Vesuvianite from the Somma-Vesuvius Complex : New Data and Revised Formula

At present, the vesuvianite group of minerals consists of eight members, six of which are distinguished by the dominant cation in the Y1(A,B) five-coordinated site. We investigated two vesuvianite samples from the type locality by electron microprobe analysis, Mössbauer and infrared spectroscopy, TGA/DSC, MAS NMR, single-crystal and powder X-ray diffraction. The crystal structures of these samples (# 27844 and 51062 from the Vesuvius collection, Fersman Mineralogical Museum, Moscow) have been refined to R1 = 0.027 and R1 = 0.035, respectively. Both samples have the space group P4/nnc; a = 15.5720(3) and 15.5459(3), c = 11.8158(5) and 11.7988(4), respectively. In both samples low-occupied T1 and T2 sites are populated by minor B and Al, which agrees with their high-temperature origin. According to our experimental results, the general revised crystal-chemical formula of vesuvianite can be written as X19Y1Y12(Z2O7)4(ZO4)10(W)10, where X are sevento nine-coordinated sites of Ca with minor Na, K, Fe2+ and REE impurities; VY has a square pyramidal coordination and is occupied predominantly by Fe3+ with subordinate Mg, Al, Fe2+ and Cu2+; VIY has octahedral coordination and is predominantly occupied by Al with subordinate Mg, Fe2+, Fe3+, Mn2+, Mn3+, Ti, Cr and Zn; ZO4 = SiO4, sometimes with subordinate AlO4 and/or (OH)4, and W = OH, F, with minor O and Cl. The idealized charge-balanced formula of the vesuvianite end-member without subordinate cations is Ca19Fe(Al10Me2)(Si2O7)4(SiO4)10O(OH)9, where Me = Fe2+, Mg2+, Mn2+.


Introduction
The vesuvianite-group minerals are widespread in different contact rocks (including skarns formed during contact or regional metamorphism of limestones; in garnetized gabbros, mafic and ultramafic rocks, and serpentinites) of metamorphic, volcanic and hydrothermal origin [1].They crystallize in a wide range of PT conditions (0-8 kbar and 200-800 • C) at the greenschist up to granulite facies of metamorphism and can be considered as a geothermometer [2].Because of their crystal structure flexibility, vesuvianite-group minerals can contain variable amounts of di-and trivalent cations, and they are stable under reducing and oxidizing conditions [3].
However, the general formula of vesuvianite given in the International Mineralogical Association (IMA) List of Minerals (http://ima-cnmnc.nrm.se/imalist.htm),(Ca,Na) 19 (Al,Mg,Fe) 13 (SiO 4 ) 10 (Si 2 O 7 ) 4 (OH,F,O) 10 [36], does not explicitly show the occupancies of the Y1 site.This simplified formula is now out of date since it refers to five vesuvianite-group minerals: vesuvianite sensu stricto, manganvesuvianite, cyprine, magnesiovesuvianite and alumovesuvianite.For this reason, we report the results of a multimethodological study of two vesuvianite samples from the type locality of this mineral, i.e., the Somma-Vesuvius complex, Campania, Italy.The studied samples nos.27844 and 51062 originate from the Vesuvius collection of the Fersman Mineralogical Museum, Moscow.The aim of this work is to establish a correct formula of vesuvianite, which does not contradict the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) guidelines [37].
In samples 51062 and 27844 (Figure 1), euhedral vesuvianite occurs in association with grossular and clinozoisite or as a monomineralic aggregate, respectively; the large sizes of the crystals indicate their skarn provenance.In particular, the association of vesuvianite with garnet, wollastonite, gehlenite or anorthite in the first sample is typical for zoned skarns of the Somma-Vesuvius complex [42].It is worth nothing that the occurrence of clinozoisite in the sample 51062 represents, to the authors' knowledge, the first record of this mineral at Somma-Vesuvius.carbonate rocks to form skarns.According to fluid inclusion studies [42], the estimated temperature of the formation skarn minerals at Somma-Vesuvius complex is in the range 800-900 °C.
In samples 51062 and 27844 (Figure 1), euhedral vesuvianite occurs in association with grossular and clinozoisite or as a monomineralic aggregate, respectively; the large sizes of the crystals indicate their skarn provenance.In particular, the association of vesuvianite with garnet, wollastonite, gehlenite or anorthite in the first sample is typical for zoned skarns of the Somma-Vesuvius complex [42].It is worth nothing that the occurrence of clinozoisite in the sample 51062 represents, to the authors' knowledge, the first record of this mineral at Somma-Vesuvius.

Materials and Methods
Both vesuvianite samples studied in this work (27844 and 51062) form well-shaped shortprismatic crystals up to 6 mm long (Figure 1) with the main prismatic forms {100} and {110} terminated by the pyramidal {101} and pinacoidal {001} faces.The crystals are lustrous and semitransparent.Their color varies from greenish brown (27844) to brownish green and orange-brown in the marginal zone (51062).
Chemical analyses of both samples (Table 2, each analysis is the mean from 4 to 7 points) were obtained using a HITACHI S-3400N scanning electron microscope (Tokyo, Japan) equipped with INCA Wave 500 WDS spectrometer.The system was operated at 20 kV and 10 nA, and the electron beam was focused to a 5 µm spot.The following standards were used: rutile (Ti), wollastonite (Si, Ca), forsterite (Mg), fluorite (F), cryolite (Na), halite (Cl), spessartine (Mn, Al), hematite (Fe), danburite (B), synthetic Ce-glass (Ce), synthetic Nd-glass (Nd).The content of boron was measured at the Faculty of Geology, University of Warsaw, by a CAMECA SX100 instrument operated at 6 kV and 80 nA for 50-100 s counting at each point (Kα line, PC2 crystal (Ni/C-LSM), standard deviation 0.25 wt % B2O3).No other elements having atomic numbers above eight were found above their detection

Materials and Methods
Both vesuvianite samples studied in this work (27844 and 51062) form well-shaped short-prismatic crystals up to 6 mm long (Figure 1) with the main prismatic forms {100} and {110} terminated by the pyramidal {101} and pinacoidal {001} faces.The crystals are lustrous and semi-transparent.Their color varies from greenish brown (27844) to brownish green and orange-brown in the marginal zone (51062).
Chemical analyses of both samples (Table 2, each analysis is the mean from 4 to 7 points) were obtained using a HITACHI S-3400N scanning electron microscope (Tokyo, Japan) equipped with INCA Wave 500 WDS spectrometer.The system was operated at 20 kV and 10 nA, and the electron beam was focused to a 5 µm spot.The following standards were used: rutile (Ti), wollastonite (Si, Ca), forsterite (Mg), fluorite (F), cryolite (Na), halite (Cl), spessartine (Mn, Al), hematite (Fe), danburite (B), synthetic Ce-glass (Ce), synthetic Nd-glass (Nd).The content of boron was measured at the Faculty of Geology, University of Warsaw, by a CAMECA SX100 instrument operated at 6 kV and 80 nA for 50-100 s counting at each point (Kα line, PC2 crystal (Ni/C-LSM), standard deviation 0.25 wt % B 2 O 3 ).No other elements having atomic numbers above eight were found above their detection limits.The content of H 2 O in 27844 was determined by means of thermogravimetric analysis using NETZSCH STA 449 F3 Jupiter thermoanalyzer (Exton, PA, USA) during 10 • C/min heating of a 68.4 mg sample from room temperature to 1100 • C in a dynamic argon atmosphere, with aluminum oxide standard.
Reflections collected 14,428 5687 The 27 Al nuclear magnetic resonance (NMR) spectrum was obtained at room temperature by means of a Bruker Avance III 400 WB spectrometer (Billerica, MA, USA) at 104.24 MHz.The magicangle spinning (MAS) spin rate of the rotor was 20 kHz.For these investigations, a single-pulse sequence with a pulse length of 86 kHz was used, with the recycle delay of 1 s and 4096 scans.The 1H NMR spectrum was obtained at room temperature using a Bruker Advance III 400 WB spectrometer operating at 400.23 MHz.The MAS spin rate of rotor was 20 kHz.A single-pulse sequence was used with a pulse length of 100 kHz, recycle delay of 20 s and 32 scans.
The Fe 2+ /Fe 3+ ratio is given in accordance with the Mössbauer spectrometry data.Gamma-resonance studies of 27844 and 51062 were carried out using a commercial Mössbauer spectrometer (WissEl Wissenschaftliche Elektronik GmbH, Starnberg, Germany) in order to clarify the valence state of iron and the coordination environment(s) of the Fe.The spectrometer was equipped with a standard radioactive source, 57 Co, in a rhodium matrix (Ritverc).The absorption-mode measurements have been performed at room temperature.The samples were crushed in an agate mortar, distributed homogeneously within the punch-holder and packed in the form of a tablet with a density of 112 mg/cm 2 in order to optimize the conditions for measurement.The spectral fitting of the experimental Mössbauer spectrum was carried out by the method of least squares under the assumption of Lorentz-shaped spectral lines.Isomer shifts were measured relative to metallic iron.The mathematical analysis was performed with the commercial Mössbauer software SpectRelax Version 2.1 (Moscow State University, Moscow, Russia).
A FTIR spectrum of 27844 was collected using a Bruker ALPHA spectrometer at room temperature in the range 360-3800 cm −1 (KBr pellet, resolution 4 cm −1 , 16 scans).A pure KBr pellet was used as a reference.
Single-crystal X-ray diffraction experiments were carried out using an Agilent Technologies Xcalibur Eos diffractometer (Agilent Technologies, Santa Clara, CA, USA) operated at 50 kV and 40 mA.A hemisphere of three-dimensional data was collected at 293 K for each sample using monochromatic Mo Kα radiation, with frame widths of 1 • and 20 s count for each frame.The crystal-to-detector distance was 45 mm.For the investigated samples, P4/nnc was chosen as the most probable space group.Only 79 reflections violating the absence conditions in the range 2σ(I) < I < 188σ  [46].Empirical absorption corrections were applied in the CrysAlisPro program complex [47] using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.The final difference Fourier map showed no features higher than 0.53/0.56e − for 27844 and 51062, respectively.The experimental details and crystallographic parameters are given in Table 2. Crystallographic information files (CIFs) are deposited in the electronic Supplementary Materials.

Chemical Composition
The crystal-chemical formulae of the investigated samples (Table 3) were calculated on the basis of 19 X-cations (Ca + Na + REE).Both compositions are in good agreement with the previous data for vesuvianite from the Monte-Somma-Vesuvius complex [38,48].The TGA curve (Figure 2) of 27844 contains one major step of weight loss, accompanied by two endothermic effects at 980 • C and 1030 • C in the DSC-curve, which can be assigned to separated dehydroxylation processes for the O(11)H and O(10)H groups [23,49].According to [49,50] one-step weight loss in the temperature range of 820-1090 • C is characteristic for low-temperature vesuvianite.Weight loss in the range of 990-1150 • C responds to the high-symmetry vesuvianite.The total weight loss for 27844 is 1.60 wt %, which corresponds to 5.22 OH-groups per formula unit.As a result of dehydroxylation, vesuvianite transforms into the mixture of wollastonite, grossular, quartz and an amorphous glass-like substance.

Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC)
The TGA curve (Figure 2) of 27844 contains one major step of weight loss, accompanied by two endothermic effects at 980 °C and 1030 °C in the DSC-curve, which can be assigned to separated dehydroxylation processes for the O(11)H and O(10)H groups [23,49].According to [49,50] one-step weight loss in the temperature range of 820-1090 °C is characteristic for low-temperature vesuvianite.Weight loss in the range of 990-1150 °C responds to the high-symmetry vesuvianite.The total weight loss for 27844 is 1.60 wt %, which corresponds to 5.22 OH-groups per formula unit.As a result of dehydroxylation, vesuvianite transforms into the mixture of wollastonite, grossular, quartz and an amorphous glass-like substance.
The 1 H MAS NMR spectrum (Figure 3) of 27844 contains two peaks centered at 6.9 and 1.

Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC)
The TGA curve (Figure 2) of 27844 contains one major step of weight loss, accompanied by two endothermic effects at 980 °C and 1030 °C in the DSC-curve, which can be assigned to separated dehydroxylation processes for the O(11)H and O(10)H groups [23,49].According to [49,50] one-step weight loss in the temperature range of 820-1090 °C is characteristic for low-temperature vesuvianite.Weight loss in the range of 990-1150 °C responds to the high-symmetry vesuvianite.The total weight loss for 27844 is 1.60 wt %, which corresponds to 5.22 OH-groups per formula unit.As a result of dehydroxylation, vesuvianite transforms into the mixture of wollastonite, grossular, quartz and an amorphous glass-like substance.
The 1 H MAS NMR spectrum (Figure 3) of 27844 contains two peaks centered at 6.9 and 1.

X-ray Crystallography
The unit-cell dimensions of 27844 were determined from the X-ray powder-diffraction pattern (Figure 5) by Rietveld refinement using the program Topas 4.2 [64] and are in a good agreement with the single-crystal XRD data: a = Å, c = 11.8156(6)Å, V = 2867.3(4)Å 3 .A1).
Structural models of 27844 and 51062 are close to that reported in [48], with site designations according to [15].We pay particular attention to the occupancy of the Y1 site that plays a key role in the diversity of the vesuvianite-group minerals.
Structural models of 27844 and 51062 are close to that reported in [48], with site designations according to [15].We pay particular attention to the occupancy of the Y1 site that plays a key role in the diversity of the vesuvianite-group minerals.
In the crystal structure of vesuvianite, the five-coordinated Y1 site (Table 1 and Figure 6) can be occupied by Fe 3+ , Fe 2+ , Mn 3+ , Mn 2+ , Cu, Mg and Al [16,23,30,51,52,55,65].In most cases, including previously studied vesuvianite from the Somma-Vesuvius volcanic complex, Fe 3+ dominates in the Y1 site [48].In accordance with the 27 Al MAS NMR data, five-coordinated Al 3+ in 27844 is absent, and the final occupancy of the Y1 site is determined by Fe and Mg.In the structures of 27844 and 51062, iron is predominant at the Y1 site: (Fe 0.72 Mg 0.28 ) 1.00 and (Fe 0.73 Mg 0.27 ), respectively, or, taking into account Mössbauer data, Fe 3+ 0.50 Mg 0.28 Fe 2+ 0.22 and Fe 3+ 0.47 Mg 0.27 Fe 2+ 0.26 , respectively.Scattering of the Y2 site in both samples is close to 13 ē, and the average <Y2-O> bond lengths are equal to 1.896 and 1.895 Å for 27844 and 51062, respectively, which agrees well with the site occupancy by Al atoms only.The octahedral Y3 site has minor admixtures of iron (mainly Fe 3+ ), which is confirmed by a slightly higher mean length of the <Y3-O> bonds: 1.966 and 1.961 Å, respectively.The total refined Y3 site occupancies are (Al 0.90 Fe 0.10 ) 1.00 and (Al 0.91 Fe 0.09 ), respectively.Both the contents and oxidation states of iron have been confirmed by the Mössbauer data.
Structural models of 27844 and 51062 are close to that reported in [48], with site designations according to [15].We pay particular attention to the occupancy of the Y1 site that plays a key role in the diversity of the vesuvianite-group minerals.
In the crystal structure of vesuvianite, the five-coordinated Y1 site (Table 1 and Figure 6) can be occupied by Fe 3+ , Fe 2+ , Mn 3+ , Mn 2+ , Cu, Mg and Al [16,23,30,51,52,55,65].In most cases, including previously studied vesuvianite from the Somma-Vesuvius volcanic complex, Fe 3+ dominates in the Y1 site [48].In accordance with the 27 Al MAS NMR data, five-coordinated Al 3+ in 27844 is absent, and the final occupancy of the Y1 site is determined by Fe and Mg.In the structures of 27844 and 51062, iron is predominant at the Y1 site: (Fe0.72Mg0.28)1.00 and (Fe0.73Mg0.27),respectively, or, taking into account Mössbauer data, Fe 3+ 0.50Mg0.28Fe2+ 0.22 and Fe 3+ 0.47Mg0.27Fe2+ 0.26, respectively.Scattering of the Y2 site in both samples is close to 13 ē, and the average <Y2 − O> bond lengths are equal to 1.896 and 1.895 Å for 27844 and 51062, respectively, which agrees well with the site occupancy by Al atoms only.The octahedral Y3 site has minor admixtures of iron (mainly Fe 3+ ), which is confirmed by a slightly higher mean length of the <Y3 − O> bonds: 1.966 and 1.961 Å, respectively.The total refined Y3 site occupancies are (Al0.90Fe0.10)1.00 and (Al0.91Fe0.09),respectively.Both the contents and oxidation states of iron have been confirmed by the Mössbauer data.The mean <Z − O> bond lengths and scattering factors of the tetrahedral Z1, Z2 and Z3 sites are in agreement with the full occupancies of these sites by Si atoms only.
In the refinement model, the 7-to 9-coordinated X1, X2, X3 and X4 sites are completely populated by Ca.The X sites have full occupancies except for the eightfold-coordinated X4 position that is half-populated and is situated in the structure channels.According to [66], REEs incorporate into the X3 site, but their low content (1.5% of the X3 site occupancy) cannot be precisely determined by the structural analysis.Similar reasoning applies also to the admixture of Na in the X4 site [22].
Incorporation of small amounts of boron into the vesuvianite structure does not lead to the splitting of the O7 site and appearance of the O12 site as it does in wiluite [67].The low-occupied T2 site (Figure 7) is populated by about 25% boron, which prevented us from the determination of its coordination.However, the presence of 3-coordinated boron is confirmed by FTIR.The observed scattering factors of the T1 site of 1.4 and 1.0 e − correspond to the refined occupancies Al 0.106 and Al 0.072 , respectively.Due to the low occupancies of the T1 site, the mean <T1-O> bond lengths, 1.827 and 1.830 Å, are significantly longer than the theoretical one for tetrahedrally coordinated Al with full occupancy.At the same time, the presence of small amounts (~0.45 apfu) of tetrahedrally coordinated boron at the T1 site cannot be excluded.
All oxygen sites have been refined as fully occupied by oxygen atoms, excluding the O7 site that possesses anomalously high displacement parameters.In the final model, the O7 site was refined as (O 0.75 F 0.25 ) 1.00 with plausible displacement parameters.Small amounts of Cl determined by the electron microprobe (0.05 and 0.04 apfu for and 51062, respectively) are probably situated in the O10 site [19].
Taking into account all of the data given above, the final crystal-chemical formulae of the 27844 and 51062 samples can be written, as follows: X1 (Ca) 2.00 X2 (Ca) 8.00 X3 (Ca) 8.00 X4 (Ca 0.97 Na 0.03 ) The Fe content in each Y site was assigned according to the crystal-structure data, whereas the Fe 2+ /Fe 3+ ratio is given in accordance with the Mössbauer spectroscopy data.
Minerals 2017, 7, 248 10 of 16 In the refinement model, the 7-to 9-coordinated X1, X2, X3 and X4 sites are completely populated by Ca.The X sites have full occupancies except for the eightfold-coordinated X4 position that is half-populated and is situated in the structure channels.According to [66], REEs incorporate into the X3 site, but their low content (1.5% of the X3 site occupancy) cannot be precisely determined by the structural analysis.Similar reasoning applies also to the admixture of Na in the X4 site [22].
Incorporation of small amounts of boron into the vesuvianite structure does not lead to the splitting of the O7 site and the appearance of the O12 site as it does in wiluite [67].The low-occupied T2 site (Figure 7) is populated by about 25% boron, which prevented us from the determination of its coordination.However, the presence of 3-coordinated boron is confirmed by FTIR.The observed scattering factors of the T1 site of 1.4 and 1.0 e -correspond to the refined occupancies Al0.106 and Al0.072, respectively.Due to the low occupancies of the T1 site, the mean <T1 − O> bond lengths, 1.827 and 1.830 Å, are significantly longer than the theoretical one for tetrahedrally coordinated Al with full occupancy.At the same time, the presence of small amounts (~0.45 apfu) of tetrahedrally coordinated boron at the T1 site cannot be excluded.
All oxygen sites have been refined as fully occupied by oxygen atoms, excluding the O7 site that possesses anomalously high displacement parameters.In the final model, the O7 site was refined as (O0.75F0.25)1.00 with plausible displacement parameters.Small amounts of Cl determined by the electron microprobe (0.05 and 0.04 apfu for 27844 and 51062, respectively) are probably situated in the O10 site [19].

Infrared Spectroscopy
The IR spectrum of 27844 (Figure 8) is typical for high-symmetry vesuvianite-group minerals.It contains strong bands of Si-O stretching in the Si2O7 (at 1018 and 976 cm −1 ) and SiO4 groups (at

Infrared Spectroscopy
The IR spectrum of 27844 (Figure 8) is typical for high-symmetry vesuvianite-group minerals.It contains strong bands of Si-O stretching in the Si 2 O 7 (at 1018 and 976 cm −1 ) and SiO 4 groups (at 914 cm −1 with shoulders at 900 cm −1 and 870 cm −1 ), as well as Si-O-Si and O-Si-O bending vibrations, partly mixed with the M-O stretching vibrations, where M = Al, Mg, Fe, Ti, Mn (below 700 cm −1 ).The band at 799 cm −1 is assigned to bending vibrations of the M• • O-H groups.The region of O-H stretching vibrations is dominated by the peak at 3565 cm −1 (D band according to [68], which corresponds to the vibrations of the O11-H1 bond in the presence of F in the neighboring O11 site [69].The weak bands at 3450 cm −1 and 3629 cm −1 are related to the OH groups coordinated by Ti and to the O11-H1•••O7 fragment with the angle <120°, respectively [69].The weak band at 3210 cm −1 (J band) corresponds to the OH groups oriented along the c-axis [68] forming strong hydrogen bonds.
Two weak bands at 1568 and 1478 cm −1 are assigned to the BO3 3− anions (the T2-centered triangle) with shortened (as compared to wiluite) B-O bonds.The shoulders at 1170 cm −1 and 1075 cm −1 are tentatively assigned to boron atoms having 4-fold coordination.

Discussion
Generalization of the vesuvianite formula is an important step for the development of the nomenclature for this group of minerals.The tentative vesuvianite formula, (Ca,Na)19(Al,Mg,Fe)13(SiO4)10(Si2O7)4(OH,F,O)10, does not specify the occupancies of the Y1(A,B) site [36], which are responsible for the observed species diversity of the vesuvianite-group minerals [16,21,29].According to the IMA CNMNC guidelines for compositional criteria [37], 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 [16].In this context, recently approved members of the vesuvianite group with Mn 3+ , Al 3+ , Mg 2+ , Cu 2+ as predominant cations at the Y1 site can be confused with vesuvianite itself, according to the present formula of this mineral.Therefore, it is crucially important to determine a new vesuvianite formula, which would be in accordance with the above-mentioned IMA rule.
In most cases, vesuvianite with the P4/nnc symmetry has only Fe and Mg at the Y1 site [55].There are only three publications available that report vesuvianite with the Fe 2+ /Fe 3+ ratio determined by direct chemical or spectroscopic methods (Figure 9) [21,29,63].Our data demonstrate the predominance of Fe 3+ at the Y1 site for both vesuvianite samples from the type locality, which is consistent with previous conclusions [38,48].According to our experimental results, the new formula of vesuvianite can be written as VII−IX X19 V Y1 VI Y12(ZO7)4(ZO4)10(W)10, where X are seven-to ninecoordinated sites of Ca with minor Na, K, Fe 2+ and REE impurities; V Y has square pyramidal coordination of dominant Fe 3+ and subordinate Mg, Al, Fe 2+ and Cu 2+ ; VI Y has octahedral coordination The region of O-H stretching vibrations is dominated by the peak at 3565 cm −1 (D band according to [68], which corresponds to the vibrations of the O11-H1 bond in the presence of F in the neighboring O11 site [69].The weak bands at 3450 cm −1 and 3629 cm −1 are related to the OH groups coordinated by Ti and to the O11-H1• • • O7 fragment with the angle <120 • , respectively [69].The weak band at 3210 cm −1 (J band) corresponds to the OH groups oriented along the c-axis [68] forming strong hydrogen bonds.
Two weak bands at 1568 and 1478 cm −1 are assigned to the BO 3 3− anions (the T2-centered triangle) with shortened (as compared to wiluite) B-O bonds.The shoulders at 1170 cm −1 and 1075 cm −1 are tentatively assigned to boron atoms having 4-fold coordination.

Discussion
Generalization of the vesuvianite formula is an important step for the development of the nomenclature for this group of minerals.The tentative vesuvianite formula, (Ca,Na) 19 (Al,Mg,Fe) 13 (SiO 4 ) 10 (Si 2 O 7 ) 4 (OH,F,O) 10 , does not specify the occupancies of the Y1(A,B) site [36], which are responsible for the observed species diversity of the vesuvianite-group minerals [16,21,29].According to the IMA CNMNC guidelines for compositional criteria [37], 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 [16].In this context, recently approved members of the vesuvianite group with Mn 3+ , Al 3+ , Mg 2+ , Cu 2+ as predominant cations at the Y1 site can be confused with vesuvianite itself, according to the present formula of this mineral.Therefore, it is crucially important to determine a new vesuvianite formula, which would be in accordance with the above-mentioned IMA rule.

Conclusions
By revising vesuvianite-group of minerals [16,21,29] two vesuvianite samples 27844 and 51062 from type locality were investigated by electron microprobe analysis, Mössbauer and infrared spectroscopy, TGA/DSC, MAS NMR, single-crystal and powder X-ray diffraction.
Present studies reveal square-pyramidal Y1 position as a key site in the diversity of vesuvianitegroup minerals.On the basis of this approach we determine vesuvianite formula as Ca19Fe 3+ (Al10Me 2+ 2)(Si2O7)4(SiO4)10O(OH)9, which would not contradict present CNMNC guidelines for compositional criteria.
The vesuvianite from the Somma-Vesuvius complex demonstrate disorder inside the structural channels and overall P4/nnc symmetry, which partially connected with the incorporation of boron at additional T1, T2 sites and agrees with the estimated temperature of crystallization at 800-900 °C.

1
The symbols and the assignment of the quadrupole doublets are given in accordance with the data from[21,29,61] based on the analysis of Mössbauer spectra of 17 vesuvianite samples from different localities.Taking into account Fe-O distances and site multiplicities, quadrupole doublet d with anomalously low quadrupole splitting could not be assigned to Fe at distorted Y1 sites.

Figure 6 .
Figure 6.The crystal structure of 27844 projected along (A) c axis and (B) b axis.T-sites have low occupation: T1 ≈ 10% and T2 ≈ 25%.The O11 site with mixed (O0.75F0.25)occupancy represent by blue circles, oxygen atoms are red circles.The mean <Z − O> bond lengths and scattering factors of the tetrahedral Z1, Z2 and Z3 sites are in agreement with the full occupancies of these sites by Si atoms only.

Figure 6 .
Figure 6.The crystal structure of 27844 projected along (A) c axis and (B) b axis.T-sites have low occupation: T1 ≈ 10% and T2 ≈ 25%.The O11 site with mixed (O0.75F0.25)occupancy represent by blue circles, oxygen atoms are red circles.The mean <Z − O> bond lengths and scattering factors of the tetrahedral Z1, Z2 and Z3 sites are in agreement with the full occupancies of these sites by Si atoms only.

Figure 6 .
Figure 6.The crystal structure of 27844 projected along (A) c axis and (B) b axis.T-sites have low occupation: T1 ≈ 10% and T2 ≈ 25%.The O11 site with mixed (O 0.75 F 0.25 ) occupancy represent by blue circles, oxygen atoms are red circles.

Figure 7 .
Figure 7. Graphs of observed electron density along the cation rods (fourfold axis is centered at 0.75; 0.75; z) in the crystal structures of: (A) low-symmetry P4/n vesuvianite from Akhmatovskaya Pit and (B) 51062.Projections are onto the (010) plane, contour intervals are 0.1 eÅ −3 .Displacement ellipsoids are drawn at the 50% probability level.

Figure 7 .
Figure 7. Graphs of observed electron density along the cation rods (fourfold axis is centered at 0.75; 0.75; z) in the crystal structures of: (A) low-symmetry P4/n vesuvianite from Akhmatovskaya Pit and (B) 51062.Projections are onto the (010) plane, contour intervals are 0.1 eÅ −3 .Displacement ellipsoids are drawn at the 50% probability level.

− 1
with shoulders at 900 cm −1 and 870 cm −1 ), as well as Si-O-Si and O-Si-O bending vibrations, partly mixed with the M-O stretching vibrations, where M = Al, Mg, Fe, Ti, Mn (below 700 cm −1 ).The band at 799 cm −1 is assigned to bending vibrations of the M•••O-H groups.

Table 1 .
Dominant components in crystallographic sites of vesuvianite-group minerals 1 .

Table 2 .
Single-crystal data, data collection and structure refinement parameters.
Refinements of both structures in space groupsP4/n and P4nc did not indicate any reasons for the symmetry lowering from the P4/nnc space group.The crystal structures of 27844 and 51062 were refined to R 1 = 0.027 and R 1 = 0.035 for 1653 and 1622 unique observed reflections with |F o | ≥ 4σF, respectively, using the SHELX program

Table 4 .
Parameters of the Mössbauer spectra of 27844 and 51062 at 293 K.

Table 4 .
Parameters of the Mössbauer spectra of 27844 and 51062 at 293 K.

Table A1 .
Powder X-ray diffraction data for 278441.