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This study is part of the H2020-GEMex Project, a collaboration between Europe and Mexico for investigating super-hot geothermal systems. The novelty of this work is in its integrated methodological approach which bridges the past to the present and contributes to the reconstruction of conceptual models of deep geothermal systems, thus favoring appropriate strategies for deep exploration. The Las Minas mining area (Veracruz State) where Cu-Fe-Au ore-deposits have been historically exploited, is considered as an exhumed Neogene geothermal system analogous to the deeper part of the Los Humeros geothermal field (Puebla State). We used an integrated approach, which involved structural and kinematic, petrological, geochemical, geochronological and fluid inclusion analyses of the pre-volcanic Paleozoic-Cretaceous succession, intruded by Miocene-Pliocene granitoids and mafic dykes. Our emphasis was on the (paleo) fluid-rocks interaction and the related structural control. The results indicate that saline and hypersaline fluids (up to 70 wt. % NaCl equiv.) of magmatic derivation and aqueous-carbonic fluids issued during metamorphic or metasomatic reactions with T up to 600-650°C were channeled along and in the intersection between the NW- and NE-trending faults and their damage zones. The faults are characterized by strike-slip to oblique-slip/normal movements. Fluids were also stored in carbonate rock-volumes at the contact with the granitoids. The distribution of the ore-deposits and hydrothermal alteration related to the interaction of geothermal fluids and hosting rocks indicates the importance of conduits (such as fractures and faults) in the reservoirs. Hence, understanding the relationships between ore-deposits, hydrothermal alteration and structures can increase our knowledge of the present geothermal fluids' deep circulation in the Los Humeros geothermal field.

Banded Ca-carbonate veins in travertine deposits are efficient recorders of the compositional fluctuations of geothermal fluids flowing (or flowed) from deep reservoirs up to the surface, within fault zones. In this view, these veins represent key tools for decoding those factors that influenced the geochemical variations. We have analyzed veins developed in fractures channeling geothermal fluids forming travertine deposits. The studied veins cut a fossil travertine fissure ridge, near the Larderello geothermal area (Iano area, southern Tuscany) where geothermal fluid circulation is favored by NE-trending strike-to-oblique-slip faults and their intersections with NW-trending normal ones. U-Th dating indicates that fluid circulation occurred from (at least) 172 ka to 21 ka. In this time span, the geothermal fluid changed in composition, and the banded Ca-carbonate veins recorded these variations in terms of mineralogical and stable isotope composition and temperature ( T ) of deposition. We also documented for the first time the occurrence of Mn-rich black tree-shaped structures within the veins. Mineralogy coupled with stable and clumped isotope measurements allows the reconstruction of some features (i.e., crystal texture, temperature, and CO2 origin) and the inference of the processes (i.e., pH, T, and pCO2 variations) that have controlled the fluid evolution through time. Multiple-stage and one-stage deposition processes have played an important role in modifying the stable isotope composition of banded Ca-carbonate veins; temperature coupled with pCO2 also influenced their mineralogical composition. Interpreted in the context of the tectonic setting, the data show that the NW-trending faults have mainly controlled travertine deposition. Their intersection with NE-trending faults, interpreted as transfer faults, highlights the important role of transfer zones in channeling the geothermal fluids.

This study is focused on a barian titanian phlogopite found in an alkaline ultramafic dyke transecting Mesozoic limestones of the Gargano Promontory (Apulia, Italy). The rock containing the barian titanian phlogopite, an olivine-clinopyroxene-rich lamprophyre with nepheline and free of feldspars, has been classified as monchiquite. The present study combines chemical analyses, single crystal X-ray diraction and Raman spectroscopy. Chemical variations suggest that the entry of Ba into the phlogopite structure can be explained by the exchange Ba + Al = K + Si. The crystal structure refinement indicates that the Ti uptake is consistent with the Ti-oxy exchange mechanism. The structural parameters associated with the oxy substitution mechanism are extremely enhanced and rarely reported in natural phlogopite: (a) displacement of M2 cation toward the O4 site (~0.7); (b) M2 octahedron bond-length distortion (~2.5); (c) very short c cell parameter (~10.14 Å). Raman analysis showed most prominent features in the 800-200 cm?1 region with the strongest peaks occurring at 773 and 735 cm?1. Only a weak, broad band was observed to occur in the OH-stretching region. As concerns the origin of the barian titanian phlogopite, the rock textural features clearly indicate that it crystallized from pockets of the interstitial melt. Here, Ba and Ti enrichment took place after major crystallization of olivine under fast-cooling conditions, close to the dyke margin.

Fluorine-, boron-, and magnesium-rich metamorphosed xenoliths occur in the Campanian Ignimbrite (CI) deposits at Fiano (southern Italy), at about 50 km northeast of the sourced volcanic area. These rocks originated from Mesozoic limestones of the Campanian Apennines, embedded in a fluid flow. The studied Fiano xenoliths consist of ten fluorophlogopite-bearing calc-silicate rocks and five carbonate xenoliths, characterized by combining mineralogical analyses with whole rock and stable isotope data. The micaceous xenoliths are composed of abundant idiomorphic fluorophlogopite, widespread fluorite, F-rich chondrodite, fluoborite, diopside, Fe(Mg)-oxides, calcite, humite, K-bearing fluoro-richterite and grossular. Out of the five mica-free xenoliths, two are calcite marbles, containing very subordinate fluorite and hematite, and three are weakly meamorphism carbonates, composed of calcite only. The crystal structure and chemical composition of fluorophlogopite approach those of the end member. The Fiano xenoliths are enriched in trace elements with respect to the primary limestones. Comparison between the REE patterns of the Fiano xenoliths and those of both CI and Somma-Vesuvius marble and carbonate xenoliths shows that the Fiano pattern on the whole both overlaps that of Somma-Vesuvius marble and carbonate xenoliths, and yet reproduces the trend of CI rocks. δ13C and δ18O values depict the same trend of depletion in the heavy isotopes observed in the Somma-Vesuvius nodules, related tothermometamorphism. Trace element distribution, paragenesis, stable isotope geochemistry and data modelling point to infiltration of steam enriched in F, B, Mg, As into carbonate rocks at temperature of ca. 300-450°C during the emplacement of the CI.

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.

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 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.