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Fluorine-rich metamorphosed xenoliths associated to the distal Campanian Ignimbrite (CI) tephra (Campania region, southern Italy) have long attracted the interest of the mineralogical community (i.e. Scacchi, 1890; Zambonini, 1919; Masi & Turi, 1972; Balassone et al., 2002). These rocks derive from the Mesozoic carbonate lithotypes of the Campanian Apennine, embedded in the pyroclastic flow and presently resting at the bottom of the tuff formations. The sedimentary protoliths suffered from the action of hot, volatile-rich pyroclastic flow, and consequently were affected by thermal metamorphism to various degrees. Their peculiarity is the occurrence of F- and Mg-bearing phases, with an ubiquitous presence of neoformed fluorite. The occurrence at Fiano quarries (the so-called “Tufare”) is the classical locality for these rock xenoliths. Similar rocks were also found in the CI of the Caserta area, and in the Latium region (Colli Albani). This research aims both at a crystal chemical study of fluorophlogopite occurring in the Fiano xenoliths, and at a geochemical and volcanological survey, to constrain the petrogenesis processes related to this rare F-, Mg-rich assemblage and to CI. The analysed samples belong to two lithotypes: mica-bearing clasts and variably metamorphosed carbonate blocks. The former lithotype is represented by abundant fluorite, followed by fluorophogopite, F-rich chondrodite, fluoborite, diopside and (Fe,Mg)-oxides. Minor to trace contents of calcite, humite, tremolite, and grossular also occur. Carbonate rocks can show calcite only, or also trace amounts of fluorite. The Fiano micas composition approaches that of the Vesuvius micas from the 1872 eruption (Balassone et al., 2013). The micas belong to the 1M polytype and have crystal chemical features typical of fluorophlogopites i.e., low c lattice parameter (~ 10.13 Å), (~ 2.060 Å) and (~ 3.135 Å) distances. New data on minor to ultratrace elements amounts found in the studied clasts, together with Carbon and Oxygen isotope data will be reported and interpreted in a petrogenetic and volcanological frame. Balassone G., Franco E., Mattia C.A., Petti C. & Puliti R. 2002. Re-examination of fluosiderite, an unknown mineral from southern Italy: equal to fluorine-rich chondrodite. Eur. J. Min., 14, 151-155. Balassone G., Scordari F., Lacalamita M., Schingaro E., Mormone A., Piochi M., Petti C. & Mondillo N. 2013. Trioctahedral micas in xenolithic ejecta from recent volcanism of the Somma-Vesuvius (Italy): crystal chemistry and genetic inferences. Lithos, 160-161, 84-97. Masi U. & Turi B. 1972. Frazionamento isotopico del carbonio e dell'ossigeno negli inclusi calcarei metamorfosati del "Tufo grigio campano" Auct. di Fiano (Salerno). Per. Min., 41, 291-310. Scacchi A. 1890. La regione vulcanica fluorifera della Campania. Mem. R. Com. Geol. It., I, 1-48. Zambonini F. 1919. Il tufo pipernoide della Campania e i suoi minerali. Mem. Descr. Carta Geol. It., 7, 130 pp.

The hydrated copper-aluminium sulphate cyanotrichite, ideally Cu4Al2(SO4)(OH)122H2O, often occurs in sky blue clumps or aggregates of sub-millimeter sized fibrous crystals. The problem of indistinguishable admixing of variable amounts of carbonate-cyanotrichite with cyanotrichite, the close association with other copper sulphates (chalcoalumite, brochantite) and the very small size of the acicular crystals hampered to date an ab initio structure determination from conventional X-ray diffraction. In light of these difficulties, we have taken advantage of the recent development of precessed automated electron diffraction tomography (ADT) combined with synchrotron powder X-ray diffraction to investigate the crystal structure of cyanotrichite. Through ADT investigation, two similar monoclinic cell were determined, corresponding to cyanotrichite (a = 10.16, b = 2.90, c = 12.64 Å and β = 92.4°) and carbonatecyanotrichite (a = 10.16, b = 2.91, c = 12.42Å and β = 98.4°). A structure model was obtained ab initio by direct methods in space group C2 from electron diffraction data and tested with the Rietveld method against X-ray powder diffraction profiles. All reflections in the powder pattern were indexed with the two cyanotrichite-like phases, according to electron diffraction data. The Rietveld analysis, consistently with electron diffraction investigations, indicates that the refined structural model has based on Al(OH)6 octahedra interconnected through common edges to build infinite columns running along b. Each Al-columns is coupled by sharing the remaining edges to two Cu-columns based on Cu distorted octahedra giving rise to ribbons along b. These ribbons are linked by SO4 tetrahedra to form corrugated layers.

Sideronatrite - Na2Fe3+(SO4)2(OH)•3(H2O) - is a secondary iron-bearing, hydrous sulfate formed during oxidation of pre-existing sulfides. It is a typical sedimentary product in some soils and evaporatic deposits. Sideronatrite is formed in acid, sulfate-rich environments, and plays an important role in the acid drainage mobilization of metals and in the monitoring of water quality. The dehydration behavior of sideronatrite from Sierra Gorda (Chile) was investigated by in situ high-T FTIR spectroscopy. The structural variations induced by thermal treatment were monitored by FTIR using the bending (2) mode of the water molecule around 1650 cm-1, the stretching (3 and 1) modes around 3580 and 3420 cm-1, and the corresponding combinations modes in the wavenumber region 4000-6000 cm-1. The reaction sequence that describes the thermal decomposition of sideronatrite in the range RT-400°C , is the following: sideronatrite → metasideronatrite I → metasideronatrite II → Na3Fe(SO4)3 Na2Fe(SO4)2(OH)∙3H2O Na2Fe(SO4)2(OH)∙H2O Na2Fe(SO4)2(OH) Fe - Na - SO4 During the transformation sideronatrite → metasideronatrite I (Ventruti et al. 2010), only two interstitial water molecules, coordinated by the Na atoms, are lost, while the characteristic framework of the structure of sideronatrite, i.e. the [Fe3+(SO4)2(OH)]∞ chain, remains unchanged both in metasideronatrite I and in metasideronatrite II structures. The loss of OH- group corner-sharing between two adjacent Fe3+-octahedra in sideronatrite chain, involves the break down of the [Fe3+(SO4)2(OH)]∞ chains and the subsequent formation of an intermediate amorphous transitional phase from which the Na3Fe(SO4)3 compound will form later. The first and second reaction (i.e. sideronatrite to metasideronatrite I and metasideronatrite I to metasideronatrite II) are certainly topotactic, whereas the formation of Na3Fe(SO4)3 is a typical nucleation and growth reaction process which originates from metasideronatrite II decomposition through the following reaction: 6Na2Fe(SO4)2OH →4 Na3Fe(SO4)3 + Fe2O3+ 3H2O. The presence of Fe2O3 in the observed diffraction pattern is thus justified as a co-product of the decomposition process. In the recrystallization of Na3Fe(SO4)3 from the amorphous phase, the OH– group is replaced by a (SO4)2– group according to: OH– → (SO4)2– + Na+ to preserve the electro-neutrality of the structure. Such (SO4)2– group, together with two bridging (SO4)2– tetrahedral, already present in sideronatrite chain, provides a further linkage between two consecutive Fe3+-octahedra in Na3Fe(SO4)3 (Fig. 1), to build up infinite [Fe3+(SO4)3] chains, which extend along the c axis.

The crystal structure of tobelite 2M2, a dioctahedral mica with ideal composition (NH4,K)Al2(Si3Al)O10(OH)2, has been refined for the first time via single crystal XRD by Mesto et al. (2006). Those authors refined two single crystals of tobelite in the C2/c S.G., to R-values 6.0% and 7.2%. The average cell parameters were: a = 9.026, b = 5.203, c = 20.721, beta = 100.05°. However, residues in the Fourier density maps pointed to some kind of structure disorder. In the present work a HRTEM investigation was undertaken in order to elucidate the nature of the disorder. This turned out to be quite a difficult task due to the high sensitivity of the samples to electron beam damage (see also Nieto 2002, Fig. 5). After few seconds of exposure to the electron beam, even at very low magnification (i.e., low beam dose), “buttonhole” shaped features develop parallel to (001). They look like inflated pockets, probably due to NH4 + diffusion and accumulation, before volatilization, within the interlayer. In spite of these difficulties, numerous stacking faults parallel to (001) were detected at low magnification and some of them were characterized by high resolution imaging. Other kinds of inhomogeneities, such as intergrowths with other micas have not been observed in the studied samples at the present state of the investigation. Full stacking sequence characterization of micas by HRTEM requires structural images along two not equivalent direction of the [UW0] zone type of the same faulted area (Kogure 2002). For the samples considered here, this turned out as very demanding, because of the aforementioned instability of tobelite under prolonged exposure. This notwithstanding, assuming that stacking faults belonging to subfamily A are absent – a reasonable assumption since diffraction patterns taken along [-130] do not show any streaks or spot splitting along [31l]* rows – most of the near atomic resolution images of stacking faults taken along [010] can be interpreted as interruptions of the alternating ± 60° rotations of the 2M2 stacking sequence resulting from the occurrence of 180° rotations – i.e. nanotwins given by 180°-rotations along c*. These findings can explain the presence of residues in the Fourier density maps in single crystal structure refinements and hopefully would help to improve the disorder modelling for future structural refinements

Trioctahedral mica samples, collected at Cava St. Antonio (Mt. Vulture, Italy) were studied by combining electronmicroprobe and C-H-N elemental analyses, single-crystal X-ray diffraction refinement, Mössbauer and Fourier transform infrared spectroscopies. Electron-microprobe analyses show the crystals to be quite homogeneous with TiO2 ~3 wt% and F ranging from 0.42 to 0.59 wt%. Quantitative analyses of H combined with ferric/ferrous ratios from Mössbauer data allowed reliable crystal-chemical formulae to be derived. The results suggest that the entry of both Ti4+ and Fe3+ in the structure occurs through R-oxy substitution mechanisms involving deprotonation at O(4). This inference is supported by X-ray structure-refinement results (notably the c cell-parameter, the off-centering of the M2 cation towards O(4), the bond-length distortions of the cis-M2 octahedron) obtained using anisotropic thermal parameters in space group C2/m. The amount of oxy-substitutions from both electron-microprobe and X-ray data is in agreement with carbon-hydrogen-nitrogen analyseswhich give an average anion composition (OH1.25O0.65F0.10). Polarized-light infrared spectroscopy shows a complex OH-stretching spectrum which is composed of several overlapping (at least five) components. These can be assigned to the main octahedral local configurations that are compatible with the chemical composition. Pleochroic Fourier transform infrared spectroscopy measurements done along the principal optical directions show that the O-H bond axis is tilted from [001] and provide the average orientation of the O-H dipole in the structure: O-H^α ~ 23° and O-H^γ ~ 56°.

The iron sulfate FeOHSO4 studied here has been obtained as a dehydration product of the thermal decomposition of metahohmannite Fe2(H2O)4[O(SO4)2]1 during a synchrotron real time powder diffraction experiment. This compound is commonly described in the orthorhombic system [ Pnma, a = 7.33, b = 6.42, c = 7.14 Å, and Z = 4] with Fe3+ cations coordinating two hydroxyl groups and four sulfate O atoms2. Each hydroxyl group is bonded to two iron atoms. However a careful analysis of the experimental powder diffraction pattern exhibited unexpected missing peaks for h = 2n +1, whereas sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the OD character of the FeOHSO4 compound was the key element leading to the successful interpretation of the features of the observed powder pattern and to the understanding of the misfit with respect to the calculated pattern. In fact, FeOHSO4 seems to belong to a family of OD structures formed by equivalent layers with symmetry Pbmm. Only two maximum-degree order (MDO) polytypes are possible. The form MDO1 results from a regular alternation of stacking operators 21/2 and 2-1/2, and yields the known orthorhombic structure; the form MDO2 results from the sequence 21/2

The incorporation of OH- in trace but measurable amounts in the structure of nominally anhydrous minerals is to date well documented. OH- occurring in this manner may constitute the dominant reservoir of hydrogen in the Earth's interior and is believed to play an important role in the physical properties of the mantle. It may also affect the evolution of the hydrosphere through its influence on mantle melting and isotopic fractionation [1]. In Ti-garnets, hydrogen may be incorporated via the hydrogarnet substitution, where a SiO4 unit may locally be replaced by a H4O4-tetrahedron. However, more complex mechanisms have also been proposed [2]. In addition, the uptake of fluorine into the structure is more complex than the simple exchange reaction F- OH- [3]. In this study, Secondary Ion Mass Spectrometry (SIMS) has been used to analyse hydrogen -quantified conventionally as H2O (wt%)-, fluorine and lithium in a suite of Ti garnets of different origin and geological provenance. From Electron Probe Micro-Analysis (EPMA) and structure refinements from single crystal X-ray diffraction (SCXRD) data, such garnets were expected to have tetrahedral substitutions and a hydrogarnet component. FTIR preliminary spectra in the OH- stretching region evidenced that, comparatively, the samples were characterized by various degrees of hydration. The results of our SIMS analyses confirmed the presence of significant amount of H2O (0.091 - 0.459 wt%), low concentration of F (0.0089 - 0.048 wt%) as well as of Li2O (0.0024 - 0.0139 wt%). The comparison with data obtained by X-ray site scattering refinement and Mössbauer spectroscopy allowed to ascertain that the combination of VITi4+VIM3+-1 IVFe3+IVSi-1 (schorlomite substitution) and VIM2+VITi4+VIFe3+-2 (morimotoite substitution), with M3+ = Fe3+, Al3+, and M2+ = Fe2+, Mg2+, Mn2+, can explain most of the chemical variation of tetrahedral and octahedral sites. However, the accurate evaluation of H and F by SIMS favoured the assessment of minor substitution mechanisms. A good qualitative agreement between SIMS and FTIR data was found.

Mica is one of the most common minerals present on the Earth Crust (~ 4 vol%) in all kinds of rocks: metamorphic, sedimentary and igneous ones. In addition, mica can be found in the lower crust as well as in the upper and deeper mantle, in association with other minerals. It is of paramount importance in the fields of mineralogy, crystal chemistry, petrography, petrology, geochemistry and volcanology. Indeed, due to the peculiarity of its atomic organisation, mica is able to record in its structure information about the geological history the host rock underwent. Actually, its complex chemical composition and the frequently occurring structural disorder often prevent or limit the knowledge of the structural details suitable for comparative crystal-chemical studies. This is particularly true for trioctahedral micas of volcanic origin, essentially owing to the different structural role played by some cations (e.g., Al, Fe, Ti), their isomorphic substitutions, and their different oxidation states (Fe, Ti) combined with frequent grain-to-grain and/or inter-grain chemical heterogeneity. In mica literature the importance of Fe3+-oxy mechanisms in determining the explosive nature of volcanism is often highlighted. Since the first applications of secondary ion mass spectrometry (SIMS) to geologic samples, it has been evident that this technique can play a key role in the characterisation of minerals and rocks, owing to its high spatial resolution and sensitivity from H to U. However, SIMS suffers from a major intrinsic drawback, i.e., the so-called «matrix-effects» affecting sputtering-ionisation phenomena, which cannot be eliminated but only calibrated. To date, the available standards for micas are very rare and insufficient to face the variety of their chemical compositions. Moreover, the «matrix effects» are further complicated by the existence of crystallographic orientation effects relative to the ion beam, which are related to its strongly anisotropic structure. By means of an analytical procedures developed by us, it is possible to use the ion microprobe to obtain accurate concentrations of the light elements, i.e., H, Li, (Be, B), F (Cl) in mica by means of a small number of silicate standards and mathematical corrections on the ion yields. Examples of SIMS applications to the solution of the complex crystal chemistry in micas from volcanic areas of Southern Italy are presented. In particular, the possibility to measure hydrogen by SIMS has resulted to be fundamental in assessing the correct structural formula and substitution mechanisms occurring in micas, which in turn constitutes the basis of a sound derivation of petrologic/thermodynamic parameters in mica-bearing systems; while the knowledge of Li- and F-contents have contributed to understanding the processes the host rock underwent. For instance the finding of fluorophlogopite in the volcanic rocks of Biancavilla (Sicily) provided evidence that metasomatic processes affected the pre-existing benmoreitic rocks of the Etnean volcanic area.

The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 21/2 and 2−1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 21/2|21/2 |21/2... sequence of symmetry operators and yields a monoclinic structure with space group P21/c, aM = 7.33, bM = 7.14, cM = 7.39 Å, and β = 119.7°. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit (Rp = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

Mt. Vulture trioctahedral micas-1M mainly consist of phlogopite-annite solid solutions with a little component of true micas (Schingaro et al., 2001 EJM, 13,1057). However, both Li-free, and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. In the present contribution two single crystals are considered, which are representative of both compositions. The crystals, labelled LC7-27o and LC7R1 (Rinaldi, 2004, Ph.D thesis) were studied by combining chemical determinations (EPMA, SIMS) and structural investigations (SCXRD). Electron Probe Micro-Analyses yielded XMg = 0.87, TiO2 = 1.07 wt% for sample LC7-27o, and XMg = 0.65, TiO2 = 5.39 wt% for sample LC7R1, while SIMS provided Li2O = 0.0046, F = 1.78, H2O =3.45 wt% for LC7-27o and Li2O = 0.140, F = 3.51, H2O = 2.17 wt% for LC7R1. Relevant structural data are the following: a = 5.346(1), b = 9.257(3), c = 10.312(3) Å, ? = 99.97°(2) for LC7-27o, and a = 5.310(1), b = 9.193(2), c = 10.096(3) Å, ? = 100.03°(3) for LC7R1. Anisotropic structure refinements in space group C2/m converged at R = 4.34, Rw = 4.39 and at R = 3.77, Rw = 4.09 for the Li-bearing and Li-free sample, respectively. When considering bond distances, both samples can be described as homo-octahedral whereas taking into account mean atomic numbers, LC7R1 is meso-octahedral. Significant differences occur between the two samples for some of the distorsion parameters commonly used for micas. In particular, LC7R1 displays higher values of BLDM(2), ShiftM(2) and lower values of ?K-O4 and tK-O4 than those of LC7-27o. These differences can be ascribed to the different extent of F- and Ti-substitutions affecting their structure. The final cation distribution was obtained by combining EPMA, SIMS and SCXRD data after taking into account the crystal chemical model illustrated by Scordari et al. (2004, 32nd IGC) and after careful analysis of the effect of several normalization schemes on mica formula recalculation. It turned out that in sample LC7-27o Ti is incorporated according to the Ti-vacancy mechanism, whereas more than one Ti-substitution occur in LC7R1.