103,365 research outputs found
The P-C phase transition of synthetic amphiboles in the NCMSH system: an FTIR study
No full textDuring the last years, a series of experimental studies have been done to characterize the structural features of synthetic amphiboles in the Li2O-Na2O-MgO-SiO2-H2O (LNMSH system) under different T, P and X conditions. Camára et al. (2003) and Iezzi et al. (2004) showed that the Li-free Na0.8(Na0.8Mg1.2)Mg5Si8O22(OH)2 compound is P21/m at room-T, and Camára et al. (2003) characterized the displacive P21/m C2/m phase-transition by single-crystal XRD. The phase transition of this amphibole has a macroscopic second order character, with a transition temperature (TC) of 257 °C.
Iezzi et al. (2005) did a similar investigation on the Na(LiMg)Mg5Si8O22(OH)2 composition by means of synchrotron radiation HT-XRPD, and showed that this compound also undergoes a displacive phase transition as a function of T, but the transition has a tricritical character and TC = 326 °C. Furthermore Iezzi et al. (2006) showed that all amphiboles in the LNMSH system, independently of their Na-Li contents at the B site, have the P21/m symmetry at room conditions.
Natural Group 5 amphiboles which belong to the B(Na,Li)2, B(Na,Mn)2 and B(Ca,Mn)2 solid-solutions, whether their A site is filled or empty, have monoclinic C2/m symmetry. In contrast, the A-site filled synthetic samples belonging to the B(Na,Mg)2 and B(Li,Mg)2 solid-solutions, have P21/m symmetry at room T, and undergo phase-transitions at variable TC, depending on their chemical composition (Welch et al. 2007). The P21/m symmetry is restricted to amphiboles with a double-chain composition close to Si8 and is probably stabilized by small cations only at the B-group sites or by a B compositions where large monovalent cation (Na) and a small divalent cation (Mg) have a ratio close to 1:1. However, the dimension of the C-group cations also play a significant role in determining the transition behavior (cf. Welch et al. 2007 for more discussion).
In this work, we explore the effects on the phase transitions of a large divalent cation as Ca which progressively replaces BMg. The amphibole symmetry was monitored using in situ low- and high-T FTIR spectroscopy. Samples were prepared as conventional KBr disks (Iezzi et al., 2006); low-T spectra were collected using a nitrogen-cooled cryostat, while high-T spectra were collected using a Specac microfurnace. Previous work (Iezzi et al., 2004, 2006) show that OH-stretching FTIR spectroscopy is particularly useful in characterizing the symmetry changes associated with the P C transition in these amphiboles. For varying T conditions there are well-defined modifications of the room-T pattern, consisting of progressive band broadening and band splitting. Inspection of the data shows that for increasing BCa in the amphibole structure there is a linear decrease of the transition temperature TC for the Mg-richer samples, as also evidenced by the companion X-Ray diffraction study (Iezzi et al., this meeting).
References
Cámara F., Oberti R., Iezzi G., Della Ventura G. (2003) Phys. Chem. Miner., 30, 570-581.
Della Ventura G., Iezzi G., Bellatreccia F., Càmara F., Oberti R. (2004). Mitt. Oster. Min. Ges., 149: 23.
Iezzi G., Della Ventura G., Oberti R., Cámara F., Holtz F. (2004) Am. Miner., 89, 640–646.
Iezzi G., Tribaudino M., Della Ventura G., Nestola G., Bellatreccia F. (2005) Phys. Chem. Miner., 32, 515–523.
Iezzi G., Della Ventura G., Tribaudino M. (2006) Am. Miner., 91, 425-429.
Welch M.D., Cámara F., Della Ventura G., Iezzi G. (2007) Rev. Mineral. Geochem., 67, in press
Synchrotron infrared spectroscopy of synthetic Na(NaMg)Mg(5)Si(8)O(22)(OH)(2) up to 30 GPa: Insight on a new high-pressure amphibole polymorph
No full textThis paper describes a high-pressure synchrotron infrared (IR) Spectroscopy Study of the synthetic amphibole Na(NaMg)Mg(5)Si(9)O(22)(OH)(2). This compound has P2(1)/m symmetry at room conditions; its IR OH-stretching spectrum Consists Of two main bands at 3743 and 3715 cm(-1), which are assigned to the two symmetrically independent O-H groups in the P Structure (sample 403, Iezzi et al. 2004a). For increasing pressure, both bands shift toward higher frequency, suggesting a shortening of the O-H bond. In addition, the two bands progressively merge to give a single, symmetric and broad absorption band at 20-22 GPa. This behavior suggests that at 20-22 GPa there is a unique O-H group in the structure, indicative of a C-lattice type. The IR data thus show that the examined sample undergoes a P2(1)/m C2/m phase-transition at that pressure. Upon release of pressure, the initial two-band pattern is immediately recovered indicating that the pressure-induced phase-transition is reversible, as already observed for the same kind of transition induced by temperature. By analogy with structurally related pyroxenes, and taking into account the possible crystal Structural topologies of amphiboles, we suggest that the C2/m polymorph stable at high pressure is characterized by fully kinked double-chains
The analysis of fibrous amphiboles by FTIR spectroscopy in the OH-stretching region
No full textThe mechanism through which fibrous amphiboles may give rise to mesothelioma is still not completely understood. Several factors have been taken into account, and these include the morphological aspect (length:width ratio), the chemical composition, and a variety of surface properties which are ultimately responsible for the mineral-cellule interactions (e.g. van Oss et al., 1999). Studies in vitro demonstrate that the morphology of the fiber has a strong role in determining its biological danger, because very thin and long crystallites are hardly eliminated by the alveolar macrophages. However, a second factor which seems to influence the biological attack of a fiber is its Fe content; iron can in fact be involved in complex biochemical reactions with oxygen and cause DNA damages. It follows that the rapid determination of the chemical composition of a fiber, and of its Mg/Fe ratio in particular, is extremely important in environmental studies. The direct analysis can be achieved only by micro-chemical tools such as by EDS- or WDS-equipped electron microscopes. However, these techniques are extremely expensive and often unsuitable when dealing with extremely fibrous (diameter 2+ at the OH-coordinated M(1,3) sites (Della Ventura et al. 1996, 1997, Iezzi et al. 2005, 2007). The digitized spectra were fitted by interactive optimization followed by least-squares refinement; all bands were modelled as symmetric Gaussians. Della Ventura et al. (1996, 1997) showed that the binary site-occupancies at M(1) and M(3) in the amphibole structure are related to the observed intensities of the four (A to D) components in the principal IR OH-stretching spectrum. Using the original equations of Burns and Strens (1966):
M(1,3)Mg = 3IA + 2IB + IC and M(1,3)M2+ = IB + 2IC + 3ID (with M2+ = Fe2+)
where IA-ID are the intensities measured for the corresponding A to D bands, one can derive the (Mg, M2+) site populations at M(1,3) with a high degree of confidence. This method is particularly suitable for asbestiform materials which cannot be properly characterized by EMP. The present work shows that the above spectroscopic tool can be applied to a large variety of amphibole types. For species were significant (Mg, Fe) are present at M(4) (i.e. anthophyllite –cummingtonite - grunerite) an additional information (e.g. Mössbauer) is however required for a complete characterization of the sample.
References
Burns, R.G. and Strens, R.G.J. (1966) Science, 153, 890-892.
Della Ventura, G., Robert, J.-L., Hawthorne, F.C. (1996) Geochimica and Cosmochimica Acta, vol. spec. 5, 55-63.
Della Ventura, G., Robert, J.-L., Raudsepp, M., Hawthorne, F.C., Welch, M. (1997) American Mineralogist, 82, 291-301.
Iezzi, G., Della Ventura, G., Hawthorne, F.C., Pedrazzi, G., Robert, J.-L., Novembre, D. (2005) European Journal of Mineralogy, 17, 733-740.
Iezzi, G., Della Ventura, G., Bellatreccia, F., Lo Mastro, S., Gunther, M., Bandly, (2007) Mineralogical Magazine, in press.
van Oss, C.J., Naim, J.O., Costanza, P.M., Giese, R.F. Jr., Wu, W., Sorling, A.F. (1999) Clays and Clay Minerals, 47, 697-707
Crystal fabric evolution in lava flows: results from numerical simulations
No full textThe analysis of the preferred orientation of crystals (crystal fabric) in magmatic rocks has become a widely used technique for the reconstruction of the flow history. However, little is known about the evolution of the fabric during flow. Here, numerical simulations are used to study the fabric evolution of low-concentration, laminar magmatic flows (e.g. lava flows). The fabric evolution of (a) particle populations with a specified shape (cube, tablet, prism, and transitional shape) and (b) crystal populations from a lava flow is analyzed in different flow geometries (simple shear, hyperbolic and pure shear flows) assuming plane strain. Results show that fabric analysis of the whole crystal population gives little information about flow kinematics, whereas the comparative analyses of crystal sub-populations with different shapes allow us to recognize the flow geometry. Simple shear flow produces oscillating to pseudo-stable fabrics. The fabric strength is lower with respect to that of hyperbolic and pure shear flows and the preferred orientation of crystals does not coincide with the flow direction, except for large strain and specified shapes. Sub-fabrics with opposite sense of shear may also develop, depending on the crystal shape and finite strain. Pure shear and hyperbolic flows show stable to pseudo-stable fabrics. The preferred crystal orientation may or may not coincide with the flow direction according to whether flow is in pure or hyperbolic shear. Results from numerical simulations are comparable with those from experimental models and natural examples. The fabric strength depends on the number of crystals and caution must be used in extrapolating the results beyond the scale of observation. The finite strain in a sample from a lava flow from the Aeolian Islands is determined by the comparative analysis of the calculated and measured fabric parameters (fabric intensity and crystal preferred orientation). Criteria to discriminate among fabrics produced by different flow types are also provided
The potentialities of powder-diffraction from neutron scattering in the crystal-chemistry of amphiboles
No full textNeutron powder diffraction is nowadays a widely used technique in solid-state chemistry and physics. The low attenuation of neutrons and strong scattering power even at high “Q”, allow the collection of diffraction data from a wide range of reciprocal space, also under non-ambient conditions. The coverage of a wide range of scattering vectors, resulting from the very short neutron scattering lengths [about 10-15m vs 10-10m for X-rays] determines an insignificant variations of scattering amplitude with scattering vector Q (i.e. Bragg angle). The capability of neutrons to discriminate between iso-electronic or quasi-iso-electronic ions in crystal structures has been exploited in an increasing number of studies of solid-solutions in rock-forming minerals. In addition, the long scattering length of hydrogen (or deuterium) makes neutron diffraction the method of choice to locate proton sites and to refine their thermal displacement parameters. Rietveld structure refinements of neutron diffraction data leads to an easier separation of the information on thermal motion from that of site occupancy and a more accurate location of atoms in the crystal cell[1]. On the other hand, neutron powder diffraction techniques require a relatively large amount of material (hundreds of mg). We recently utilised the potentiality of neutron powder diffraction on amphiboles by studying a. ANaB(NaMg)CMg5Si8O22(OH,D)2 amphibole, hydrothermally synthesized at 850 °C and 0.3 GPa[2]. Neutron Time-of-Flight powder-diffraction data were collected at the ROTAX ToF diffractometer of the pulsed spallation source ISIS, U.K. The instrument uses a “white” beam with neutron wavelengths between 0.7-5.1 Å. The primary flight path is 14.0 m. The amphibole powder sample was packed into an 8 mm diameter V-can. Diffraction patterns were collected at 297 and 8 K using three stationary detector banks covering the 2θ ranges of 12-45°, 57-87°, and 100-143°, respectively. The ToF technique actually allows each detector to collect the entire diffraction pattern, hence redundancy (in T and θ) ensures a high precision of the measurements. Rietveld structure refinements of the patterns from all three detector banks were carried out simultaneously using GSAS[3]; the starting model was that obtained at room T by Cámara et al. (2003)[4]. Scale factor, background, cell parameters, and peak-profile (with a double exponential pseudo-Voigt function) were refined first. Then the atom parameters (positions, occupancies, and thermal displacements) were refined. All atoms were refined isotropically; thermal parameters were refined by grouping them on the basis of their environment and constraining the same shift within each group. The C- and T-sites were considered completely occupied by Mg and Si, respectively. In contrast, Na and Mg at the B sites were refined starting with values derived from the EPMA (and IR data). ANa and BNa were constrained to be equal during the refinement. The populations of hydrogen and deuterium at the H1 and H2 sites were also refined. At the end of the refinement, the shifts in all parameters were less than their standard deviations. The space group of the amphibole is P21/m at both temperatures, as confirmed by the presence of b-type reflections (h + k = 2n + 1). The unit-cell volumes at room T and at 8 K are 896.78(2) and 890.80(2)Å3, respectively, with a relative reduction of less than 1%. Accurate structural positions for the hydrogen atoms were obtained from the diffraction data. The O5A-O6A-O5A and O5B-O6B-O5B angles, diagnostic of the A- and B-chains kinking along the c-axis, are 190.0° and 159.2° at 293 K and 193.8° and 156.8° at 8 K, respectively. The orientation and magnitude of the thermo-elastic strain ellipsoid was calculated. A comparison between the low-temperature data reported here and the high-temperature data for a similar amphibole composition, reported by Cámara et al. (2003) up to 643 K, is discussed. The excellent agreement between the structural model refined at room-conditions by neutron powder diffraction and single-crystal X-ray diffraction (Cámara et al. 2003), respectively, confirms the reliability of the neutron powder diffraction even with complex structures with low symmetry.
[1] Rinaldi, R. (2002) European Journal of Mineralogy, 14,195-202.
[2] Iezzi, G., Gatta, D.G., Kockelmann, W.A., Della Ventura, G. Rinaldi, R., Schäfer, W., Piccinini, M., Galliard, F. (2005). American Mineralogist, 90, 695-700.
[3]Larson, A.C., Von Dreele, R.B. (2001) GSAS: General Structure Analysis System. Document LAUR 86-748, Los Alamos National Laboratory, NM, USA.
[4] Cámara, F., Oberti, R., Iezzi, G., Della Ventura, G. (2003) Physics and Chemistry of Minerals, 30, 570-581
Crystal-chemical characterization of fibrous amphiboles by FTIR and Mössbauer spectroscopies
No full textThe mechanism through which asbestos amphiboles may give rise to mesothelioma is still not completely understood. Several factors have been taken into account, and these include the morphological aspect of the fiber, the chemical composition, and a variety of surface properties which are ultimately responsible for the mineral-cellule interactions[1]. Studies in vitro demonstrate that the morphology of the fiber has a strong role in determining its biological danger, because very thin and long crystallites are hardly eliminated by the alveolar macrophages. However, recent studied show that lung injury after asbestos exposure is associated with an oxidative stress that is catalyzed by iron in the fiber[2]. It follows that the rapid determination of the chemical composition of asbestiform amphiboles, and of its Mg/Fe ratio in particular, is extremely important in environmental studies. The direct analysis can be achieved only by micro-chemical tools such as by EDS- or WDS-equipped electron microscopes. However, these techniques are extremely expensive and often unsuitable when dealing with extremely fibrous (diameter < 3 μm) materials. Therefore a rapid and easily accessible method is highly desirable in large scale environmental monitoring. The best alternative to EMPA is provided by FTIR spectroscopy, a technique which can be used on both single crystals (down to few μm in dimension) and powders. Here we present the results of a new calibration based on the analysis of a large set of well-characterized fibrous and prismatic natural amphiboles spanning a very large variety of chemical compositions and geological occurrences. All samples were previously studied using X-ray diffraction and EMPA. FTIR spectra in the principal OH-stretching region were collected on KBr disks prepared with a mineral:matrix = 5:150 mg ratio. Most spectra show four prominent bands which can be assigned to the combination of Mg and F2+ at the OH-coordinated M(1,3) sites[3,4,5,6]. The digitized spectra were fitted by interactive optimization followed by least-squares refinement; all bands were modelled as symmetric Gaussians. [3,4] showed that the binary site-occupancies at M(1) and M(3) in the amphibole structure are related to the observed intensities of the four (A to D) components in the principal IR OH-stretching spectrum. Using the original equations of [7]:
M(1,3)Mg = 3IA + 2IB + IC and M(1,3)M2+ = IB + 2IC + 3ID (with M2+ = Fe2+)
where IA-ID are the intensities measured for the corresponding A to D bands, one can derive the (Mg, M2+) site populations at M(1,3) with a high degree of confidence. This method is particularly suitable for asbestiform materials which cannot be properly characterized by EMP. The present work shows that the above spectroscopic tool can be applied to a large variety of amphibole types. For species were significant (Mg, Fe) are present at M(4) (i.e. anthophyllite–cummingtonite–grunerite) an additional information (e.g. Mössbauer) is however required for a complete characterization of the sample.
[1] van Oss, C.J., Naim, J.O., Costanza, P.M., Giese, R.F. Jr., Wu, W., Sorling, A.F. (1999) Clays and Clay Minerals, 47, 697-707.
[2] Xinchao, W., Wu, Y., Stonehuerner, J.G., Dailey, L.A., Richards, J.D., Jaspers, I., Piantadosi, C.A., Ghio, A.J. (2006) Am. J. Respir. Cell. Mol. Biol., 34, 286–292.
[3] Della Ventura, G., Robert, J.-L., Hawthorne, F.C. (1996) Geochimica and Cosmochimica Acta, vol. spec. 5, 55-63.
[4] Della Ventura, G., Robert, J.-L., Raudsepp, M., Hawthorne, F.C., Welch, M. (1997) American Mineralogist, 82, 291-301.
[5] Iezzi, G., Della Ventura, G., Hawthorne, F.C., Pedrazzi, G., Robert, J.-L., Novembre, D. (2005) European Journal of Mineralogy, 17, 733-740.
[6] Iezzi, G., Della Ventura, G., Bellatreccia, F., Lo Mastro, S., Gunther, M., Bandly, (2007) Mineralogical Magazine, in press.
[7] Burns, R.G. and Strens, R.G.J. (1966) Science, 153, 890-892
Synthetic P21/m amphiboles in the system Li2O-Na2O-MgO-SiO2-H2O (LNMSH)
No full textWe describe here the synthesis of amphiboles along the nominal Na(NaMg)Mg5Si8O22(OH)(2)- Na(LiMg)Mg5Si8O22(OH)(2) join, at 800 degrees C, 0.4 GPa. High amphibole yields (> 90%) plus minor quartz and enstatite have been obtained at all compositions; amphibole crystals are acicular and their size rarely exceeds 20-30 X 0.5-3 mu m. TEM analysis shows the presence of h+k odd reflections in all samples, indicative of a P-lattice. By similarity with closely related amphiboles from the literature (c,g., Oberti et al. 2000; Camara et al. 2003) it P2(1)/m space group was assigned to the amphiboles synthesized here. Refined cell-parameters from X-ray powder-patterns show a linear decrease as a function of increasing Li at M4, a and P being the most affected parameters. The four infrared OH-stretching spectra all show two main bands at 3741-3748 and 3712-3716 cm(-1), respectively. They are assigned to two independent O-H groups in the P2(1)/m structure, interacting with a strongly delocalized Na-A. The spectra show in addition two minor absorptions at about 3688 and 3667 cm(-1), respectively; these bands are assigned to vacant A-sites in the structure and indicate slight departure of the nominal composition toward cummingtonite. The present work shows that one apfu of Na-B can also be completely replaced by one apfu of Li-B (M+), in synthetic Na(M+Mg)Mg5Si8O22(OH)(2) and that all compositions have P2(1)/m symmetry at ambient conditions
Synthetic ANaB(NaxLi1 ¡ xMg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study
No full textThe high-pressure behavior of three synthetic amphiboles crystallized with space group P2(1)/m at room conditions in the system Li(2)O-Na(2)O-MgO-SiO(2)-H(2)O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions (A)Na (B)(Na (x) Li(1 - x) Mg(1)) (C)Mg(5) Si(8) O(22)(OH)(2) with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O-H dipoles in the P2(1)/m structure, bonded to the same local environment (M1M3)Mg(3)-OH-(A)Na, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2(1)/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole (B)(Na(0.2)Li(0.8)Mg(1)) in the frequency range 50 to 1,400 cm(-1) also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479-482, 2006a) regarding a new high-pressure amphibole polymorph
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