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1 Dipartimento di Scienze della Terra, Sapienza Università di Roma, P.le A. Moro, 5, I-00185 Rome, Italy
2 CNR-IGAG, Istituto di Geologia Ambientale e Geoingegneria, Sede di Roma, Via Bolognola 7, I-00138 Rome, Italy
3 CNR-IGG, Istituto di Geoscienze e Georisorse, Sede di Roma, P.le A. Moro, 5, I-00185 Rome, Italy
4 Department of Geological Engineering, Hacettepe University, Ankara, Turkey
5 Department of Geological Engineering, Ankara University, Ankara, Turkey
6 Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242, U.S.A.
Correspondence: * E-mail: umran-dogan{at}uiowa.edu
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ABSTRACT |
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, 0, 0), which is empty in erionite-Ca. In addition, site Ca1 shows low occupancy and Ca3 is vacant. The oxidation and coordination state of Fe, whose occurrence was revealed by chemical analysis, have been clarified by exploiting room- and low-temperature 57Fe Mössbauer spectroscopy. The majority of Fe (95%) was attributed to Fe3+-bearing, superparamagnetic, oxide-like nanoparticles with dimensions between 1 and 9 nm, and the remaining 5% was attributed to hematite particles with size 
10 nm, both located on the crystal surface.
Key Words: Oregon erionite • erionite-K • crystal structure • iron topochemistry • field emission • scanning electron microscopy • laboratory parallel-beam transmission powder diffraction • Rietveld refinement • 57Fe Mössbauer spectroscopy
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INTRODUCTION |
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) cages connected along the z direction by double 6-rings (D6R) (Staples and Gard 1959). Erionite cages (23-hedra) are formed by linking adjacent columns via single 6-rings (S6R) at the level of -cages (Fig. 1
). The erionite framework can also be described by the stacking along the z direction of 6-rings following the AABAAC sequence. Intergrowth of erionite with offretite (OFF) may occur, due to the close similarity between the two zeolites. In fact, offretite has an AABAAB sequence, and stacking faults corresponding to partial substitution of C with B 6-rings have been observed (Schlenker et al. 1977). Erionite is hexagonal, space group P63/mmc (Kawahara and Curien 1969) and has an average formula K2(Na,Ca0.5)8[Al10Si26O72] · 30H2O (Coombs et al. 1997). A large chemical variability is typical of this mineral, and for this reason three different species are identified, according to the most abundant extra-framework cation: erionite-Na, erionite-K, and erionite-Ca (Coombs et al. 1997; Passaglia et al. 1998; Gualtieri et al. 1998; Dogan and Dogan 2008).
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The Fe presence/absence, as well as its content, oxidation, and coordination state, and behavior, are of particular relevance. In fact, it is well known from amphibole asbestos that Fe has been claimed to be one of the causes of carcinogenesis by participating in Fenton chemistry and producing free radicals (e.g., Kane et al. 1996; Fubini and Otero Aréan 1999; Kamp and Weitzman 1999; Robledo and Mossman 1999). Similarly, erionite toxicity was ascribed to ion-exchanged and/or surface-deposited Fe that participates in Fenton chemistry (Eborn and Aust 1995). Moreover, it was proposed that even upon inhalation of Fe-free fibers, they can imbibe Fe via ion-exchange because protein injury can lead to metal ion release (Carr and Frei 1999). In any case, whether Fe is carried on from erionite or is captured in the lungs, it was highlighted that the Fenton chemistry occurs on the surface of the fibers and that the coordination state of Fe is critical for inducing cytotoxicity and genotoxic damage (Fach et al. 2003; Ruda and Dutta 2005). Moreover, the molecular mechanisms by which mineral fibers induce genotoxic damage remain unclear (or poorly understood so far), since the appearance of malignant mesothelioma in humans has been found to be also dependent on genetic predisposition (Dogan et al. 2006, 2008; Bertino et al. 2007; Carbone et al. 2007).
Morphostructural/biological activity relationships in erionite is a crucial subject still to be clarified, and a detailed crystal-chemical investigation would be a fundamental first step. The present work reports new data on the crystal chemistry of erionite-K from Rome, Oregon, from combined field emission scanning electron microscopy (FESEM), 57Fe Mössbauer spectroscopy (MS), and laboratory parallel-beam transmission powder X-ray diffraction data (PXRD).
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EXPERIMENTAL METHODS |
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). They consist of the {10
0} (hexagonal prism) and {0001} (pinacoid) forms. However, the prevailing morphology is that of very thin fibrils arranged to form fibers and bundles up to 50 µm of length. The diameter of individual fibrils forming the erionite bundles is of the order of 0.5 µm or less (Fig. 2b). No clear evidence of any crystalline phase other than erionite was observed.
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) and 77 K (Fig. 3b) in a liquid nitrogen cryostat, using a conventional spectrometer system operated in constant acceleration mode with a 57Co source of nominal strength of 25 mCi in an Rh matrix. Data were recorded in a multichannel analyzer using 512 channels for the velocity range –10 to 10 mm/s. To have good statistics, up to 10 million counts per channel were collected. After velocity calibration against a spectrum of high-purity 
-iron foil (25 µm thick), the raw data were folded to 256 channels. The spectra were fitted using the Recoil 1.04 fitting program (Lagarec and Rancourt 1998). A first cycle was done fitting pure Lorentzian line shapes, and results were quite satisfying (
2 < 1 and linewidths up to 0.50 mm/s). In spite of this, a second refining cycle was performed fitting quadrupole splitting distributions (QSD), following the successful approach of Rancourt and Ping (1991) and Rancourt (1994a, 1994b). Several fitting models with unconstrained parameters were tried to get a best fit. Results of both the Lorentzian and the QSD approach were in agreement and this gave a good confidence in the physical correctness of the distribution analysis. Uncertainties were calculated using the covariance matrix and errors were estimated to be approximately ±5% for absorption areas (Table 1).
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- geometry. It is fitted with an incident-beam Göbel mirror, diffracted-beam radial Soller slits, and a PSD VÅNTEC-1 detector. Diffraction data were collected in the angular range 5–140 °2, with a step-size of 0.0214 °2, using CuK radiation and were evaluated by the Rietveld method using the GSAS crystallographic suite of programs (Larson and Von Dreele 1985) coupled to the EXPGUI graphical user interface (Toby 2001). The background was fitted with a 36-term Chebyshev polynomial of the first kind to model the amorphous contribution arising from the capillary. Peak-shapes were fitted by the TCH (Thompson, Cox, Hastings) pseudo-Voigt function (Thompson et al. 1987), modified for asymmetry (Finger et al. 1994). Refined variables were GV and GW (tan -dependent and angle-independent, respectively) Gaussian parameters, LX and LY (1/cos - and tan -dependent) Lorentzian parameters, and S/L and H/L asymmetry parameters (constrained to be equal in magnitude). Peak positions were corrected for sample displacement from the focusing circle. An absorption parameter was refined. Starting structural parameters were those of Alberti et al. (1997) consisting, apart from the framework, of four cationic sites [K (hereafter renamed as K1), Ca1, Ca2, and Ca3] and six H2O-molecules sites (OW7, OW8, OW9, OW10, OW11, and OW12). Immediately Ca3 site occupancy reduced to zero and, for this reason, the site was removed from the refinement. Difference Fourier maps calculated from the refined model located a further cationic site at special position , 0, 0 corresponding to the K site reported by Schlenker et al. (1977) for dehydrated erionite and hereafter labeled K2. Bond distances and angles were completely unrestrained. Displacement parameters for K1 and K2, Ca1 and Ca2, and OW8, OW9, OW10, OW11, and OW12 were constrained to be equal in magnitude (Ow7 unconstrained as lying on mm2). However, after a few cycles of refinement the Uiso isotropic displacement parameter of OW7 was set and kept fixed to the value of 0.08 Å2 reported by Alberti et al. (1997), because of strong correlation with the corresponding site occupancy. Site populations for all cationic and H2O-molecules sites were refined. Attempts to model the presence of preferred orientation by means of the generalized spherical harmonics description of Von Dreele (1997) produced a small improvement of the fit as a result of a J texture index of 1.105. Convergence was reached at Rp = 2.64%, Rwp = 3.31%, RF2 = 4.31%, and reduced 2 = 4.52. At this point the structural data were used and recycled in a further refinement using TOPAS v. 3.0 (Bruker AXS 2005). This program implements the Fundamental Parameters Approach FPA (Cheary and Coelho 1992) and is claimed to be able to provide a more accurate description of the peak-shape. In fact, keeping the same constraints on displacement parameters, we were able to significantly reduce the agreement indices to Rp = 1.93%, Rwp = 2.60%, RBragg = 0.80%, and reduced 2 = 3.54. No significant structural differences were observed between the two refinements. Experimental details and miscellaneous data of the latter refinement are reported in Table 2, positional and displacement parameters of non-hydrogen atoms in Table 3, relevant bond distances and angles in Table 4, bond valences (according to Brese and O’Keeffe 1991) in Table 5, and experimental, calculated, and difference plots in Figure 4.
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RESULTS |
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TopAbstractIntroductionExperimental methodsResultsDiscussionAcknowledgmentsReferences cited |
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), but the crystal chemical formula based on the second one is more accurate—because the oxides were not measured for Sample 1—and close to erionite stoichiometry K2(Na, Ca0.5)8 (Al10Si26O72)· 30H2O.
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It is worth noting that, from comparison of the Mössbauer absorption spectra collected on the erionite-K from Rome, Oregon, with those collected on synthetic, microporous zeolites (with known Fe content), the average Fe content of our sample should not be higher than 1 wt%. Many attempts to quantify Fe content of other erionite samples (Dogan and Dogan 2008) gave values very different from point to point, supporting in this way the hypothesis of Fe coming from heterogeneous impurities. Passaglia et al. (1998) reported compositions of both erionite and offretite samples from Oregon. Figure 5 shows corresponding extra-framework cation compositions of the present erionite sample and Passaglia et al. (1998) samples.
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The 57Fe Mössbauer spectrum of erionite collected at RT (Fig. 3a
) shows a broad absorption signal, which was initially modeled by a close quadrupole doublet with an isomer shift (IS) of 0.34 mm/s relative to -iron and a quadrupole splitting (QS) of 0.37 mm/s, indicating the presence of high-spin Fe3+ ions. However, the large value of linewidth (0.75 mm/s) suggested the presence of at least two overlapping signals. A second model with two doublets gave better resolution, with a first doublet (80% of Fetot) characterized by IS = 0.34 mm/s, QS = 0.34 mm/s, and linewidth of 0.50 mm/s, and a second doublet (20% of Fetot) characterized by IS = 0.38 mm/s, QS = 1.17 mm/s, and linewidth of 0.40 mm/s. On the basis of the IS values, the two doublets may be ascribed to Fe3+ octahedrally coordinated (Meagher et al. 1988). A third model was also tried, and a third doublet (5% of Fetot) with IS = 0.16 mm/s, QS = 0.64 mm/s, and linewidth of 0.20 mm/s was refined, but the area of the third doublet showed an error larger than its value and the model was rejected.
Structural analysis
Microstructural parameters were extracted by evaluating the integral breadths βi of the individual reflections from the X-ray diffraction data. A volume-weighted mean column height Lvol, defined as βi = 
/Lvol cos , of 1037(19) Å was refined. Moreover, an 0 microstrain (lattice strain), defined as βi = 40 tan , of 0.0479(14) was obtained.
Framework.
The two independent tetrahedral sites T1 and T2 (building, respectively the D6R and the S6R) are characterized by a disordered Si/Al distribution. In fact, <T1-O> – <T2-O> is –0.021 Å, clearly indicating the preferential partition of Al on T2. From Jones’ (1968) determinative curve a distribution of Al as 14.3% of T1 (corresponding to 3.44 apfu) and 27.8% of T2 (corresponding to 3.33 apfu) has been calculated. Therefore, a total of 6.77 Al apfu was estimated, as compared to 6.95 apfu from recalculated chemical data. However, it is well known that, in the case of a disordered Si/Al distribution, the curve of Jones provides systematically underestimated values. Moreover, according to the regression equation of Passaglia et al. (1998) [R = Si/(Si+Al) = 5.65149 – 0.00213 x Volume (Å3)], an R of 0.788 has been determined. This value is slightly smaller than 0.807 from chemical data and 0.817 from the equation of Jones (1968).
No linear correlation between –sec(T-O-T) and 
(T-O), the deviation of an individual T-O bond distance from <T-O>, predicted from Extended Hückel Molecular-Orbital (EHMO) theory and observed by Schlenker et al. (1977) in dehydrated erionite has been observed. Moreover, a check carried out on data of Alberti et al. (1997) confirms that this relationship does not hold for such erionite samples.
Individual T-O-T bond angles are consistent with those reported by Alberti et al. (1997) and Gualtieri et al. (1998), the exception being the anomalous Agate Beach sample showing a T2-O6-T2 angle significantly smaller than the average value of 168(6)°. As expected, differences are observed in dehydrated erionite (Schlenker et al. 1977) due to the partial collapse of the framework.
Despite the very good agreement indices obtained from the refinement, an attempt to detect the presence of offretite stacking faults was carried out following the procedure of Schlenker et al. (1977). However, after a few cycles of refinement the population of the extra Si and O sites required to describe the B'C' ring decreased to zero, providing indication of the absence or very limited presence of stacking faults.
Extra-framework cations and H2O molecules.
The cancrinite cage is filled by K, as reported by Gard and Tait (1973), Alberti et al. (1997), and Gualtieri et al. (1998). Structure refinement indicates an almost complete occupancy of the K1 site. The K atom is 12-fold coordinated by six O2 atoms at 2.926(3) Å and six very distant O3 atoms at 3.367(3) Å. However, the presence of the latter O atoms provides ca. 0.2 valence units (v.u.) to reach a reasonable total of 0.906 v.u., confirming that the inclusion of higher coordination shells is important in the evaluation of the bond valence in the case of alkali cations (Adams 2001).
Significant differences are reported with respect to the crystal structure of Alberti et al. (1997) with respect to the position and site population of the erionite cage. This is most likely due to the presence of significant K to be located within this cage. Three cationic sites and six H2O sites (similar to Alberti et al. 1997) have been identified. Ca1 and Ca2 are both approximately located at the same z coordinate of the corresponding sites reported by Alberti et al. (1997) and Gualtieri et al. (1998) (note the inversion of Ca1 and Ca2 labels in Gualtieri et al. 1998), but they are slightly displaced from the threefold axis. This displacement provides more favorable coordination by the O atoms of both water molecules and framework. Large displacement parameters for both cationic and H2O-molecule sites reasonably indicate that the determined positions represent only an average between neighboring split sites, mutually exclusive, occupied by different chemical species in the case of cationic sites, and by different H2O-molecule distributions required for correct coordination. Attempts to model such positional disorder using split sites, not imposing constraints on bond distances and angles, failed due to high correlations between the refined parameters. The shortest Ca1–Ca2 contact of 3.314 Å does not imply any constraint on the occupancy of both sites. Further the cationic site labeled as K2 has been observed at special position , 0, 0. This corresponds to the K site found by Schlenker et al. (1977) in dehydrated erionite and to the Ca4 site found by Gualtieri et al. (1998) in Lady Hill, Shourdo, and Tunguska samples. As indicated in the experimental methods section, Ca3 site scattering (s.s.) decreased immediately to zero during the first refinement cycles. OW11, which is reported by Alberti et al. (1997) to be approximately coplanar with Ca3, shows x and y coordinates slightly displaced toward the axis of the cage. Both behaviors could be reasonably explained by the presence of a very small site occupancy at Ca3, impossible to model with the present powder data set. This hypothesis is also supported by the fact that standard deviations of fractional coordinates of OW11 are significantly higher than those of other H2O-molecule sites showing a comparable site population.
According to bond-valence analysis (Table 5), Ca1 and Ca2 both offer different possible coordination for different cationic species. In particular, taking into account the mutual exclusion due to very short contacts among H2O molecules, Ca1 provides a very favorable distorted sixfold coordination for Mg [Ca1-OW11 x2 of 2.05(11) Å, Ca1-OW9 x2 of 2.16(5) Å, and Ca1-OW8 x2 of 2.37(8) Å] (Fig. 6). Calcium could alternatively receive a possible eightfold coordination, but that is prevented by short OW9–OW10 contacts. Thus, this is the only chemically reasonable location for Mg that is the carrier of a s.s. of 9.6 e– compared to the refined value of 13(2). The relevant standard deviations of fractional coordinates, site occupancy, and displacement parameter could result from correlation of parameters. Therefore, we may conclude that Ca1 is filled by Mg. It is worth noting that Mg is exclusively coordinated by water molecules, as found in natural and synthetic zeolites. It should be pointed out that a few short contacts occur within the sixfold coordination, in particular OW8–OW9 of 2.10(2) and OW8–OW11 of 2.15(5) Å. However, this fact can be explained, as previously indicated, considering that the determined positions represent only an average between very close neighboring split, mutually exclusive sites, occupied by different H2O molecules.
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). Considering this site as occupied by all available Na and Ca (Mn) cations, we obtain a s.s. of 39.7 e– exactly coincident with the refined value.
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-cage to the center of an eight-membered ring. H2O molecules are distributed among six sites. Total refined s.s. for H2O molecules corresponds to 30.5 water molecules pfu, a value consistent with literature data (i.e., Coombs et al. 1997; Alberti et al. 1997; Passaglia et al. 1998) but higher than the 25.7 H2O molecules obtained from thermogravimetric analysis. However, scarcity of material could lead to significant error in the TG analysis. Site population is significantly different with respect to that reported by Alberti et al. (1997), but this clearly reflects the different cationic content of the erionite cage. The most striking difference, apart from site occupancies, is related to the OW8 position, which is significantly displaced along the z axis. This is required to provide a favorable coordination for Ca1.
Similarly to Alberti et al. (1997), a few OW-OW very short contacts prevent the contemporaneous presence of OW9, OW10, and OW12, as well as OW7 and OW11. It is worth noting that the refined sum of OW7, OW11, OW9, and OW12 occupancies [1.11(4) and 1.07(3), respectively] slightly exceeds unity. This can be an effect of correlation between displacement parameters and occupancy as well as, for OW11, that its position and occupancy are not well defined due to the electronic contribution of a neighboring low-occupancy cationic site.
Therefore, we can consider the above OW-OW contacts as never occurring because the sites cannot be simultaneously occupied. To investigate the geometry of the hydrogen bond network possibly occurring, O-O contacts were also identified. Selecting the 2.5–3.2 Å range, as reported by Chiari and Ferraris (1982), OW8, OW11, and OW12 were found to be linked exclusively to H2O molecules.
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DISCUSSION |
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). All channels are equidimensional with elliptical 8-ring openings of 0.36 x 0.51 nm (Hernandez et al. 2000). The hypothesis of surface Fe, that is, external Fe3+-O clusters bound by either weak physisorption or strong chemisorption, must therefore be explored. Attributing this Fe to impurities, the total refined cationic s.s. (105.2 e–) and the s.s. retrieved from recalculated chemical data (106.5 e–) are in very good agreement (Table 4).
The most common iron oxide and hydroxide with Fe3+ octahedrally coordinated are -Fe2O3 (hematite) and -FeO(OH) (goethite), respectively. Magnetically ordered materials such as hematite and goethite generally give a six-line (sextet) 57Fe Mössbauer spectrum, which was not observed in our spectrum. However, in the case of very small particles (<10 nm) the magnetic sextet collapses into a doublet, and the phenomenon is known as superparamagnetic relaxation (Kündig et al. 1966; Meagher et al. 1988; Bachari et al. 2007). Since the superparamagnetism influence decreases with temperature, another spectrum of erionite was collected at 77 K (Fig. 3b). The low-temperature spectrum is not very different from the RT one and it was modeled by two quadrupole doublets and, tentatively, one magnetic sextet. The IS of all doublets and sextet is about 0.5 mm/s confirming the presence of high-spin Fe3+ ions. The numerical difference with the RT spectrum is due to an effect known as second-order Doppler shift, which makes the IS increase when temperature decreases. The absorption area of the sextet is very small (5% Fetot) and comparable with the error estimation, thus making its presence questionable, but the magnetic field measured for it is about 54 T, a value typical of hematite. On this basis, two Fe species may be identified: magnetic hematite particles with size 10 nm, corresponding to about 5% of total Fe, and superparamagnetic oxide-like nanoparticles with size <10 nm, corresponding to about 95% of total Fe. Due to both the extremely limited particle size and the very low total Fe content, it is now clear why the diffractometric study, though detailed, could not reveal any signal from iron oxide.
The chemical activity of surface Fe oxide nanoparticles in zeolites is known to be very important, and is extensively studied because of catalytic applications (i.e., Bachari et al. 2007; Zecchina et al. 2007). In particular, Bachari et al. (2007) collected a Mössbauer spectrum virtually identical to ours from their synthetic zeolite sample at 77 K. Measuring spectra at extremely low temperature, they were able to further characterize the sizes of their nanoparticles, which clustered around 3 and 9 nm, and concluded that particles of such dimensions may agglomerate on the external surface of silica walls. In our case, the lower constraint is the channel size (<0.5 nm), so that we can reasonably assume that the Fe3+-bearing, superparamagnetic, oxide-like nanoparticles are located on the external surface of silica walls and have dimensions between 1 and 9 nm.
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT RECEIVED December 3, 2008; MANUSCRIPT ACCEPTED April 27, 2009
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