Search

The sorption properties of zeolites are controlled by several factors, i.e. Si/Al ratio of the aluminosilicate framework [AlSiO4]-, the type and position of the extraframework (EF) cations, and the applied temperature. Here we investigate the flexibility of CHA framework as a function of EF cation-content and temperature (20–350 °C). Two CHA forms (Na- and Cu-CHA) with Si/Al = 2 were analysed. The main objectives were: (i) to shed light on the HT behaviour of Na-CHA, for which contrasting results exist in literature; (ii) define the role of temperature and EF cation-type in the response to the heating stimuli. We show that at 75 °C, Na-CHA undergoes a severe contraction of the unit-cell volume (-12%) accompanied by a symmetry lowering (R-3m to I2/m). The transformation is reversible, if the dehydrated Na-CHA is exposed to ambient conditions. In contrast, Cu-CHA experiences a significant different dehydration path, which involves minor changes of the CHA framework, and a net positive thermal-expansion after dehydration. The implications of the observed transformations for gas separation processes are finally discussed. [ABSTRACT FROM AUTHOR]

Gismondine-Sr, recently discovered in the Hatrurim Complex in Israel, has been recognised in a xenolith sample from the Bellerberg volcano in Germany. The empirical crystal-chemical formula indicates elevated K content: (Sr1.74Ca1.05Ba0.09K1.56Na0.49)Σ4.93 [Al7.98Si8.06O32]⋅9.62H2O. Additionally, Ba-rich gismondine and amicite have been found in the low-temperature mineral association of the pyrometamorphic rock from the Hatrurim Complex. The Raman spectra of the studied zeolites and the crystal structure of gismondine-Sr from the second occurrence are presented. A review of zeolites with GIS framework-type structure leads to the following conclusions: (1) garronite-Na and gobbinsite are equivalent and constitute a solid solution with garronite-Ca; (2) gismondine-Ca, -Sr, and amicite belong to one mineral series; (3) two zeolites series with different R-factors (defined as Si/(Si+Al+Fe)) can be distinguished within GIS topology: the garronite series (R > 0.6) including garronite-Ca and gobbinsite, with general formula (MyD0.5(x–y)) [AlxSi(16–x)O32]⋅nH2O, where M and D refer to monovalent and divalent cations, respectively; and the gismondine series, including amicite, gismondine-Sr and gismondine-Ca, with R ≈ 0.5, and the general formula (MyD0.5(8–y))[Al8Si8O32]⋅nH2O. The Raman band between 475 cm–1 and 485 cm–1 is distinctive for the garronite series, whereas the band around 460 cm–1 is characteristic of the gismondine series. On the basis of these findings, a revision of GIS zeolites nomenclature is suggested. [ABSTRACT FROM AUTHOR]

A new mineral, gismondine-Sr with ordered gismondine framework type [B2212 no. 20, Z = 1; a = 14.0256(2) Å, b = 10.45900(10) Å, c = 13.79360(10) Å, V = 2023.44(4) Å3] and the ideal chemical formula Sr4(Si8Al8O32)·9H2O was discovered in amygdaloidal voids of partly melted gehlenite hornfels at Halamish locality, Hatrurim Basin of the Hatrurim Complex, Negev Desert, Israel. Gehlenite hornfels is mainly composed of gehlenite, wollastonite, and garnet of the grossular-andradite-schorlomite series. In a low-temperature association occur minerals such as thomsonite-Ca, flörkeite, analcime and minerals of the tobermorite supergroup. Gismondine-Sr forms spherulitic aggregates up to 180 μm and, rarely, pseudotetragonal bipyramidal crystals up to 50 μm. Empirical crystal-chemical formula of gismondine-Sr is (Sr2.02Ca1.09Ba0.02K0.72Na0.62)Σ4.47Al7.91Si8.09O31.85·9H2O. It is the strontium analog of gismondine-Ca and the second orthorhombic zeolite with the GIS structure topology. Crystals are transparent to translucent and feature vitreous luster. The mineral exhibits a white color, imperfect cleavage in [101] direction, a brittle tenacity, and uneven fracture. The Mohs hardness was estimated at approximately 4. Gismondine-Sr is biaxial negative, α = 1.488(3), β = 1.492(3), γ = 1.495(3), 2Vobs = 70–80°. The Raman spectrum is characterized by a band at 465 cm−1, which is also the main band in gismondine-Ca. The structure refinement using SC-XRD (R1 = 0.0353) reveals the ordered distribution of framework cations and the disordered arrangement of extraframework cations. The aluminosilicate framework is built by crankshaft chains with 8-membered apertures channels parallel to [101] and [101]. In gismondine-Sr, the 8-membered rings are elliptically deformed and the T-O-T angle of the upward and downward tetrahedra in the double crankshaft chains is smaller compared to that for gismondine-Ca. Consequently, a slight rotation of the double crankshaft chains has been noticed. Similar observations have been made in partially dehydrated and the pressure-modified gismondine-Ca. The present study suggests that, in addition to high-pressure and dehydration, the elliptical deformation of the channels in GIS also arises as a consequence of the extraframework cations and H2O content. Thus, the extraframework content influences the aluminosilicate framework leading to the orthorhombic symmetry. [ABSTRACT FROM AUTHOR]

For the first time, the structural investigation of a Pb-exchanged zeolite (Pb13.4(OH)10Al17.4Si54.6O144 ∙38H2O) with STI framework type, revealed a highly unusual and intriguing sudden volume increase under continuous heating. Understanding the fundamental mechanisms leading to such an unusual behaviour is essential for technological applications and interpretation of chemical bonding in zeolites. The dehydration was tracked in situ from 25 to 450 °C by single crystal X-ray diffraction, infrared and X-ray absorption spectroscopy. Further interpretation of the experimental observations was supported by ab initio molecular dynamics simulations. Initially, Pb-STI unit-cell volume contracts (ΔV = − 3.5%) from 25 to 100 °C. This agrees with the trend observed in STI zeolites. Surprisingly, at 125 °C, the framework expanded (ΔV = + 2%), adopting a configuration, which resembles that of the room temperature structure. Upon heating, the structure loses H2O but no de-hydroxylation occurred. The key mechanism leading to the sudden volume increase was found to be the formation of Pbx(OH)y clusters, which prevent the shrinking of the channels, rupture of the tetrahedral bonds and occlusion of the pores. This zeolite has therefore an increased thermal stability with respect to other STI metal-exchanged zeolites, with important consequences on its applications. [ABSTRACT FROM AUTHOR]

The crystal structure of mimetite Pb5(AsO4)3Cl, a phosphate with apatite structure‐type has been investigated in situ at 123, 173, 273, 288, 353 and 393 K. A careful inspection of the diffraction pattern and subsequent structure refinements indicated that mimetite transforms from the monoclinic to the hexagonal polymorph with increasing temperature. At 123 K, a monoclinic superstructure, mimetite‐2M, with cell parameters a = 20.4487 (9), b = 7.4362 (2), c = 20.4513 (9) Å, β = 119.953 (6)°, V = 2694.5 (2) Å3 and space group P21 was observed. From 173 to 353 K, the reflections of the supercell were evident only along one direction of the corresponding hexagonal apatite‐cell and the structure transforms to the polymorph mimetite‐M with space group P21/b and unit‐cell parameters a = 10.2378 (3), b = 20.4573 (7), c = 7.4457 (2) Å, β = 120.039 (5)°, V = 1349.96 (9) Å3. Only at higher temperature, i.e. 393 K, does mimetite adopt the hexagonal space group P63/m characteristic of apatite structure‐types. The role of the electron lone pairs of Pb atoms in the phase transition was investigated through the analysis of the electron localization function (ELF) calculated based on the DFT‐geometry optimized structures of the three polymorphs. The changes in spatial distribution of the 6s2 electron density during the phase transitions were explored by means of the Wannier Function Centres (WFCs) derived from ab initio molecular dynamics trajectories. In the high‐temperature hexagonal structure the 6s2 electrons are spherically symmetric relative to the position of Pb atoms. At low temperature the maximum of 6s2 electron density is displaced relative to the position of Pb atom contributing to the polar interaction in the monoclinic polymorphs. [ABSTRACT FROM AUTHOR]

Karlleuite, ideally Ca2MnO4, is a newly approved accessory mineral found in the xenolith sample within the basaltic lava from the Caspar quarry, Bellerberg volcano, Eifel, Germany. It usually occurs as thin tabular/plate crystals, which range from 40 to 80 μm in diameter, and is associated with other members of the perovskite supergroup such as srebrodolskite, brownmillerite, sharyginite, perovskite, and lakargiite distributed within rock-forming minerals represented by reinhardbraunsite, fluorellestadite, fluorapatite, larnite, gehlenite, and several hydrated Ca aluminosilicates. Karlleuite crystals are brown with sub-metallic lustre, a light brown streak, and a good cleavage along (001). It is non-fluorescent, brittle and has an uneven fracture, a Mohs hardness of 3.5 and calculated density Dx = 3.79 g/cm3. The empirical formula of the holotype karlleuite calculated based on O = 4 atoms per formula is (Ca1.97Ce3+0.06)2.03(Mn4 +0.39Ti0.36Fe3+0.19Al0.09)1.03O4, which shows that it is a multicomponent phase characterised by various substituents at the octahedral site. Karlleuite is tetragonal I4/mmm (no. 139), with a = 3.7683(2) Å, c = 11.9893(8) Å, V = 170.254(17) Å3, and Z = 2. The calculated strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 5.995 (43), 2.742 (100), 2.665 (91), 2.023 (25), 1.998 (28), 1.884 (61), 1.553 (38), 1.371 (24). The new mineral is the first natural phase which exhibits a first order of Ruddlesden-Popper type structure, which indicates a modular nature and consists of Ca(Mn, Ti, Fe, Al)O3 perovskite layers, packed between CaO rock-salt layers arranged along the c-axis. Raman spectroscopy supports the interpretation of the chemical and structural data. Mineral association, structural data, as well as the study of the synthetic Ca-Mn-O system suggest that karlleuite could form under high-temperature conditions, above 1000˚C. [ABSTRACT FROM AUTHOR]

Zeolites show remarkable properties that can be tuned through cation exchange of their original extraframework content. In this respect, the response of the modified zeolite to the heating stimuli, in terms of structural modifications and thermal stability, can drastically change and is, therefore, an important factor to consider. In this study, the dehydration mechanism of a natural levyne previously exchanged with Cd2+ has been monitored in situ by single crystal X-ray diffraction. The initial dehydration trend between 50 and 175 °C is similar to that observed for the pristine material, levyne-Ca. The water loss is accompanied by extraframework cation migration within the zeolitic cavities and the unit-cell volume slightly contracts from 3503.8(1) to 3467.8(6) Å3. From 200 to 250 °C, a pronounced drop of the unit-cell volume (− 7%) is observed. The dehydrated structure at 250 °C corresponds to levyne B topology of natural levyne, characterized by the statistical rupture of the T–O–T bonds of the double six-ring membered cage. However, in contrast to levyne-Ca, the fraction of broken connections reached 50% instead of 37%, and no additional structural modifications were detected up to 350 °C. At 400 °C, diffraction data pointed to the onset of the structural collapse. At this temperature, the measured unit-cell volume was 8% smaller compared to that of the RT structure. The corresponding contracted structure did not rehydrate after exposure to humid conditions for 21 days. [ABSTRACT FROM AUTHOR]

The dehydration and hydration behaviour of a natural zeolite with YUG framework type was investigated as a function of water vapour pressure (P H2O) and temperature (T) by in situ single crystal X-ray diffraction. The chemical composition of the sample is CaAl 2 Si 6 O 16 ∙ n H 2 O, with n varying as a function of T and P H2O. Under ambient conditions (T = 302 K and relative humidity RH = 60%) the structure contains 4 H 2 O per formula unit. After exposure to dry air for 30 min, 1 H 2 O is released. Prolongation of the exposure time to 60 min does not induce further dehydration. The H 2 O can easily be up taken by exposing the crystal to ambient conditions for few minutes. The temperature-dependent experiments were conducted under N 2 flow from 299 to 673 K, in steps of 50 K. In the investigated T range, the unit-cell volume is reduced by 13% and the space group Pc is maintained. Two main structural changes were observed: i) at 523 K, the release of 2.5 H 2 O is accompanied by the doubling of one unit-cell parameter (a -axis); ii) at 573 K, the structure becomes anhydrous, the c -axis triples, and two T-O-T connections of the framework break, leading to the formation of an interrupted framework. This topology (called HT-B phase) was never reported before. Moreover, the breaking process takes place by a different mechanism with respect that reported for other zeolites, and it is here described for the first time. [Display omitted] • The H 2 O content of the investigated YUG zeolite at RH = 60%, T = 26 °C is 4H 2 O pfu. • The structure immediately loses up to 1H 2 O when exposed to dry air. • Further H 2 O is not absorbed after exposure to high-humidity conditions for 15 days. • The YUG zeolite was investigated from RT to 673K in situ by SC-XRD. • A new phase with an interrupted framework was detected at 573 K. [ABSTRACT FROM AUTHOR]

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of T-O-T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T > 300 °C was found to be monoclinic (space group A 2/ m ). In addition, at 400 °C, a new T-O-T connection occurred, analogous to that in the B phase of barrerite. Na-stellerite and Na-stilbite were at RT monoclinic, space group F 2/ m . Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A 2/ m . Compared to natural stellerite and stilbite different T-O-T connections ruptured leading to a different topology of the B phase. The total volume contraction was −16% at 350 °C compared to −8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525 °C. The structure corresponded to the D phase of natural barrerite and Na-barrerite. All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed. [ABSTRACT FROM AUTHOR]

The thermal stability of barrerite (Na 8.2 K 3.0 Ca 2.2 Mg 0.2 Al 17 Si 55 O 144 ·50H 2 O) from Kuiu Island, Alaska, was investigated from 25 °C to 500 °C by in situ single-crystal X-ray diffraction under dry conditions. Corresponding experiments were performed on Na-exchanged barrerite Na 17 Al 17 Si 55 O 144 ·45H 2 O from the same locality. The natural sample showed a gradual transformation of the A phase with dehydration to the B phase associated with rupture of T-O-T links and formation of new connections. Though, the high temperature modification (phase D) was not observed in the studied temperature range. In contrast to natural barrerite (space group Amma ), Na-exchanged barrerite was found to be monoclinic F 2/ m at room temperature and transformed to A 2/ m (same cell setting) with beginning dehydration at 50 °C. Its dehydration trend, monitored by unit-cell volume, corresponded to structural modifications observed for natural barrerite. The D phase of this sample was produced by ex situ experiments at 525 °C under ambient conditions. A new model for transformation from the disordered B phase to the ordered highly condensed D phase (space group A 2 1 ma ) is presented. In addition, the new in situ equilibrated single-crystal experiments are compared with dynamic X-ray powder diffraction studies. [ABSTRACT FROM AUTHOR]

Elpidite Na 2 ZrSi 6 O 15 ·3H 2 O is a natural zirconosilicate with space group Pbcm . A sample from Mt. Saint Hilaire (Canada) was investigated between 25 and 350 °C under different humidity conditions (RH = 0% and at RH = 100%). The largest structural channels are eight- and ten-rings delimited by corner-sharing SiO 4 tetrahedra and ZrO 6 octahedra. Under dry conditions, the structure started liberating water at 75 °C. At 100 °C, when it lost one H 2 O molecule, elpidite underwent a structural change from space group Pbcm to Cmce . The structure continuously released water up to 225 °C until it became anhydrous. From 225 °C to 350 °C no structural changes were observed. Associated with the transition from Pbcm to Cmce symmetry, the unit-cell volume reduced by 2.7%. An additional decrease is observed up to 225 °C when the contraction reached 3.4% of the starting volume. After release of all H 2 O molecules, increase of temperature led to gradual expansion of the unit cell from 2940.79 (7) Å 3 at 225 °C to 2947.4 (5) Å 3 at 350 °C. Supplementary ex-situ heating experiments showed that the structure was retained up to 950 °C but after heating at 1200 °C the sample melted. Under humid conditions a dehydration-trend similar to that found under a dry N 2 -atmosphere was observed but the structural changes occurred at slightly higher temperatures. Immersion of a dehydrated sample in water for 30 days led to only marginal rehydration and the Cmce symmetry was preserved. [ABSTRACT FROM AUTHOR]

Fluoralforsite, ideally Ba5(PO4)3F, (space group P 63/ m (#176), Z = 2, a = 10.0031(2) Å, c = 7.5382(2) Å and V = 653.23(3) Å3), is a new mineral species of the apatite group – a Ba-analogue of fluorapatite and a F-analogue of alforsite. It was discovered in rankinite paralava filling cracks in pyrometamorphic gehlenite hornfels near the tributary of wadi Zohar and Gurim Anticline, Hatrurim Basin, Negev Desert, Israel. Fluoralforsite occurs in small intergranular spaces between large gehlenite and garnet crystals and in enclaves inside large rankinite crystals with other Ba minerals such as walstromite, zadovite, bennesherite, gurimite, mazorite, barioferrite and baryte. It forms tiny transparent, colourless crystals up to 50 μm with a white streak and a vitreous lustre. The cleavage was not observed. It exhibits a brittle tenacity and a conchoidal fracture. The estimated Mohs hardness is 4–4½, and its calculated density is 4.57 g/cm–3. Fluoralforsite is uniaxial (–) with refractive indices (589 nm) n ω = 1.689(3) and n ɛ = 1.687(3). The empirical crystal-chemical formula for the holotype calculated on the basis of 8 cations is: (Ba3.81Ca0.97Na0.07K0.05Sr0.05Fe0.05)Σ5(P5+2.32V5+0.29S6+0.22Si0.17)Σ3O12(F0.85Cl0.13)Σ0.98. The crystal structure was refined from single-crystal X-ray diffraction data with R 1 = 0.0192. The structural investigation indicated an ordered arrangement of Ba/Ca at the M 1 site within individual columns running along the c -axis, but a disordered distribution among adjacent columns throughout the structure, which enables the maintenance of the P 63/ m space group. Fluoralforsite was formed at the final stage of crystallisation as a result of a reaction between the primary mineral assemblages and residual melt. [ABSTRACT FROM AUTHOR]

Inhalation of erionite has been proven to be the cause of the extended epidemic of malignant mesothelioma occurring in Central Anatolia, Turkey, and of cases of lung diseases in the U.S.A. Its carcinogenicity is three orders of magnitude greater than that of regulated asbestos. Here we report the results of the investigation of the structural and crystal chemical modifications occurring in erionite leached with artificial lysosomal fluid (ALF) and Gamble's solution. ALF leaching produces a migration of Na+ ions from Ca1 to Ca2 extraframework cationic site, without the occurrence of any significant modification of the chemical composition of the fibers. In contrast, leaching with Gamble's solution induces a complex ionic-exchange process resulting in a temporary partial replacement of Na+ by Ca2+, coming from the fluid, which is fixed at a third Ca3 cationic site. Subsequently, the exchange process reverses. In fact, Ca2+ is removed from Ca3 and Na+ migrates back to Ca1, the structure being indistinguishable from the starting, unleached material. Such processes seem to be accompanied by a progressive amorphization of fibers. Present data could provide valuable background for a more detailed comprehension of the morphostructural/biological activity relationships inducing pathogenicity. [ABSTRACT FROM AUTHOR]

detailed morphological, crystal-chemical, and structural characterization of erionite from the type locality of Durkee, Oregon, has been carried out by combining field emission scanning electron microscopy (FESEM) and laboratory parallel-beam transmission X-ray powder diffraction (XRPD). According to the crystal-chemical formula (Na5.38K1.99Mg0.24)[Al7.66Si28.34O72.09]⋅29.83H2O, the sample has been classified as erionite-Na. The Rietveld refinement has indicated that the extraframework cations are located at three Ca1, Ca2, and Ca3 sites, the first one containing all available Mg. Moreover, the absence of the additional K2 site found in both dehydrated erionite and erionite-K has been demonstrated for this erionite sample. Furthermore, our results revealed the absence of Fe and Ca although previous investigations have reported the presence of a variable content of both these elements in erionite samples from Durkee. This is relevant information because it is well known from amphibole asbestos that Fe2+ has been claimed to be one of the causes of carcinogenesis by participating in Fenton chemistry and producing free radicals. [ABSTRACT FROM AUTHOR]

Abstract: The dehydration dynamics and the thermal stability of a sample of erionite-K, (Na2.00K2.87Mg0.80Ca0.86) [Al6.95Si29.05O72.64]·30.5H2O, have been investigated in the 303–1253K thermal range by in situ X-ray powder diffraction. The structural breakdown starts to occur at 1113K and is completed at 1193K. The dependence of cell parameters and volume from temperature is fairly complex. As general trends, the c-parameter expands whereas the a-parameter contracts at a faster rate and, as a net result, the cell volume contracts. The smallest cell volume is observed at 1093K just before the incoming of the structural breakdown. Microstrain ε 0 is remarkably constant from 303K up to approximately 1000K. At this temperature, ε 0 starts to increase, at a fast rate, from ca. 0.05 to a maximum value of ca. 0.09 at 1113K, in correspondence of the starting of the structural breakdown. As the structural breakdown proceeds, microstrain is increasingly released until the complete structure disruption is attained. The most relevant framework modifications with increasing temperature are: (a) the squeezing of the 8-member ring (8MR) leading to a reduction of the Crystallographic Free Area (CFA) from ca. 14 to ca. 12.4Å2; (b) the expansion of the 6-member ring (6MR) shared between the cancrinite cage and the erionite cavity permitting the cation diffusion between them; (c) the expansion of the base of the erionite cavity to host the migrating Ca2 cation. The evolution of the distribution of the site scattering (s.s.) of the extra framework (EF) cations as a function of temperature is quite complicated. It may be roughly described as a general depletion of the Ca1, Ca2, and K1 sites counterbalanced by an increase of the K2 s.s. The observed reduction of the site scattering of the K1 site has been explained by invoking an “internal ion exchange” mechanism whose detailed description is difficult because of the simultaneous occurrence of three cationic species (Na, K, and Ca). In fact, the process should lie anywhere between two extremes, the partial substitution of Na for K (at a 50:50K:Na proportion at high temperatures) and the complete substitution of Na+Ca for K (at a 55:45 Na:Ca proportion at high temperatures). The full depletion of the various water molecule sites is attained in the 413–573K thermal range, in good agreement with TG data, the only exception being OW7 that shows significant s.s. up to the structural breakdown. This fact has been justified considering OW7 as a site partially occupied by both water molecules and EF cations. [Copyright &y& Elsevier]

The document titled "Correction to: Karlleuite Ca2MnO4 – a first mineral with the Ruddlesden-Popper type structure from Bellerberg volcano, Germany" in the journal Mineralogy & Petrology addresses errors in the original article. The corrections involve the structure of authors' names, the addition of Biljana Krüger as a corresponding author, and the removal of e-mail addresses. The corrected version of the article is now available online. [Extracted from the article]

Small-pore zeolites are successfully employed as catalysts, sorbents and molecular sieves. Their physiochemical properties can be tuned by modifying their extraframework cation (EF) composition via ion exchange. In this study, we investigate the crystal structure of a Cd-exchanged levyne (LEV) intergrown with erionite (ERI) by combining Single Crystal X-ray Diffraction (SCXRD), Molecular Dynamic simulations (MD) and Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). Data obtained from the different techniques consistently indicate that Cd2+ in LEV is arranged in a nearly ordered fashion. In contrast, strong disorder of the EF species (Cd2+ and H 2 O) is observed in the ERI cavities. Here, Cd2+ forms aqueous complexes that are more mobile in comparison to LEV , where Cd2+ bonds to both H 2 O and framework-oxygen atoms. The formation of Cd-clusters is excluded based on EXAFS analysis. Finally, to discriminate between thermal and static disorder, we propose a new approach based on combined MD and geometry optimization. Image 1 • The crystal structure of Cd-LEV and Cd-ERI was determined by SCXRD. • Cd2+ has a fairly regular octahedral coordination in Cd-LEV. • Cd2+ is strongly disordered in the eri cavity. • Cd aqueous complexes form in Cd-ERI. • The disorder of the EF species was resolved by combining MD simulations and XAFS. [ABSTRACT FROM AUTHOR]

The modification of natural zeolites via ion exchange is an efficient technique used to improve their performances and tune their properties for specific applications. In this study, a natural levyne- Ca intergrown with erionite was fully exchanged by Ag+ and its structure [with idealized chemical composition Ag6(Si,Al)18O36·18H2O] was investigated by combining a theoretical and experimental approach. Single-crystal X-ray diffraction data demonstrated that Ag-levyne maintained the R3m space group, characteristic of the natural levyne. Ag ions distribute over partially occupied sites along the threefold axis and, differently from the pristine material, at the wall of the 8-membered ring window of the lev cavity. The lack of ~30% of Ag ions that could not be located by the structural refinement is ascribed to the strong disorder of the extraframework occupants. The structural results obtained by Molecular Dynamics simulations are in overall agreement with the experimental data and showed that, on average, Ag+ is surrounded by ~2 H2O and 1 framework oxygen at distances between 2.43 and 2.6 Å. Molecular Dynamics trajectories indicate that the occurrence of silver inside the D6R cage depends on the water content: silver occupancy of D6R cages is estimated to be 83, 30, and 0% when the structure contains 3, 2.5, and 2 H2O per Ag ion, respectively. The cation-exchange process, as demonstrated by scanning electron microscopy and energydispersive spectroscopy (SEM-EDS) spectrometry, affects the intergrown erionite as well. A structural characterization of the Ag-erionite phase (with dimension <100 μm) was possible by means of a CuKα micro-focus source: structure solution pointed to P63/mmc space group, indicating no change with respect to natural erionite. In agreement with previous studies, K ions in the cancrinite cage could not be exchanged, whereas Ag+ is found in the eri cavity. [ABSTRACT FROM AUTHOR]

Inhalation of fibrous erionite particles has been linked to malignant mesothelioma. Accordingly, erionite is considered the most carcinogenic mineral. The reactivity and the nature of erionite biotoxicity has been the subject of intensive research. Despite very close chemical and structural relationships between erionite and offretite, the reactivity of offretite in lung fluids remains unknown. In this paper, the interaction of erionite and offretite surfaces with simulated lung fluids was investigated by means of in situ atomic force microscope (AFM). To simulate different environments in the lungs, artificial lysosomal fluid (ALF) and Gamble's solution were used. In ALF (4.15 < pH < 4.31) the dissolution of erionite and offretite surfaces was detected, as well as an evident removal of particles (mainly attributed to impurities) from the crystal faces. Instead, the growth of a layer of a yet unknown phase on the surface of both zeolites was observed during the interaction with Gamble's solution (7.4 < pH < 8.48). The thickness of this layer reached a few tens of nanometers and covered all the observed areas. The understanding of the observed processes is of paramount importance, since they could be potentially involved in the mechanisms triggering the toxicological effects of erionite fibres. [ABSTRACT FROM AUTHOR]

Erionite is a mineral belonging to zeolites group occurring with prismatic or fibrous habit. The exposure of humans to erionite fibres has been unambiguously linked to the development of several pulmonary diseases, e.g. malignant mesothelioma, and at present the erionite is classified as Group I carcinogenic mineral (WHO, 1986). Moreover, a second zeolite called offretite, typically prismatic but closely related to erionite from the chemical and structural point of view, has also been found under asbestiform habit (Mattioli et al., 2018). Despite the great number of researches, the relationships between mineralogical features and biological activity of erionite have not been fully understood. In opposite, there are no studies regarding the potential hazard of offretite fibres, and it is unclear whether the mineralogical distinction between erionite and offretite has any health implications.Recently, the biodurability and dissolution of erionite fibres after leaching with artificial simulated lung fluids (SLFs) (e.g. Gualtieri et al., 2018) was investigated. Although these studies were extremely useful to determine the eventual response of erionite fibers in lung environment, at present, information on modifications occurring at nanometric scale at the fibres surface are still missing. Considering that the chemical reactions involved in pathogenesis mainly take place at the solid-liquid interface, this kind of information is of paramount importance. With this aim, the interactions of erionite and offretite with SLFs were investigated in real-time by means of in situ AFM.To simulate different environments in the lungs, Artificial Lysosomal Fluid (ALF) and Gamble's solution were used (Stopford et al., 2003). Significant morphological changes on zeolite-surfaces at nanometric scale were observed. In ALF (4.15 < pH < 4.31) superficial dissolution of erionite and offretite crystals occurred. In particular, an evident removal of adhered particles (mainly impurities with diameter ranging from <30 nm to ~2 µm) from the zeolites surface was detected. Instead, a layer of an unknown phase, likely amorphous, on the surface of both zeolites, grew during interaction with Gamble's solution (7.4 < pH < 8.48). The thickness of this layer reached few tens of nanometers but covered all the investigated area. These results highlight how different and complex the response of fibers surface are with respect to the bulk mineral. The understanding of these aspects is crucial since they could potentially be involved in the reactions triggering the carcinogenic process.WHO. 1986. Asbestos and Other Natural Mineral Fibers. Environmental Health Criteria, 53: pp. 69-107. Geneva, World Health Organization.Mattioli, M., Giordani, M., Arcangeli, P., Valentini, L., Boscardin, M., Pacella, A., and Ballirano, P. 2018. Prismatic to Asbestiform Offretite from Northern Italy: Occurrence, Morphology and Crystal-Chemistry of a New Potentially Hazardous Zeolite. Minerals, 8(2), 69.Gualtieri, A. F., Pollastri, S., Gandolfi, N. B., and Gualtieri, M. L. 2018. In vitro acellular dissolution of mineral fibres: A comparative study. Scientific reports, 8(1), 7071.Stopford, W., Turner, J., Cappellini, D., and Brock, T. 2003. Bioaccessibility testing of cobalt compounds. Journal of Environmental Monitoring, 5(4), 675-680. [ABSTRACT FROM AUTHOR]

The structural transformations occurring as a function of increasing temperature in zeolites are of interest because the porous structure, and therefore the physical properties, can significantly change. Zeolites with THO framework type are small-pore materials, which received attention because of their applications in catalytic processes. The majority of THO zeolites (synthetic and natural) are characterized by an ordered distribution of the cations at the tetrahedral sites. To date, few cases of disordered thomsonite have been reported. In this study, we investigated the dehydration behaviour of a natural thomsonite with disordered Si/Al distribution and chemical composition Ca 3 · 34 Na 2 · 66 Si 11 Al 9 O 40 ∙12H 2 O. The structure was determined from room temperature (RT) to 698 K in order to compare the thermal behaviour with that reported for the ordered variety. Accurate structural analysis was performed by in situ single crystal X-ray diffraction. The dehydration starts at 348 K. Up to 498 K, thomsonite gradually releases four H 2 O. From 498 to 573 K, additional four H 2 O are lost and the space group changes from orthorhombic (Pbmn) to monoclinic (P 2 1 /n). This partially hydrated phase is characterized by a unit-cell volume contraction of −3% with respect to the RT phase and by a rearrangement of the extraframework cations in the zeolitic pores. The thermally treated thomsonite is able to reabsorb 50% of the lost H 2 O and transforms to the orthorhombic phase, equivalent to that observed at lower temperatures. However, the diffraction pattern analysis indicated a high degree of mosaicity, most probably due to the residual stress accumulated during the phase transformation. [Display omitted] • A Si/Al disordered thomsonite was investigated from RT to 698 K in situ by SC-XRD. • A phase transformation from orthorhombic to monoclinic occurred at 573 K. • The monoclinic phase is partially hydrated. • The structure is able to reabsorb 50% of the lost H 2 O. [ABSTRACT FROM AUTHOR]

Two samples of fibrous erionite from different localities of Oregon, USA were suspended in FeCl 2 solutions at different concentrations, at pH ca. 5, in anaerobic conditions. Comparison between released and acquired charges under the form of Fe confirms that erionite binds Fe (II) by ion exchange with the extra framework (EF) cations, mainly Na. The Fe (II) binding process exhibited by both studied samples here investigated indicates a direct correlation between the extent of the ion-exchange process and the Fe (II) concentration of the solution used for fibres incubation. In the sample from Rome no further Fe (II) acquisition was observed for concentrations exceeding 500 μM FeCl 2 . XPS investigation revealed that, when comparing surface and bulk composition of the samples, no Fe enrichment of the fibre surface was observed. Moreover, the location of Fe (II) within the erionite cage has been devised by combining chemical and structural data. Results highlight that, for both samples, Fe (II) is fixed at the Ca3 site being six-fold coordinated to water molecules. The occurrence of Fe (II) within the erionite cage causes a gradual migration of the other EF cations and in addition, induces a small rearrangement of the water molecules positions. It is worth mentioning that, in Fe-loaded zeolites at rather low Fe content, the Fe sites with very low nuclearity, located in well-defined crystallographic positions, represent the active sites for the formation of reactive oxygen species. Therefore, identification of segregation of Fe (II) at Ca3, which is coupled with the high surface area of erionite, provides very important information for the understanding of the molecular mechanism/s inducing its strong carcinogenicity. [ABSTRACT FROM AUTHOR]

Structural properties of Pb-exchanged zeolites are of interest because of their applications in environmental remediation and in industrial processes. In this study, we report on a Pb-exchanged stellerite (Pb-STI), with particular focus on the cationic species, which form inside the zeolitic pores as a result of the exchange experiments. The produced zeolite had chemical composition Pb 13.4 (OH) 10 Al 17.4 Si 54.6 O 144 ∙38H 2 O, indicating that Pb was uptaken in form of [Pb(OH) x ](2-x) species. The STI framework maintained the Fmmm space group characteristic of the type material. However, the extraframework occupants, Pb2+, H 2 O and OH−, were characterized by a strong positional-disorder. The latter was resolved and interpreted combining Extended X-ray Absorption Fine Structure (EXAFS) analysis with Molecular Dynamics (MD) simulations. On average, Pb2+ ions are coordinated by 2 OH− and 1H 2 O at distances <2.5 Å, whereas bonds to framework oxygen-atoms were found only at longer distances (>2.8 Å). Pb2+ adopts mainly a sided distorted coordination, indicating a stereochemical activity of the lone pair electrons. The obtained results were compared with those of other mono-cationic forms of STI zeolites. Based on the analysis of the framework distortion experienced after the incorporation of different metal ions, considerations are drawn on the potential effect of Pb2+ on the thermal stability of STI framework type zeolites. [Display omitted] • A STI zeolite was Pb-exchanged in 0.5 M Pb(CH 3 COO) 2 solution at pH = 5-6. • No framework distortion from the Fmmm symmetry was observed. • The EF species (Pb2+, OH−, and H 2 O) were highly disordered in the cavities. • Pb2+ is sided coordinated by OH− and H 2 O. • No significant interactions occur between Pb and framework oxygen. [ABSTRACT FROM AUTHOR]

The exposure of humans to erionite fibres of appropriate morphology and dimension has been unambiguously linked to the occurrence of Malignant Mesothelioma. For this reason, a detailed morpho-structural investigation through Electron Microscopy techniques has been performed on erionite samples collected at two different localities, Durkee (ED) and Rome (ER), Oregon, USA. The sample from Rome has been also investigated after a prolonged leaching with Gamble's solution (ER4G) in order to evaluate the possible occurrence of morpho-structural modifications induced by this Simulated-Lung-Fluid (SLF). Here we report how the micrometric erionite fibres evolve in irregular ribbon- or rod-like bundles as a function of different nano-structural features. The reasons for the observed morphological variability have been explained by considering the structural defects located at ED surface fibrils (bi-dimensional ribbons) and the presence of nontronite, an iron-bearing clay mineral embedding the ER fibrils (mono-dimensional rods). ER4G shows a decrease in width of the rod-like fibres due to their partial digestion by SLF leaching, which synchronously dissolves nontronite. The reported results represent a valuable background toward the full comprehension of the morphological mechanisms responsible for potentially damage of lung tissue through the potential relocation of fibers to extrapulmonary sites, increasing the carcinogenic risk to humans. [ABSTRACT FROM AUTHOR]