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In this paper, we review 10 years of research on high-temperature oxidation process on amphiboles with variable composition. One notable feature that has emerged from our experiments is that iron oxidation is reversible over a relatively large thermal range before the kinetic energy of delocalized electrons and H+ ions becomes high enough to allow their ejection from the crystal. Experiments under different oxidation conditions showed that the sodic amphibole riebeckite follows two distinct paths: under oxidizing conditions, it undergoes both Fe oxidation and dehydrogenation and the resulting oxo-riebeckite is stable up to ~ 900 °C; under vacuum conditions, neither Fe oxidation nor dehydrogenations occur, and the amphibole is stable up to 800 °C, via a rearrangement of the octahedral cations. In situ HT Raman measurements on grunerite provided the first atomic-scale proof for the thermal activation of polarons in Fe-amphiboles; further studies showed that this process is a general feature of Fe2+-bearing amphiboles. For riebeckite, it starts at 227 °C and is complete at 377 °C under both reducing and oxidizing conditions. Above 377 °C, external oxygen triggers the expulsion of H+ and e− from the crystal. The temperature range observed for the development of charge carriers accurately fits the temperatures for the development of high-conductivity layers in warm and cold subduction zones and provides the atomic-scale picture for large-scale processes such as the development of anomalous conductivity layers at convergent-plate margins. Our work shows that Raman spectroscopy may provide relatively straightforward access to the properties of rock-forming minerals of geophysical interest. [ABSTRACT FROM AUTHOR]

In this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mossbauer spectroscopy and powder X-ray diffraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400-450 °C), then it undergoes a rapid (within ~50 °C) dehydrogenation, forming oxo-riebeckite, which is stable up to ~900 °C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (Ea) for the riebeckite to oxo-riebeckite transition is 166 ± 6 kJ/mol. Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750-800 °C); however, in the 550 < T < 700 °C range, it undergoes a significant rearrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700-800 °C range has the same chemical formula as riebeckite but has a disordered and nonstandard cation distribution at the octahedra, i.e., ^(1)(Fe3+Fe2+)M(2)(Fe3+Fe2+)M(3)Fe2+; we call this phase "CR3+ disordered riebeckite". For T > 800 °C, it decomposes to aegirine + fayalite + cristobalite + H2O. External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures. [ABSTRACT FROM AUTHOR]

Human exposure to Volatile Organic Compounds (VOCs) and their presence in indoor and working environments is recognized as a serious health risk, causing impairments of varying severities. Different detecting systems able to monitor VOCs are available in the market; however, they have significant limitations for both sensitivity and chemical discrimination capability. During the last years we studied systematically the use of Fourier Transform Infrared (FTIR) spectroscopy as an alternative, powerful tool for quantifying VOCs in air. We calibrated the method for a set of compounds (styrene, acetone, ethanol and isopropanol) by using both laboratory and portable infrared spectrometers. The aim was to develop a new, and highly sensitive sensor system for VOCs monitoring. In this paper, we improved the setup performance, testing the feasibility of using a multipass cell with the aim of extending the sensitivity of our system down to the part per million (ppm) level. Considering that multipass cells are now also available for portable instruments, this study opens the road for the design of new high-resolution devices for environmental monitoring. [ABSTRACT FROM AUTHOR]

In this work, we address the kinetics of dehydrogenation occurring at high temperatures (HT) in riebeckite, a sodic amphibole with the ideal composition Na 2 Fe 2 3 + Fe 3 2 + Si 8 O 22 (OH) 2. ${\rm{N}}{{\rm{a}}_2}{\rm{Fe}}_2^{3 + }{\rm{Fe}}_3^{2 + }{\rm{S}}{{\rm{i}}_8}{{\rm{O}}_{22}}{({\rm{OH}})_2}.$ We performed isothermal experiments on both powders and single-crystals up to 560 °C and monitored the O-H stretching signal by Fourier transform infrared (FTIR) spectroscopy. Single-crystals show an initial increase in IR absorption intensity due to increasing vibrational amplitudes of the O-H bond stretching, not observed for powders. The OH-intensities vs. time were fitted using the formalism for first-order reactions. The calculated activation energies for H+ diffusion in riebeckite are 159 ± 15 kJ/mol for powders and 216 ± 20 kJ/mol for single crystals, respectively. The exponential factor m in the Avrami-Erofeev equation obtained for crystals ranges between 1.02 and 1.31, suggesting that, unlike powders, the dehydration process in crystals is not a purely first-order reaction. This implies that a second energy barrier must be considered, i.e., diffusion of H+ through the crystal. FTIR imaging showed that H+ diffusion occurs mainly perpendicular to the silicate double-chain. Our results confirm that the release of H+ from riebeckite occurs after the irreversible Fe2+-to-Fe3+ exchange, thus at temperatures >550 °C. To be effective, the process needs the presence of external oxygen that, by interacting with H+ at the crystal surface, triggers the release of H2O molecules. This implies that oxidizing conditions are required for the amphibole to be an efficient water source at depth. [ABSTRACT FROM AUTHOR]

The thermal behavior of melanterite from the Fornovolasco mine (Tuscany, Italy) has been investigated via differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature X-ray powder diffraction (XRPD) and Fourier-transform infrared spectroscopy (FTIR). The DTA curve showed endothermic peaks at 70, 100, 260, 500–560 and 660 °C whereas the TG curve evidenced a total mass decrease of ~68%, in keeping with the loss of all H2O and SO4 groups. Rietveld refinements were performed for all the collected patterns in the 25–775 °C range and converged at 1.57 ≤ R (%) ≤ 2.75 and 1.98 ≤ Rwp (%) ≤ 3.74. The decomposition steps FeSO4·7H2O → FeSO4·4H2O (25 ≤ T ≤ 50 °C) → FeSO4·H2O (50 < T ≤ 100 °C) → FeOHSO4 (75 < T ≤ 200 °C) → Fe2(SO4)3 (400 < T ≤ 500 °C) → Fe2O3 (500 < T ≤ 775 °C) were obtained. The high-temperature infrared analysis confirmed that melanterite undergoes a three-step dehydration in the 25–300 °C temperature range. The FeOHSO4 phase is stable over a wide range of temperature and transforms partially to Fe2(SO4)3 without the formation of Fe2O(SO4)2. The findings highlight a different behavior of the studied sample with respect to the synthetic salt. [ABSTRACT FROM AUTHOR]

A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca9Mg(PO4)6[PO3(OH)], from Palermo Mine (USA), was achieved by means of a combination of electron microprobe analysis (EMPA) in WDS mode, single-crystal neutron diffraction probe (NDP) and single-crystal X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The crystal-chemical characterization resulted in the empirical formula (Ca8.682Na0.274Sr0.045)Σ9.000(Ca0.034□0.996)Σ1.000(Mg0.533Fe2+0.342Mn2+0.062Al0.046)Σ0.983(P1.006O4)6[PO3(OH0.968F0.032)Σ1.000]. Crystal-structure refinement, in the space group R3c, converged to R1 = 7.12% using 3273 unique reflections from NDP data and to R1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are a = 10.357(3) Å, c = 37.095(15) Å and V = 3446(2) Å3, and from XRD, the parameters are a = 10.3685(4) Å, c = 37.1444(13) Å and V = 3458.2(3) Å3. NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO3(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca3(PO4)2 and displays minor band shifts due to slightly different P-O bond lengths and to the presence of additional elements in the structure. A comparison between whitlockite, isotypic phases from the largest merrillite group, and its synthetic counterpart Ca3(PO4)2 is provided, based on the XRD/NDP and FTIR results. [ABSTRACT FROM AUTHOR]

The new mineral species braccoite, ideally NaMn2+5 [Si5AsO17(OH)](OH), has been discovered in the Valletta mine dumps, in Maira Valley, Cuneo province, Piedmont, Italy. Its origin is probably related to the reaction between ore minerals and hydrothermal fluids. It occurs as subhedral crystals in brown-red coloured thin masses, with a pale-yellow streak and vitreous to resinous lustre. Braccoite is associated with tiragalloite, for which new data are provided, as well as gamagarite, hematite, manganberzeliite, palenzonaite, quartz, saneroite, tokyoite, unidentified Mn oxides, organic compounds, and Mn arsenates and silicates under study. Braccoite is biaxial positive with refractive indices α = 1.749(1), β = 1.750(1), γ = 1.760(1). It is triclinic, space group P1̄ , with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, α = 92̄691(2), β = 117.057(4), γ = 105.323(3)°, V = 740.37(4) Å3 and Z = 2. Its calculated density is 3.56 g/cm3. The ten strongest diffraction lines of the observed powder X-ray diffraction (XRD) pattern are [d in Å, (I), (hkl)]: 3.055 (69)(22̄1), 3.042 (43)(102), 3.012 (65)(32̄1̄ ), 2.985 (55)(23̄1̄ ), 2.825 (100)(213̄), 2.708 (92)(220), 2.627 (43)(23̄2̄), 2.381 (58)(41̄1̄ ), 2.226 (25)(214̄) and 1.680 (433̄)(36). Chemical analyses by wavelength-dispersive spectroscopy electron microprobe gave (wt.%): Na2O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al2O3 0.04, CuO 0.02, SiO2 39.73, As2O5 6.87, V2O5 1.43, SO3 0.01 and F 0.04. H2O 2.20 was calculated on the basis of 2OH groups p.f.u. Raman spectroscopy confirmed the presence of (SiO4)4-, (AsO4)3- and OH groups. The empirical formula, calculated on the basis of Σ cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal structure, is Na1.06(Mn2+4.46Mn3+0.32Mg0.19V3+0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V5+0.15)O17(OH)](OH0.98F0.02); the simplified formula is Na(Mn,Mg,Al,Ca)5[Si5(As,V,Si)O17(OH)](OH,F). Single-crystal XRD allowed the structure to be solved by direct methods and revealed that braccoite is the As-dominant analogue of saneroite. The structure model was refined on the basis of 4389 observed reflections to R1 = 3.47%. Braccoite is named in honour of Dr Roberto Bracco (b. 1959), a systematic minerals collector with a special interest in manganese minerals. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2013-093). [ABSTRACT FROM AUTHOR]