Crystal ‐ Chemistry of Sulfates from the Apuan Alps (Tuscany, Italy). VII. Magnanelliite, K 3 Fe 3+2 (SO 4 ) 4 (OH)(H 2 O) 2 , a New Sulfate from the Monte Arsiccio Mine

: The new mineral species magnanelliite, K 3 Fe 3+2 (SO 4 ) 4 (OH)(H 2 O) 2 , was discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as steeply terminated prisms, up to 0.5 mm in length, yellow to orange ‐ yellow in color, with a vitreous luster. Streak is pale yellow, Mohs hardness is ca. 3, and cleavage is good on {010}, fair on {100}. The measured density is 2.82(3) g/cm 3 . Magnanelliite is optically biaxial (+), with α = 1.628(2), β = 1.637(2), γ = 1.665(2) (white light), 2 V meas = 60(2)°, and 2 V calc = 59.9°. It exhibits a strong dispersion, r > v . The optical orientation refined using X ‐ ray single ‐ crystal data to a final R 1 = 0.025, on the basis of 2411 reflections with F o > 4 σ ( F o ) and 144 refined parameters. The crystal structure is isotypic with that of alcaparrosaite, K 3 Ti 4+ Fe 3+ (SO) 4 O(H 2 O) 2 .


Introduction
The occurrence of secondary sulfate assemblages in the pyrite ores from the Apuan Alps (Tuscany, Italy) was reported in the second half of the nineteenth Century (e.g., D'Achiardi [1]).However, few species were reported, and, among them, some were doubtful.For instance, D'Achiardi [1] reported the presence of gypsum, melanterite, halotrichite, alum-(K), goslarite, and, possibly, coquimbite.Since the second half of the 2000 s, the mineralogical study of the pyrite ± baryte ± iron oxide ore deposits from the southern Apuan Alps allowed the identification of some interesting sulfate assemblages, in which rare or even new mineral species were identified.The Fornovolasco ore deposit was the first locality to be studied using modern analytical techniques.Here, Biagioni et al. [2] described the new oxy-hydroxy iron sulfate volaschioite, Fe 3+ 4O2(SO4)(OH)6•2H2O.Later, Mauro et al. [3] identified a new hydrated iron phosphate-sulfate from the Buca della Vena mine, naming it bohuslavite, Fe 3+ 4(PO4)3(SO4)(OH)(H2O)10•nH2O.The most recent discovery is a sulfate assemblage found at the Monte Arsiccio mine, in which well-crystallized sulfate specimens were recovered, including three new mineral species, i.e., giacovazzoite [4], scordariite [5], and magnanelliite.
Magnanelliite was identified on a specimen found by the mineral collector Mario Bianchini.The name honors Stefano Magnanelli (b.1959), chemist and mineral collector, for his contributions to the knowledge of the mineralogy of the Apuan Alps hydrothermal veins.He coauthored the type description of bottinoite [6] and provided the first specimens of parasterryite [7] and bianchiniite [8], as well as several samples from the Carrara marble quarries and other mining sites of the Apuan Alps, supporting the mineralogical studies of these occurrences.The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2019-010).The holotype specimen of magnanelliite is deposited in the mineralogical collection of the Museo di Storia Naturale, Università di Pisa, Via Roma 79, Calci (Pisa), Italy, under catalog number 19894.A cotype specimen is deposited in the collections of the Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA, under catalogue number 67241.
This paper presents the definition, occurrence, and crystal structure of the new mineral species magnanelliite.
The sulfate assemblage in which magnanelliite was collected occurs in an old stope of the mine where microcrystalline pyrite is exposed and deeply altered.This ongoing oxidation process has given rise to a large suite of secondary phases [5], among them, several K-Fe 3+ sulfates: krausite, goldichite, giacovazzoite, scordariite, and the latest addition, magnanelliite.

Chemical and Spectroscopic Data
A preliminary chemical analysis using a FEI Quanta 450 ESEM FEG (FEI Company, Hillsboro, OR, USA) equipped with a QUANTAX Xflash detector 6|10 (EDS mode) (Bruker, Billerica, MA, USA) did not indicate the presence of elements (Z > 9) other than K, Fe, and S.
Quantitative chemical data were collected on one polished crystal using a JEOL JXA 8200 electron microprobe (JEOL, Tokyo, Japan) operating in WDS mode at 8 kV and 20 nA, with the beam defocused to 15 μm in diameter to limit sample damage.The slightly high analytical total obtained after addition of calculated H2O content (on the basis of structural data-see below) is likely due to the partial dehydration of the sample under high vacuum during carbon coating or during electron microprobe analysis.Such a dehydration was manifested by fractures in the sample surface.The following standards (element, emission line) were used: synthetic MgSO4 (MgKα, SKα), synthetic TiO2 (TiKα), albite (AlKα, NaKα), hematite (FeKα), and sanidine (KKα).Chemical data (average of 4 spot analyses) are given in Table 1.Iron is assumed to be trivalent, in agreement with the structural study (see below).Micro-Raman spectra of magnanelliite were obtained on an unpolished sample in nearly back-scattered geometry with a Horiba Jobin-Yvon XploRA Plus apparatus (Horiba France SAS, Longjumeau Cedex, France), equipped with a motorized x-y stage and an Olympus BX41 microscope (SPOT Imaging, Sterling Heights, MI, USA) with a 10× objective.The 532 nm line of a solid-state laser was used.The minimum lateral and depth resolution was set to a few μm.The system was calibrated using the 520.6 cm −1 Raman band of silicon before each experimental session.Spectra were collected through three acquisitions with single counting times of 180 s, with the laser power filtered at 25% (i.e., 6.25 mW).No thermal damage was observed.Backscattered radiation was analyzed with a 1200 gr/mm grating monochromator.Figure 2 shows the Raman spectrum of magnanelliite.In the region between 400 and 1300 cm −1 , bands related to bending and stretching modes of SO4 groups occur.Symmetrical stretching modes ν1 occur at 947 and 1001 cm −1 , whereas antisymmetric stretching modes ν3 are at 1074, 1082, 1116, 1157, 1215, and 1240 cm −1 .Symmetric bending vibrations ν2 occur at 372, 435, 474, and 502 cm −1 , whereas bands at 576, 606, and 635 cm −1 can be attributed to ν4 antisymmetric bending modes.The band at 848 cm −1 can be interpreted as due to librations of H2O groups, in agreement with previous authors (e.g., [19,20]).Finally, bands at wavenumbers lower than 300 cm −1 (147, 206, 243, and 286 cm −1 ) can be related to lattice vibrations and Fe-O modes.The stretching of O-H bonds, related to the presence of H2O groups, is represented by relatively weak and broad bands between 3000 and 3600 cm −1 .

X-Ray Crystallography
X-ray powder diffraction data for magnanelliite (Table 2) were collected using a 114.6 mm Gandolfi camera and Ni-filtered CuKα radiation.Unit-cell parameters were not refined, owing to the multiple attribution of indices for the strongest reflections.Single-crystal intensity data were collected using a Bruker AXS Smart Breeze diffractometer (Bruker AXS Inc., Madison, WI, USA) equipped with a Photon II CCD area detector (Bruker, Billerica, MA, USA) and graphite-monochromatized MoKα radiation.The detector-to-crystal distance was 50 mm.Data were collected using ω and φ scan modes, in 0.5° slices, with an exposure time of 30 s per frame.The data were corrected for Lorentz and polarization factors and absorption using the software package Apex3 [22].
The crystal structure of magnanelliite was refined using Shelxl-2018 [23] starting from the atom coordinates of alcaparrosaite [16], with which magnanelliite is isostructural.The following neutral scattering curves, taken from the International Tables for Crystallography [24] were used: K at K(1)-K(3) sites, Fe versus Al at Fe site, S at S(1)-S(3) sites, and O at the O(1)-Ow sites.After several cycles of isotropic refinement, the R1 converged to 0.0839, indicating the correctness of the structural model.Refining anisotropic displacement parameters for all the atom positions, the R1 was lowered to 0.0335.The positions of the H atoms were found in the difference-Fourier map.The structural model finally converged to 0.0248 for 2411 reflections with Fo > 4σ(Fo) and 144 refined parameters.Details of data collection and refinement are given in Table 3. Atom coordinates and equivalent isotropic or isotropic displacement parameters are reported in Table 4.The crystallographic information file (CIF) of magnanelliite is available as Supplementary material linked to this article.

Crystal Structure Description
Seven independent cation and ten anion sites were located in the crystal structure of magnanelliite.Table 5 reports selected bond distances, and Table 6 gives the bond-valence calculations obtained using the parameters of Brese and O'Keeffe [25] and corrected for H-bonds using the relationship of Ferraris and Ivaldi [26].
Potassium is hosted at three independent sites, K(1), K(2), and K(3).K(1) and K(2) are ten-fold coordinated, whereas K(3) is nine-fold coordinated.However, if one takes into account two additional longer bonds, the K(3) site can be considered as eleven-fold coordinated, as in alcaparrosaite [16].Bond valence sums (BVS) range between 0.95 and 1.44 valence unit (v.u.).The over-bonding at the K(1) and K(2) site is similar to that observed in alcaparrosaite, where BVS values of 1.30 and 1.35 v.u.have been reported [16].
Iron is hosted at the octahedrally-coordinated Fe(1) site.Its average bond distance is 2.006 Å, with bond distances ranging between 1.920 and 2.053 Å.The difference between the longest and shortest Fe-O distances, hereafter Δd, is 0.133 Å, compared with 0.2818 Å observed in alcaparrosaite, where the occurrence of Ti 4+ increases the octahedral distortion, in agreement with the usual off-center displacement shown by this cation having a small radius and a high charge [27].The refined site scattering at Fe(1) is 25.0 electrons.Taking into account the chemical data, the site population (Fe0.972Al0.023Mg0.005)can be proposed, corresponding to 25.6 electrons.The BVS is 3.09 v.u., in agreement with the presence of Fe 3+ .In alcaparrosaite, where a mixed (Ti 4+ 0.5Fe 3+ 0.5) site occupancy occurs, the corresponding BVS is 3.542 v.u.[16].
Sulfur occurs at three independent S sites, showing S-O distances ranging between 1.442 and 1.506 Å, with <S-O> distances between 1.476 and 1.477 Å. BVS values range between 5.96 and 6.00 v.u.The examination of the BVS at the O sites reveals the presence of four under-bonded oxygen atoms hosted at O(1), O(2), OH (9), and Ow sites.Indeed, Ow is bonded to two H atoms, i.e., H (1) and H(2), and is an H2O group.It is donor in the H-bonds with O(5), through H(1), and to both O(1) and O(2), through H(2) (Table 5).In the isostructural mineral alcaparrosaite, Kampf et al. [16] 2) angle is close to the lower limit (~120° [28]), considered as indicative of bonding interactions, whereas the H(2)•••O(2) distance, i.e., 2.31(4) Å, is below the 2.5 Å limit, considered as the longest H•••A distance in H-bonds [28].The oxygen atom at the OH(9) site has a BVS of 1.37 v.u. and it is bonded to H(3), statistically occupying a position above and below the mirror plane.In this way, O(2) can also be an acceptor in the OH(9)-H(3)•••O(2) bonds.The occurrence of an (OH) − group at OH( 9) is an important difference with respect to alcaparrosaite, where O( 9) is occupied by an O 2− anion, in agreement with the BVS of 1.882 v.u.[16].In the crystal structure of magnanelliite, two symmetry-related Fe-centered octahedra are connected through corner-sharing [OH(9) is the shared site], forming a dimer.The opposite vertex is occupied by an H2O group (Ow site).The four remaining O sites are shared with (SO4) groups.Along c, dimers are connected through two S(1)-centered tetrahedra.S(3)-centered tetrahedra are bonded to only one dimer.Tetrahedral-octahedral chains running along c are connected along a through S(2)-centered tetrahedra, giving rise to undulating {010} layers.Bonding between {010} layers is achieved through three K atoms hosted within the interlayer (Figure 3).

Discussion
Magnanelliite is a new addition to the K2O-Fe2O3-SO3-H2O quaternary system, currently including the minerals giacovazzoite, goldichite, jarosite, krausite, scordariite, and yavapaiite.It is worth noting that all these species, with the exception of anhydrous yavapaiite, have been observed in the Monte Arsiccio sulfate assemblage, where they likely formed as the result of the interaction between acidic solutions, derived through pyrite oxidation, and K-bearing rocks.
Magnanelliite is related to alcaparrosaite [16] through the coupled heterovalent substitution Fe Ti 4+ + O(9) O 2− = Fe Fe 3+ + O(9) OH − .Following the definition of mineral groups given by Mills et al. [29], magnanelliite and alcaparrosaite belong to the same group, having essentially a similar structure and being formed by chemically similar elements.This group can be named the alcaparrosaite group.

Supplementary Materials:
The CIF of magnanelliite is available online at www.mdpi.com/xxx/s1.

Figure 1 .
Figure 1.Magnanelliite as (a) divergent intergrowths of steeply terminated prisms (field of view = 1.25 mm) or (b) as isolated individuals (field of view = 0.5 mm), in association with colorless alum-(K).In (c), SEM image showing a divergent intergrowth of magnanelliite.A crystal drawing (clinographic projection in non-standard orientation with a vertical) of a crystal is shown in (d).

Figure 2 .
Figure 2. Micro-Raman spectrum of magnanelliite.The position of the main bands is indicated.

Author
Contributions: C.B. conceived and designed the experiments; C.B. collected the single-crystal X-ray diffraction and micro-Raman data; A.R.K. collected physical data (density, optical properties); L.B. collected electron-microprobe data; C.B., L.B., and A.R.K. analyzed the data; C.B. wrote the manuscript, with inputs from L.B. and A.R.K. Funding: This research received support by the University of Pisa through the project P.R.A. 2018-2019 "Georisorse e Ambiente" (Grant No. PRA_2018_41).
[21]: intensity and dhkl were calculated using the software PowderCell 2.3[21]on the basis of the structural model reported in Table4.Only calculated reflections with Icalc > 5 are listed, if not observed.Observed intensities were visually estimated: s = strong; m = medium; mw = medium-weak; w = weak; vw = very weak.The eight strongest reflections are shown in bold.

Table 3 .
Crystal and experimental details for magnanelliite.