(10)
Optical class, signBiaxial; sign unknownunknownbiaxial (–)
α (589.3 nm)nav =1.791 − 1.799 (calc.) 1.760 (calc.)
β 1.82(2) 1.780(5)
γ 1.795(5)
2Vmeas (o)<10°<10° 81(2)°
2Vcalc (o) unknown
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MDPI and ACS Style

Roth, P.; Meisser, N.; Nestola, F.; Škoda, R.; Cámara, F.; Bosi, F.; Ciriotti, M.E.; Hålenius, U.; Schnyder, C.; Bracco, R. Rüdlingerite, Mn2+2V5+As5+O7·2H2O, a New Species Isostructural with Fianelite. Minerals 2020, 10, 960. https://doi.org/10.3390/min10110960

AMA Style

Roth P, Meisser N, Nestola F, Škoda R, Cámara F, Bosi F, Ciriotti ME, Hålenius U, Schnyder C, Bracco R. Rüdlingerite, Mn2+2V5+As5+O7·2H2O, a New Species Isostructural with Fianelite. Minerals. 2020; 10(11):960. https://doi.org/10.3390/min10110960

Chicago/Turabian Style

Roth, Philippe, Nicolas Meisser, Fabrizio Nestola, Radek Škoda, Fernando Cámara, Ferdinando Bosi, Marco E. Ciriotti, Ulf Hålenius, Cédric Schnyder, and Roberto Bracco. 2020. "Rüdlingerite, Mn2+2V5+As5+O7·2H2O, a New Species Isostructural with Fianelite" Minerals 10, no. 11: 960. https://doi.org/10.3390/min10110960

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