Gennaro Ventruti

Ti-rich phlogopite from Mt. Vulture (Potenza, Italy) investigated by a multianalytical approach : substitutional mechanisms and orientation of the OH dipoles

Trioctahedral mica samples, collected at Cava St. Antonio (Mt. Vulture, Italy) were studied by combining electron-microprobe and C-H-N elemental analyses, single-crystal X-ray diffraction refinement, Mossbauer and Fourier transform infrared spectroscopies. Electron-microprobe analyses show the crystals to be quite homogeneous with TiO 2 ∼3 wt% and F ranging from 0.42 to 0.59 wt%. Quantitative analyses of H combined with ferric/ferrous ratios from Mossbauer data allowed reliable crystal-chemical formulae to be derived. The results suggest that the entry of both Ti 4+ and Fe 3+ 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 analyses which give an average anion composition (OH 1.25 O 0.65 F 0.10 ). 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°.

Fluorophlogopite from Biancavilla (Mt. Etna, Sicily, Italy): Crystal structure and crystal chemistry of a new F-dominant analog of phlogopite

Abstract Fluorophlogopite, a new F-dominant mineral of the mica group, was found at Monte Calvario, Biancavilla, lower southwestern flanks of Mt. Etna volcano (Catania, Sicily, Italy). The mineral occurs in autoclasts of gray-red altered benmoreitic lavas, primarily associated with fluoro-edenite, alkali-feldspars, clino- and ortho-pyroxenes, fluorapatite, hematite, and pseudobrookite. It was formed by metasomatism of the original lava rocks from very hot fluid enriched in F, Cl, and other incompatible elements. Fluorophlogopite occurs as very thin laminae with a diameter of 200 to 400 μm. Main physical properties are pale yellow in color; yellowish-white in thin section; vitreous to resinous luster; transparent; non-fluorescent; Mohs’ hardness 2-3; brittle and malleable; perfect cleavage on {001}; biaxial (-), αcalc = 1.5430(8), β = 1.5682(5), γ = 1.5688(5) (λ = 589 nm); 2Vmeas = 17(2)°; α = acute bisectrix ⊥ (001); nonpleochroic; Dcalc = 2.830 g/cm3 (using empirical formula and single-crystal unit-cell parameters), Dcalc = 2.842 g/cm3 (using empirical formula and powder cell constants). Infrared spectrum did not show a significant absorption band in the OH-stretching region (3800-3600 cm-1) confirming that the F content of the fluorophlogopite from Biancavilla is close to the stoichiometric value. Unit-cell parameters from X-ray powder-diffraction data (114.6 mm diameter Gandolfi camera, CuKα) are a = 5.305(2), b = 9.189(3), c = 10.137(4) Å, β = 100.02(3)°. These data agree with those obtained by single-crystal X-ray studies on a very thin (~15 μm) fluorophlogopite crystal, i.e., Monoclinic (1M polytype); Space Group C2/m; a = 5.3094(4), b = 9.1933(7), c = 10.1437(8) Å, β = 100.062(5)°, V = 487.51(6) Å3, Z = 2. Structure refinements using anisotropic displacement parameters converged at R = 3.50, Rw = 4.37, Rsym = 3.72%. Electron microprobe analysis performed on the same crystal used for X-ray investigation gave: SiO2 = 45.75(39), TiO2 = 1.05(5), Al2O3 = 9.60(19), MgO = 27.92(30), MnO = 0.16(3), FeOtot = 1.25(6), BaO = 0.09(5), K2O = 8.22(11), Na2O = 0.61(30), Cl = 0.02(1) wt%. Secondary Ion Mass Spectrometry (SIMS) was used to estimate light elements [Li2O = 0.30(1) and H2O = 0.16(2) wt%] and fluorine content [F = 8.69(24) wt%]. The new mineral fluorophlogopite and its name were approved by IMA-CNMMN (2006/011).

Mechanochemical degradation of pentachlorophenol onto birnessite

The existence of a lot of worldwide pentachlorophenol-contaminated sites has induced scientists to concentrate their effort in finding ways to degrade it. Therefore, an effective tool to decompose it from soil mixtures is needed. In this work the efficiency of the phyllomanganate birnessite (KBi) in degrading pentachlorophenol (PCP) through mechanochemical treatments was investigated. To this purpose, a synthesized birnessite and the pollutant were ground together in a high energy mill. The ground KBi-PCP mixtures and the liquid extracts were analyzed to demonstrate that mechanochemical treatments are more efficient in removing PCP than a simple contact between the synthesized birnessite and the pollutant, both in terms of time and extent. The mechanochemically induced PCP degradation mainly occurs through the formation of a surface monodentate inner-sphere complex between the phenolic group of the organic molecules and the structural Mn(IV). This is indicated by the changes induced in birnessite MnO(6) layers as a consequence of the prolonged milling with the pollutant. This mechanism includes the Mn(IV) reduction, the consequent formation of Mn(III) and new vacancies, and free Mn(2+) ions release. The PCP degradation extent is limited by the presence of chloro-substituents on the aromatic ring.

Thermal behavior of a Ti-rich phlogopite from Mt. Vulture (Potenza, Italy): An in situ X-ray single-crystal diffraction study

Abstract The structural evolution of a trioctahedral mica from Cava St. Antonio, Mt. Vulture, Potenza, Italy, has been studied in the temperature range 100-1023 K using in situ single-crystal X-ray diffraction techniques. The sample used shows a Ti-rich composition close to the phlogopite-annite join with the following crystal-chemical formula: (K0.85Na0.11Ba0.03)(Al0.17Mg1.98Mn0.01Fe2+0.29Fe3+0.37Ti0.18)(Si2.75Al1.25) O10.66(F0.13OH1.20Cl0.01). In the present study, the chemical and structural changes and the deprotonation process involved during heating have been characterized. Analysis of the data showed that a, b, and c cell parameters expand almost linearly up to 823 K, while the β angle remains constant. A sharp decrease in the unit-cell dimensions was observed at 873 and 1023 K. Linear trends obtained during reversal experiments from 1023 K to room temperature demonstrated the irreversibility of these events. Structure refinements of single-crystal XRD data collected at 100, 200, 298, 473, 673, 873, and 1023 K converged to 2.14 ≤ R (%) ≤ 8.47, 2.47 ≤ Rw (%) ≤ 10.83. In the temperature range 100-673 K, the thermal expansion along the c direction is mainly due to interlayer thickness dilation. The tetrahedral ring approaches the ideal hexagonal shape with increasing temperature to match the expanding octahedral sheet. In the range 873-1023 K, a strong shrinking of the interlayer is associated with the shortening of the M1-O4 and M2-O4 distances and to the consequent reduction of octahedral thickness. Such structural features indicate the occurrence of Fe oxidation process, involving loss of structural H, which is responsible for a phase transition. Mössbauer spectroscopy supported this hypothesis.

Sideronatrite, Na2Fe(SO4)2(OH)·3H2O: Crystal structure of the orthorhombic polytype and OD character analysis

Sideronatrite, Na2Fe(SO4)2(OH)·3H2O, is a secondary hydrated sulfate occurring in desert areas as the result of pyrite alteration. It is one of the environmental indicators of soil-water processes operating in specific landscapes, and, as a consequence, an important marker of acid mine drainage pollution. Sideronatrite has been demonstrated from its peculiar diffraction pattern to belong to a family of OD structures formed by equivalent layers. In this work, a crystal with weak diffuse streaks proved to be suitable for a single-crystal X-ray diffraction study. The crystal structure was solved by direct methods and refined by full matrix leastsquares (R = 7.4% and RW = 8.0%) in the space group P212121 with a = 7.265(2), b = 20.522(6), c = 7.120(2) Å, V = 1061.5(5) Å3, and Z = 4, using 798 reflections with I > 3.0 σ(I). Sideronatrite is characterized by infinite [Fe3+(SO4)2(OH)]2- octahedral-tetrahedral chains of the type [M(TO4)2φ] running parallel to the c axis. These chains are cross-linked by a columnar system of corner-sharing, Na-distorted octahedra along c to form corrugated sheets parallel to the (010) plane. Adjacent sheets are hydrogen-bonded through water molecules coordinated by Na atoms. The present results allow a complete description of the OD character of the structure, with the derivation of the OD groupoid and MDO polytypes. Finally, chemical and structural relationships are taken into account to explain the possible paragenetic sequence concerning several sulfates associated with sideronatrite

Metasideronatrite: Crystal structure and its relation with sideronatrite

Abstract Metasideronatrite was obtained as the first dehydration product of sideronatrite, Na2Fe(SO4)2(OH)∙3H2O, from Sierra Gorda, Chile. The crystal structure of metasideronatrite was solved by direct methods and refined by full-matrix least-squares to R = 0.039, using 574 independent reflections with I > 3.0σ(I). It is orthorhombic, space group Pbnm, with a = 7.3959(8), b = 16.0979(15), c = 7.1607(8) Å, V = 852.5(2) Å3, Z = 4. The crystal-chemical formula derived from this structural study is Na2Fe(SO4)2(OH)∙H2O. The backbone of the structure is the same as that in sideronatrite: infinite [Fe3+(SO4)2(OH)]2- chains of interconnected octahedra and tetrahedra parallel to the c axis. These chains are linked primarily by Na atoms to build a 3-dimensional network of strong (Fe-O-S) and weak (Na-O) bonds. Another prominent feature of the structure is the arrangement of distorted (NaO5H2O) octahedra, which alternately share one edge and one face to form columns parallel to the [Fe3+(SO4)2(OH)] chains. Subsidiary intra-chain bonds are provided by H atoms belonging to OH- groups shared by adjacent Fe octahedra, and to the unique water molecule shared between two adjacent (NaO5H2O) octahedra. At normal conditions of relative humidity (RH) and temperature (i.e., RH > 60% and T < 40 °C), metasideronatrite rehydrates rapidly to sideronatrite. The structure solution has allowed us to: (1) investigate the strong relation between sideronatrite and metasideronitrite; (2) elucidate the mechanism involved in the transformation of metasideronatrite into the order/disorder (OD) structure of sideronatrite; and (3) get insight into the stability of this mineral from the valence-matching principle applied to the main structural unit [Fe3+(SO4)2(OH)]2 and Na+ interstitial species. The weak hydrogen bonds and the particular arrangement of the face-sharing adjacent [NaO5(H2O)] octahedra are the main factors affecting the stability of metasideronatrite.

Mechanochemical transformation of an organic ligand on mineral surfaces: the efficiency of birnessite in catechol degradation.

The aim of this work is to investigate the efficiency of the phyllomanganate birnessite in degrading catechol after mechanochemical treatments. A synthesized birnessite and the organic molecule were grounded together in a high energy mill and the xenobiotic-mineral surface reactions induced by the grinding treatment have been investigated by means of X-ray powder diffraction, X-ray fluorescence, thermal analysis and spectroscopic techniques as well as high-performance liquid chromatography and voltammetric techniques. If compared to the simple contact between the birnessite and the organic molecule, mechanochemical treatments have revealed to be highly efficient in degrading catechol molecules, in terms both of time and extent. Due to the two phenolic groups of catechol and the small steric hindrance of the molecule, the extent of the mechanochemically induced degradation of catechol onto birnessite surfaces is quite high. The degradation mechanism mainly occurs via a redox reaction. It implies the formation of a surface bidentate inner-sphere complex between the phenolic group of the organic molecules and the Mn(IV) from the birnessite structure. Structural changes occur on the MnO(6) layers of birnessite as due to the mechanically induced surface reactions: reduction of Mn(IV), consequent formation of Mn(III) and new vacancies, and free Mn(2+) ions production.

The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]

Abstract 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.

Trioctahedral micas-1 M from Mt. Vulture (Italy): Structural disorder and crystal chemistry

The crystal chemistry of trioctahedral micas-1 M from Mt. Vulture phonolitic-trachitic ignimbrites has been investigated by single crystal X-ray diffraction, microprobe analysis and plasma emission spectroscopy. Chemical analyses have shown that Mt. Vulture trioctahedral micas belong to the phlogopite-annite join, with minor component of kinoshitalite-ferrokinoshitalite solid solution. In addition, samples with remarkably different Mg/Fe ratio can coexist in the same volcano-stratigraphic level, Fe-rich samples having also higher Ti content. The crystal structure of all of the analysed samples is affected, to varying degrees, by structural disorder, due to ± b /3 slips of octahedral sheet along the [0 1 0], [3 1 0] and [3 −1 0] directions. The latter has been reported in the literature for a handful of cases. In the present work, for the first time, the disorder has been successfully interpreted and managed in the structure refinement. It is demonstrated that it leads to desymmetrization of the TOT layer and therefore of the whole structure, with reduction of symmetry from C 2/ m to C 2. For the sample mostly affected by disorder (17 %) the R -factor drops from 9.97% ( C 2/ m ) to 2.74 % ( C 2). The chemical and geometrical features show that Mt. Vulture micas are homo-octahedral, even if some structural details suggest a preferential partitioning of Ti at M2 sites. However the mechanism of Ti incorporation in the structure seems to be different in Mg-rich and Fe-rich samples. For the former the Ti-vacancy substitution mechanism seems to hold: \(2^{VI}Mg^{2}{+}\ {\longleftrightarrow}\ ^{VI}Ti^{4{+}}\ {+}\ ^{VI}[]\) For the latter, the occurrence of Li, as detected by emission spectroscopy plasma measurements, structural evidence and charge balance considerations suggest the following novel substitution mechanism, \(^{VI}Ti^{4{+}}\ {+}\ ^{VI}Li^{{+}}\ {+}\ O^{2{-}}\ {\longleftrightarrow}\ 2\ ^{VI}M^{2{+}}\ {+}\ OH^{{-}}\) , to be active in the structure.

The structure of metahohmannite, Fe23+ [O(SO4)2]·4H2O, by in situ synchrotron powder diffraction

Abstract Metahohmannite, Fe3+2[O(SO4)2]⋅4H2O, is a hydrated sulfate of ferric iron that occurs in sulfate deposits in the desert areas of Northern Chile. The compound used for this study was obtained as a dehydration product of hohmannite, Fe3+2[O(SO4)2]⋅(4+4)H2O. Intensities for the structure analysis were collected from a powdered sample using in situ synchrotron X-ray powder diffraction at ESRF (Grenoble, France). The structure was solved ab initio by profile deconvolution and the application of standard Patterson and difference Fourier maps. The structure was refined to Rp = 5.46% using the Rietveld method. Metahohmannite crystallizes in the triclinic system, space group P1̅ with unit-cell parameters a = 7.3484(5) Å, b = 9.7710(6) Å, c = 7.1521(5) Å, α = 91.684(5)°, β = 98.523 (5)°, γ = 86.390 (5)°, and Z = 2. The structure consists of four Fe3+ octahedra and four sulfate tetrahedra, which share vertices and edges to form a complex building block of Fe3+2[O(SO4)2]⋅8H2O composition. Such blocks are connected to form chains running parallel to the c axis. A complicated system of hydrogen bonds connects adjacent chains into a three-dimensional network. Finally, the crystal structures of metahohmannite, hohmannite, and amarantite are compared and the geometrical features discussed in detail.