Phys Chem Minerals (2009) 36:193–206 DOI 10.1007/s00269-008-0269-8
ORIGINAL PAPER
Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite Giancarlo Della Ventura Æ G. Diego Gatta Æ Gunter J. Redhammer Æ Fabio Bellatreccia Æ Anja Loose Æ Gian Carlo Parodi
Received: 27 June 2008 / Accepted: 21 September 2008 / Published online: 11 October 2008 Springer-Verlag 2008
Abstract We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen ˚. bridges, with OwO donor–acceptor distances [2.7 A The FTIR spectra show two main absorptions. The first at 3,602 cm-1 is polarized for E \ c and is assigned to the m3 mode. The second, at 3,531 cm-1, is also polarized for E \ c and is assigned to m1 mode. A weak component at 4,108 cm-1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several
G. Della Ventura (&) F. Bellatreccia Dipartimento di Scienze Geologiche, Universita` Roma Tre, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy e-mail:
[email protected] G. D. Gatta Dipartimento di Scienze della Terra, Universita` degli Studi di Milano, Via Botticelli 23, 20133 Milan, Italy G. J. Redhammer Division of Mineralogy, Department of Materials Engineering and Physics, University Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria A. Loose Institute of Solid State Research (IFF), Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany G. C. Parodi Laboratoire de Mine´ralogie, Museum National d’Histoire Naturelle, 61, rue Buffon, 75005 Paris, France
overlapping bands in the 1,300–1,500 cm-1 range are strongly polarized for E \ c, and are assigned to the vibrations of the CO3 group. Keywords Cancrinite Polarized FTIR Neutron diffraction Crystal-structure refinement H2O bonding system
Introduction Cancrinite, (Na,Ca)7–8(Si6Al6O24)(CO3)1.2–1.72H2O, is the parent member of a group of zeolite-type minerals, the ‘‘cancrinite group’’, which comprises a large number (20) of naturally occurring species (e.g., Bonaccorsi and Merlino 2005; Ca´mara et al. 2005; Rastsvetaeva et al. 2007a). Most of these species are mineralogical rarities and have been found only once or twice from a very restricted and peculiar type of geological occurrence, except cancrinite, davyne and vishnevite which are relatively more common minerals (Deer et al. 2004). The crystal structure of cancrinite is built up on a tetrahedral framework consisting of a pattern of close-packing of interconnected (and parallel) six-membered rings with an ABABAB stacking sequence (Jarchow 1965) and two ‘‘secondary building units’’: six and four membered tetrahedral rings (Baerlocher et al. 2001). The topological symmetry of the idealized CAN framework is P63/mmc, ˚ , and with unit-cell constants: a * 12.5 and c * 5.3 A ˚ 3 (Baerlocher et al. framework density: 16.6 T/1,000 A 2001). One large 12-membered ring (12mR) channel runs ˚ along (0001), with a ‘‘free-diameter’’ of about 5.9 A (Baerlocher et al. 2001). The so-called ‘‘cancrinite-cages’’ (also known as e-cages or undecahedral cages, [4665] following the IUPAC recommendations of McCusker et al.
123
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2001) lie around the 12mR (Fig. 1). In natural cancrinites, the Si/Al-distribution in the tetrahedral framework is fully ordered (Jarchow 1965; Bresciani Pahor et al. 1982; Grundy and Hassan 1982; Belokoneva et al. 1986; Hassan and Grundy 1991; Fechtelkord et al. 2001; Ballirano and Maras 2004; Hassan et al. 2006), and because of this, the general symmetry is reduced to P63. Recently Rastsvetaeva et al. (2007b) described a low symmetry (s.g. P3), high-sodium variety of cancrinite from Kola Peninsula. The extraframework constituents (cations like K?, Na?, Ca2?, anions like SO42-, Cl- and molecules like H2O and CO2) lie in the large 12mR channels and in the e-cages. The ideal chemical formula of Na-cancrinite is: Na8[Si6Al6O24]CO32H2O but the composition of the naturally occurring cancrinite sensu strictu (s.s.) is close to Na6Ca2[Si6Al6O24](CO3)22H2O (Deer et al. 2004). Several superstructures have been found in natural cancrinites due to substitutional (or positional) ordering of the extra-framework content (Foit et al. 1973) and/or to a periodic variation in the stacking sequence of the building-block units between those of the cancrinite-type structure (ABABAB) and the sodalite-type structure (ABCABC) (Brown and Cesbron 1973; Foit et al. 1973; Merlino and Orlandi 1977a, b; Rinaldi and Wenk 1979; Burragato et al. 1980; Grundy and Hassan 1982; Hassan and Buseck 1992, and references therein). Cancrinite and its hydroxyl-containing counterpart basic cancrinite—Na8[Si6Al6O24](OH)22H2O—have been the object of a surprisingly large number of structural studies, based on both X-ray (e.g., Smolin et al. 1981; Grundy and Hassan 1982; Hassan and Grundy 1991; Bresciani Pahor et al. 1982; Nadezhina et al. 1991; Ballirano and Maras 2004 among the others) and neutron diffraction (E´milariev and Yamzin 1982). However, despite the general Fig. 1 The crystal structure of cancrinite, based on the structural data of this study, viewed down [0001]. Si-tetrahedra are represented in gray, Al-tetrahedra in black
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Phys Chem Minerals (2009) 36:193–206
agreement about the geometry of the tetrahedral framework and the Si/Al-distribution, conflicting results have been so far reported regarding the extra-framework content. In some of the previous studies, the oxygen atoms of the H2O molecules were found lying on the 3-fold axis (Wyckoff special position 2b at 2/3,1/3, z) (Bresciani Pahor et al. 1982; Hassan et al. 2006) whereas in other studies the H2Ooxygens were located off the triad axis (Wyckoff general position 6c) (Jarchow 1965; Smolin et al. 1981; Grundy and Hassan 1982; Belokoneva et al. 1986; Ballirano and Maras 2004). In addition, to the best of our knowledge, the proton sites have never been located and, as a consequence, the H-bonding environment in cancrinite is still unknown. Conflicting results have also been reported for the CO3group, located in the 12mR channels. In some of the previous studies, only one independent CO3-group was found, with the C-site at the Wyckoff special position 2a (on the 63-axis at 0,0, z) (Jarchow 1965; Smolin et al. 1981; Belokoneva et al. 1986). In contrast, in other studies, two independent and statistically distributed CO3-groups with the C-sites at 0, 0, z and 0, 0, z0 (with z = z0 ) were reported (Grundy and Hassan 1982; Ballirano and Maras 2004; Hassan et al. 2006). The aim of this work is a reinvestigation of the crystal structure and crystal chemistry of natural cancrinite by combining single-crystal neutron diffraction and polarized infrared spectroscopy, in order to define unambiguously the real topological configuration and location of water molecules (and hydrogen bonding), the CO3-groups and the monovalent/divalent cation sites within the cancrinite-cage and the large 12mR channels. Recent studies have shown that a multi-methodological approach, based on singlecrystal neutron diffraction and polarized infrared
Phys Chem Minerals (2009) 36:193–206
spectroscopy, allows to investigate in detail the crystal chemistry of materials with very low amounts of disordered ‘‘zeolitic water’’, OH groups and light-elements (Gatta et al. 2006a, b, 2008). Neutron diffraction, in particular, allows an easier separation of the information on thermal motion from that of site occupancy in structure refinements (Dove 2002; Rinaldi 2002; Rinaldi et al. 2005; Gatta et al. 2006a, b, 2007, 2008).
Experimental methods Transparent crystals of two cancrinites, one from Cameroun (Cam) and one from Canada (C38834), were selected for this study. Microprobe analyses were done using a CAMEBAX 50X WDS-microprobe, at Camparis, Universite´ Paris VI. Operating conditions were 15 kV accelerating voltage and 15 nA beam current, with a beam size of 10 lm; counting time was 10 s on both peak and background. Natural minerals were used as standards. Single-crystal micro-FTIR spectra were collected with a NicPlan microscope, equipped with a MCT-A nitrogencooled detector and a KBr beamsplitter. Nominal resolution was 4 cm-1 and final spectra are the average of 128 scans. For polarized measurements a gold wire-grid, ZnSe substrate IR polarizer, was used. The crystal fragments were oriented using an X-ray diffractometer, and doubly polished to thicknesses ranging 10–30 lm. Millimetric crystals of the two cancrinite samples (Table 1), optically free of defects, were selected for the neutron diffraction experiments. A preliminary X-ray single-crystal diffraction investigation showed that the crystals were free of twinning, stacking and without any evidence of superstructure reflections (with respect to the ˚ and c * 5.1 A ˚ , Jarchow 1965). Diflattice: a * 12.5 A fraction data were collected at 293 K with a Huber fourcircle diffractometer installed at the SV28 beam-line at the DIDO reactor—Forschungszentrum Ju¨lich, Germany. The ˚, incident radiation, with a constant wavelength of 1.2413 A was obtained using a Cu(200) monochromator. The neutron flux density was about 2.5 9 106 ns-1 cm-2 and the tangential beam tube lead to a very small background count rate (*5 s-1). The lattice was found to be metrically ˚ hexagonal for both the crystals, with a = b = 12.595(5) A ˚ for the sample from Cameroun and and c = 5.121(5) A ˚ and c = 5.121(5) A ˚ for the sample a = b = 12.60(2) A from Canada. No evidence of superstructure has been found. For the sample from Cameroun, a total number of 2,265 reflections were collected with -12 B h B 12, -12 B kB12 and -5 B l B 3 (maximum 2h = 74.98), out of which 382 with Fo [ 4r(Fo) were unique. Technical problems at the diffraction line did not allow to collect
195
diffraction data at 2h [ 75. Two standard reflections were measured with a frequency of 450 min and the intensity variation was within r(I). Further details of the data collection are reported in Table 1. The reflection conditions agreed with the space group P63. Diffraction data were then corrected for Lorentz effect. No absorption correction was applied because of the composition and the dimensions of the sample. After correction, the discrepancy factor for the symmetry related reflections was Rint = 0.0276 (Table 1). For the sample from Canada, 2,916 reflections were collected with -15 B h B 15, -14 B kB12 and -4 B l B 6 (maximum 2h = 100.13), out of which 642 were unique with Fo [ 4r(Fo). A careful inspection of the standard reflections showed that the intensity variation throughout the experiment was within r(I). The integrated intensities were then corrected for Lorentz effect and no absorption correction was applied. Also in this case, the reflection conditions agreed with the space group P63. The discrepancy factor for the symmetry related reflections was Rint = 0.0716 (Table 1). Further details pertaining to the data collection are reported in Table 1.
Chemical composition of the examined samples Table 2 gives selected microprobe analyses and crystal– chemical formulae for the studied samples. Note that the cancrinite from Cameroun is from the locality of the sample studied by Ballirano and Maras (2004). The crystal–chemical formulae were calculated according to Ballirano et al. (1996); the weight percent of CO2 was obtained as the amount necessary to saturate the excess O. The Si:Al ratio in a.p.f.u. (atoms per formula unit) is very close to 1:1 (Table 2); the Ca content is significant in both samples, although Na is the dominant extra-framework cation. The SO4 content is almost negligible in both the samples, which can be thus classified as almost endmember carbonate-cancrinites.
Structure refinement The structure refinements to squared structure factors, based on the neutron diffraction data, were carried out at first with isotropic displacement parameters in the space group P63, using the SHELXL-97 package (Sheldrick 1997; WinGX suite, Farrugia 1999) and only considering the framework atoms (i.e., the channels were considered to be completely empty). The atomic coordinates of cancrinite from Fechtelkord et al. (2001) were used as the starting structural model of the tetrahedral framework. The scattering lengths of the atoms from the International Tables for Crystallography C (Wilson and Prince 1999) were
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Phys Chem Minerals (2009) 36:193–206
Table 1 Details of neutron data collection and refinement for the studied samples
Crystal size (mm3) Unit-cell parameters
Cancrinite from Cameroun (Cam)
Cancrinite from Canada (C38834)
2.1 9 2.3 9 3.2 ˚) a = 12.595(5) (A ˚) c = 5.121(5) (A
2.5 9 2.6 9 3.1 ˚) 12.60(2) (A
˚ 3) V = 703.5(8) (A
˚) 5.121(5) (A ˚ 3) 704.1(17) (A
Z
6
6
Space group
P63
P63
T (K)
293
293
˚) Wavelength (A
1.2413
1.2413
Neutron flux density (n s-1 cm-2)
*2.5 9 106
*2.5 9 106
2h \ 35 35 \ 2h \ 65
41 steps, pure x-scan 61 steps, pure x-scan
41 steps, pure x-scan 61 steps, pure x-scan
65 \ 2h \ 75
51 steps, pure x-scan
51 steps, pure x-scan
65 \ 2h \ 100.13
–
51 steps, x-2h scan
Time per step (s)
10
10
u, v, w
3.7, -6.4, 10.8
3.7, -6.4, 10.8
Max. 2h () and sinh/k
74.98, 0.4903
100.13, 0.6177
-12 B h B 12
-15 B h B 15
-12 B k B 12
-14 B k B 12
Scan type, steps and width
No. of measured reflections
-5 B l B 3
-4 B l B 6
2,265
2,916
No. of unique reflections
385
699
No. of unique refl. with Fo [ 4r(Fo)
382
642
No. of refined parameters
80
113
Extinction correction factor
0.015(1)
0.0040(8)
Rint
0.0276
0.0716
R1 with Fo [ 4r(Fo) wR2
0.0287 0.0631
0.0381 0.0681
GooF
1.173
1.212
Weighting scheme: a, b 0.0136, 2.510 3 ˚ Residuals (fm/A ) ±0.31 2 2 2 Rint ¼ RFobs Fobs ðmeanÞ R Fobs ; R1 ¼ RðjFobs j jFcalc jÞ=RjFobs j; h .h i . h 2 2 ii0:5 2 2 2 R w Fobs Fcalc ; w ¼ 1 r2 Fobs þ ðaPÞ2 þbP wR2 ¼ R w Fobs 2 2 ; 0 þ 2Fcalc 3; x-scan width = (u ? vtanh ? wtan2h)0.5 P ¼ Max Fobs
used. Correction for secondary isotropic extinction was applied according to Larson’s formalism (Larson 1967), as implemented in SHELXL-97 package (Sheldrick 1997). For both the cancrinites, the first cycles of the refinement showed that the model of the tetrahedral framework was correct, with a fully ordered Si/Al-distribution (Tables 2a, b, 3a, b). The maxima in the difference-Fourier map of the nuclear density allowed to locate at first the extra-framework cations (Na/Ca sites) and the CO3 groups. Two positions for the alkali cations were found in the structure: one is located in the cancrinite-cage (Na1, Table 3a, b), and the second one (Na2) lies in the large 12-
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0.02, 1.641 ±0.47
membered ring channel (Fig. 1; Table 3a, b). The Na1-site is fully occupied by sodium, whereas the Na2-site is partially occupied by Na (*97–98%) and Ca (*2–3%), in both the cancrinites (Table 3a, b). More complex is the configuration of the two non-equivalent and mutually exclusive CO3 groups [i.e., C1(OC1)3 and C2(OC2)3]: the C-sites lie on the 63-axis (at 0, 0, z) and the two CO3 groups are overlapped if viewed down [0001], as shown in Fig. 2. In the structural refinement, the occupancy factor of the OC1 and OC2 sites was fixed as a function of the co-respective C1 and C2 sites (Table 3a, b), taking into account the different site multiplicity.
Phys Chem Minerals (2009) 36:193–206
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Table 2 Chemical composition and formulae on the basis of 12(Si ? Al) apfu for the studied cancrinites. The number of averaged point analyses is given in parenthesis Cam Wt % (6)
C38834 Wt % (3)
SiO2
35.62
35.54
Al2O3
Cam apfu Si Al
6.12
C38834 apfu 6.07
29.05
29.50
CaO
5.03
5.32
BaO
0.04
0.01
SrO
0.27
0.03
Ca
0.93
0.97
K2O
0.03
0.01
Na
6.59
6.52
Na2O
Sum
5.88
5.93
12.00
12.00
19.80
19.71
K
0.01
0.00
SO3
0.52
0.20
Sr
0.03
0.00
Cl
0.01
0.03
Ba
0.00
0.00
F
0.76
0.04
Sum
7.55
7.50
CO2a
4.45
5.31
H2Ob
3.49
3.51
CO32-
1.04
1.24
99.06
99.21
SO42-
0.07
0.03
0.32 98.74
0.02 99.19
FCl-
0.41 0.00
0.02 0.01
Sum
1.52
1.30
H2O
2.00
2.00
Si/Al
1.04
1.02
-O = Cl,F Total
a
CO2 calculated from charge balance
b
H2O calculated from stoichiometry (as 2 apfu)
C38834 IMC Quarries, Blue Mtn., Nephton, Peterborough Co., ON (Canada); Cam Saton, Neende, Kribi, Cameroun
After further cycles of refinement, the difference-Fourier map of both samples showed three intense residual peaks in the cancrinite-cage (with general Wyckoff position 6c): ˚ 3) and two negative one positive (approx. ?1.5 fm/A 3 ˚ ) (Fig. 3). Therefore, further refine(approx. -0.8 fm/A ment cycles were performed assigning the oxygen scattering length to the site with the positive residual peak in the difference-Fourier map and hydrogen scattering length to the two sites with the negative residual peaks. The convergence was rapidly achieved and the variance– covariance matrixes did not show any strong correlation between the refined parameters. As shown in Fig. 4, the topological configuration of water molecules appears to be complex: the oxygen site (OW) lies off the triad axis, with the two proton sites (H1 and H2). This implies a statistical configuration with three equivalent, and mutually exclusive, water molecules around the 3-fold axis (Table 3a, b). For the structural refinement of cancrinite from Cameroun, the final least-square cycles were done with anisotropic thermal parameters for the oxygens of the tetrahedral framework and isotropic displacement parameters for the other sites (Table 3a), keeping a reliable observations/refined parameters ratio (*4.78, Table 1). In contrast, for the structural refinement of the cancrinite from
Canada, the higher number of collected Bragg reflections allows to perform the final least-square cycles of refinement with anisotropic thermal parameters for all the atoms and only the two proton sites were maintained isotropic, with observations/refined parameters ratio *5.68. All the principal mean square atomic displacement parameters were positively defined. At the end of the last cycle of ˚ 3 and refinement, no peak larger than ±0.31 fm/A 3 ˚ ±0.47 fm/A , was present in the final difference-Fourier map of the nuclear density pertaining to structural refinement of the cancrinite from Cameroun and Canada, respectively. The geometry of the water molecule is well described in both the structural refinements, with OW-H1 and OW-H2 bond distances, corrected for ‘‘riding motion’’ ˚, effect (Busing and Levy 1964), of 1.033 and 0.994 A respectively, and H1-OW-H2 bond angle of 101(2) for the cancrinite from Cameroun, whereas OW-H1 = 1.077, ˚ and H1-OW-H2 = 104(2) for the OW-H2 = 1.051 A cancrinite from Canada (Table 4a, b). The final residual R1 was 0.0287 [for 80 refined parameters and 382 unique reflections with Fo [ 4r(Fo)] for the cancrinite from Cameroun and R1 = 0.0381 [113 ref. par./642 unique refl. with Fo [ 4r(Fo)] (Table 1). Observed and calculated structure factors can be obtained from the authors upon request. Positional and displacement parameters are reported in Table 3a, b. Relevant bond lengths and geometrical parameters are listed in Table 4a, b.
Single-crystal FTIR spectroscopy Unpolarized spectra The typical single-crystal, unpolarized-light FTIR spectrum of cancrinite (sample C38834 from Canada) is displayed in Fig. 5 in the range 6,000–650 cm-1. It shows several absorption bands, the assignment of which can be based on the vast literature on these compounds (e.g., Della Ventura et al. 2005, 2007 and references therein). Briefly, the broad and very strong, multi-component absorption in the 1,200– 900 cm-1, can be assigned to the stretching modes of the tetrahedral framework. Note that these absorptions are out of scale in the spectrum of Fig. 5 due to the crystal thickness. All bands at wavenumbers[1,200 cm-1 are due to the extraframework CO3 carbonate groups and to the hydrous components within the structural voids. In particular (the band assignments are summarized in Fig. 5), the very intense groups of bands in the range 1,350–1,500 cm-1 are assigned to the m3 stretching modes of the CO3 groups, the sharp band at 1,630 cm-1 is assigned to the m2 symmetric bending mode of the water molecule, and the weak but well resolved bands in the 2,550–2,450 cm-1 range (Fig. 5), can
123
123
0.216(1)
0.325(2)
H1 (6c)
H2 (6c)
0.771(1)
0.645(2)
0.6864(9)
0.7496(3)
0.1181(5) 1/3
0
0.1175(6)
0
0.3511(1)
0.3595(1)
0.5509(1)
0.4047(1)
0.4123(2)
0.4107(2)
0.775(1)
0.653(2)
0.6883(7)
0.7514(3)
0.1192(5) 1/3
0
0.3512(2)
0.3592(2)
0.5510(2)
0.4052(2)
0.156(6)
0.141(8)
0.168(1)
0.7764(6)
0.653(2) 0.115(1)
0.655(3)
0.878(3)
0.890(4)
0.0468(3)
0.0290(3)
0.7089(4)
0.6427(4)
0.7356(5)
0.7338(4)
0.146(7)
0.144(8)
0.168(2)
0.7730(9)
0.670(1) 0.115(2)
0.666(2)
0.895(2)
0.913(3)
0.0440(5)
0.0263(5)
0.7084(6)
0.6414(6)
0.7313(8)
0.7306(7)
z
0.0007(8)
0.003(1)
0.004(1)
0.002(1)
0.005(1)
U23
0.0009(8)
0.001(1)
0.004(1)
0.006(1)
0.004(1)
U13
0.0044(7)
0.013(1)
0.010(1)
0.010(1)
0.012(1)
U12
0.020(2) 0.026(2)
0.022(2)
0.017(2)
0.010(3)
0.0171(6)
0.0164(6)
0.0199(7)
0.0165(6)
0.0070(8)
0.0060(7)
Ueq/Uiso
-0.002(4)
-0.001(1)
0.002(2) 0
0
0.001(4)
0
0.0035(7)
0.0034(7)
0.0006(7)
0.0015(6)
-0.0004(9)
-0.001(4)
-0.001(1)
-0.001(3) 0
0
-0.005(4)
0
0.0019(6)
0.0033(6)
0.0023(7)
0.0008(6)
0.0002(9)
0.041(4)
0.012(1)
0.007(2) 0.0114(7)
0.006(1)
0.008(3)
0.010(2)
0.0142(6)
0.0099(6)
0.0099(7)
0.0111(6)
0.0049(8)
0.0068(4)
0.061(3)
0.025(1)
0.042(2) 0.027(1)
0.041(3)
0.059(3)
0.055(4)
0.0141(3)
0.0142(3)
0.0169(3)
0.0111(3)
0.0067(4)
2
0.119(7) 2
0.046(4)
0.032(2)
0.095(6) 0.037(3)
0.097(10)
0.132(10)
0.125(15)
0.0097(7)
0.0069(7)
0.0235(8)
0.0123(6)
0.006(1)
0.141(9)
0.083(8)
0.026(2)
0.012(2) 0.023(1)
0.013(3)
0.018(3)
0.020(4)
0.0227(7)
0.0237(7)
0.0107(7)
0.0210(7)
0.009(1)
0.38(1)a
0.061(7)
0.019(1)
0.018(2) 0.023(1)
0.013(3)
0.025(4)
0.020(4)
0.0160(7)
0.0130(7)
0.0197(7)
0.0102(7)
0.0059(9)
0.13(1)
0.14(1)
0.38(1)a
0.38(1)a
0.02(4)Ca
0.98(4) Na,
0.405(9) 1
0.405(9)
0.363(9)
0.363(9)
1
1
1
1
1
1
0.36(1)a
0.36(1)
a
0.049(4)
0.0058(8)
0.0167(1)
0.015(2)
0.032(2)
0.021(2)
U33
0.025(2)
0.0080(8)
0.021(1)
0.023(1)
0.010(1)
0.022(1)
U22
0.97(6) Na, 0.03(6) Ca
0.0073(8)
0.017(1)
0.013(1)
0.021(1)
0.011(1)
U11
0.36(1)a
0.43(3) 1
0.43(3)
0.32(1)
0.32(1)
1
1
1
1
1
1
Site Occupancy
The anisotropic displacement factor exponent takes the form: -2p [(ha*) U11 ?…? 2hkabU12]. Ueq is defined as one-third of the trace of the orthogonalised Uij tensor. The z(Al) value was not fixed to because floating origin restraints were generated automatically by the program Shelx-97 on the basis the method of Flack and Schwarzenbach (1988) a The site occupancy factors of Ow, H1 and H2 were restrained to the same value. The maximum value physically acceptable is 1/3
0.3013(8)
OC1 (6c)
Ow (6c)
0.0592(6)
C1 (2a)
0.8735(3)
0
O4 (6c)
Na2 (6c)
0.3209(1)
O3 (6c)
0
0.0448(1)
O2 (6c)
0.0603(5) 2/3
0.1149(1)
O1 (6c)
OC2 (6c) Na1 (2b)
0.2019(1)
Al (6c)
C2 (2a)
0.0827(2)
0.3375(2)
Si (6c)
(b) Canada
0.331(2)
0.0611(4) 2/3
0.215(1)
0
C2 (2a)
OC2 (6c) Na1 (2b)
H2 (6c)
0.0609(6)
OC1 (6c)
H1 (6c)
0
C1 (2a)
0.8742(3)
0.3209(2)
O4 (6c)
0.3034(7)
0.0445(2)
O3 (6c)
Na2 (6c)
0
0.1147(2)
O2 (6c)
Ow (6c)
0.1187(6)
0.2019(2)
O1 (6c)
0.4122(3)
0.0825(2)
0.3371(3)
0.4106(2)
y
Al (6c)
x
Si (6c)
(a) Cameroun
Site (Wyck.)
˚ 2) for cancrinite from (a) Cameroun and (b) Canada Table 3 Refined positional and displacement parameters (A
198 Phys Chem Minerals (2009) 36:193–206
Phys Chem Minerals (2009) 36:193–206
199
sample Cam from Cameroun are almost identical. The higher frequency band at 3,602 cm-1 is completely polarized for E \ c, while the second component at 3,531 cm-1, although strongly polarized \ c, is less affected by rotation of the electric vector. The broad band at *3,230 cm-1 is slightly polarized for E k c; the bending mode at 1,630 cm-1 (Fig. 6) is also polarized for E \ c. The combination mode at 5,208 cm-1 is polarized for E \ c; while the 4,108 cm-1 band is completely polarized for E k c. The overlapping bands in the 1,300–1,500 cm-1 stretching region of the carbonate group are also completely polarized for E \ c (Fig. 7), in agreement with the orientation of the C–O bonds in the structure (Fig. 2).
Description of the structure Framework and large channels
Fig. 2 Configuration of the two independent, and statistically distributed, CO3-groups lying in the large 12-membered rings of the cancrinite structure, represented with isotropic (left side) and anisotropic thermal parameters (right side, probability factor: 50%). Viewed \ [0001] (above) and down [0001] (below)
be assigned to the combination modes of the CO3 group (Della Ventura et al. 2007). The doublet of well defined and intense absorptions at 3,600–3,530 cm-1 is assigned to the stretching modes of the water molecule or/and OH-groups, as it will be discussed in more detail below. The minor band at 3,234 cm-1 is assigned to the first overtone of the bending mode of the water molecule (2 m2). The speciation of hydrogen in mineral and glasses can be characterized by examination of the 4,000–6,000 cm-1 region (e.g., Ihinger et al. 1994), where the combination bands due to the coupling between the bending and the stretching modes of the water molecules and the OH-groups occur at well defined and different frequencies. On this basis, the band at 5,207 cm-1 is assigned to the combination of m2 ? m3 modes of the water molecule, while the bands at 4,104 and 3,848 cm-1 (Fig. 6) possibly indicate the presence of hydroxyl groups in the sample (Burneau and Carteret 2000; Della Ventura et al. 2007). This point will be discussed in more details below. Polarized spectra The polarized spectra in the OH region of cancrinite C38834 from Canada are shown in Fig. 6; the spectra of
The structural refinements based on the single-crystal neutron diffraction data of the two natural cancrinites confirm the general symmetry and the structural model of the Si/Al-framework of cancrinite previously described on the basis of the single-crystal and powder X-ray diffraction (e.g., Ballirano and Maras 2004 and references therein), but gives new insight into the topological configuration of the extra-framework population. The extra-framework content that lies in the large 12-membered rings channels is represented by one independent Na-site (Na2) and two independent, and statistically distributed, CO3 groups. The geometry of the CO3 groups ˚ appears to be almost regular, with C1–OC1 = 1.298(7) A ˚ and C2–OC2 = 1.300(5) A for the cancrinite from Came˚ and C2–OC2 = roun (Table 4a) and C1–OC1 = 1.283(7) A ˚ 1.288(5) A for the cancrinite from Canada (Table 4b), in agreement with the previous findings (Grundy and Hassan 1982; Ballirano and Maras 2004; Hassan et al. 2006). The atoms of the carbonate groups are not perfectly coplanar, being z(C1) = z(OC1) and z(C2) = z(OC2) (Table 3a, b). This configuration was not clearly shown in the previous studies based on X-ray diffraction data. However, the refinement of the z(C1), z(OC1), z(C2) and z(OC2) would be affected by the significantly anisotropic distribution of the nuclear density, as shown by the configuration of the displacement parameters in Fig. 2. A similar evidence was reported by Grundy and Hassan (1982). In addition, the refined site occupancies (s.o.) of C1- and C2-sites are not equal, being s.o.(C1) \ s.o.(C2) (Table 3a, b). This condition was reported only in a few studies on cancrinites (e.g. Ballirano and Maras 2004) and was also observed in NO3-cancrinite, with s.o.(N1) = s.o.(N2) (Fechtelkord et al. 2001). The CO3 groups do not show any direct (or remote) interaction with the tetrahedral framework. The two
123
200
Phys Chem Minerals (2009) 36:193–206
Fig. 3 Difference-Fourier maps of the nuclear density (scale: ˚ 3) at z = 0.168 after the fm/A first cycles of isotropic refinement of the cancrinite from Cameroun with a waterfree structure (above). Around the 3-fold axis (at x = 1/3, y = 2/3), one positive residual ˚ 3 (at peak of about ?1.15 fm/A x * 0.30, y * 0.69) and two negative residual peaks of about ˚ 3 are evident. -0.8 fm/A Difference-Fourier maps at z = 0.145 after assigning the oxygen scattering length to the site with the positive residual peak: only the two negative residual peaks are evident at x * 0.21, y * 0.65 and x * 0.33, y * 0.77, representing the proton sites (below)
independent oxygen sites of the CO3 groups (i.e. OC1 and OC2) are bonded to the Na2 site (Table 4a, b). Due to the ˚ , Table 4a, b), the two CO3 short C1–C2 distances (*1.2 A groups cannot coexist. As a consequence, the Na2-polyhedron should be described with two distinct configurations, both with coordination number CN = 7 and maximum bond ˚ (Table 4a, b). The first polyhedral condistance *2.9 A figuration is based on the bonds Na2–OC2, Na2–OC20 , Na2– O4, Na2–O3, Na2–OC200 , Na–O1, and Na2–O4; the second one on the bonds Na2–OC1, Na2–OC10 , Na2–O4, Na2–OC100 ,
123
Na2–O3, Na2–O1, and Na2–O4 (Table 4a, b). The first configuration coexists with the (C2–OC2)3 group, whereas the second one with the (C1–OC1)3 group. The cancrinite-cages and the hydrogen bonding system The water molecules and a further Na-site (Na1) lie in the cancrinite-cage. The oxygen site (OW) of the H2O molecules lies off the triad axis, giving rise to a statistical configuration with three equivalent and mutually exclusive
Phys Chem Minerals (2009) 36:193–206
201 ´˚ Table 4 Relevant bond distances (in A ) and angles () for cancrinite from (a) Cameroun and (b) Canada (a) Cameroun
Fig. 4 Configuration of the water molecules in the cancrinite-cage, Viewed down [0001] (above) and \ [0001] (below). The oxygen site (OW) of the H2O molecules lies off the triad axis, giving rise to a statistical configuration with three equivalent water molecules around the 3-fold axis, with partial site occupancy
water molecules (around the 3-fold axis) (Table 3a, b). This configuration was found in some of the previous studies on natural or NO3-cancrinites (Jarchow 1965; Grundy and Hassan 1982; Belokoneva et al. 1986; Fechtelkord et al. 2001; Ballirano and Maras 2004). However, in other studies the OW site was located at 2/3, 1/3, z (Bresciani Pahor et al. 1982; Hassan et al. 2006). Actually, the water molecule adopts a tetrahedral configuration with two lone pairs directed toward the Na cations, to form the so-called ‘‘type A lone pair coordination’’ of Chidambaram et al. (1964). The Na1–Ow–Na1 angle is, however, rather large (Table 4) with respect to the values typical for such a configuration reported by Chiari and Ferraris (1982). Based on the structural refinements of this study the geometry of the water molecule is characterized by bond distances and angles expected for water molecules in an open-framework structure (Table 4a, b). Only the H1–OW–H2 bond angle in the refinement of cancrinite from Cameroun appears to be slightly lower [101(2), Table 4a] than the expected value, but still in the range of the observed H–O-H angles in solid-state materials (Chiari and Ferraris 1982; Steiner 1998 and references therein). We cannot exclude that the low H1–OW–H2 bond angle value might be affected by a slight positional disorder of the proton sites, as confirmed by the large thermal displacement parameters of the H1 and
Si–O1
1.605(3)
O1–Si–O2
107.5(2)
Si–O2
1.609(3)
O1–Si–O4
108.5(2)
Si–O4
1.614(4)
O1–Si–O3
110.2(2)
Si–O3
1.622(4)
O2–Si–O4
112.2(2)
\Si–O[
1.612
O2–Si–O3
111.5(2)
O4–Si–O3
107.0(2)
Al–O2
1.719(3)
Al–O1
1.723(4)
O2–Al–O1
106.2(2)
Al–O3
1.740(4)
O2–Al–O3
114.2(2)
Al–O4 \Al–O[
1.743(5) 1.731
O2–Al–O4 O1–Al–O3
113.7(3) 107.8(2)
O1–Al–O4
109.1(2)
C1–OC1 (x3)
1.298(7)
O3–Al–O4
105.6(2)
C2–OC2 (x3)
1.300(5) H1–OW–H2
101(2)
Na1–OW
2.354(13)
Na1–Ow–Na1
155.6(4)
Na1–O2 (x3)
2.434(3)
Na1–O1 (x3)
2.855(2)
OW–H1
0.975(16)
Na1–OW0
2.891(4)
OWO2
3.17(1)
H1O2
2.57(4)
Na2–OC2
2.398(6)
OW–H1O2
120(2)
Na2–OC1
2.399(9)
OWO4
3.44(1)
Na2–OC20
2.417(6)
H1O4
2.56(2)
Na2–OC10
2.421(7)
OW–H1O4
151(2)
Na2–O4
2.430(5)
(OW–H1)\ (0001)
8(2)
Na2–OC100 Na2–O3
2.440(8) 2.452(5)
OW–H2
00
2.475(8)
OWO3
˚ 0.978(18)A ˚ 3.38(1)A
Na2–O1
2.545(5)
H2O3
˚ 2.46(2)A
Na2–O40
2.900(8)
OW–H2O3
157(1)
OWO2
˚ 3.22(1)A ˚ 2.62(3)A
Na2–OC2
H2O2 OW–H2O2
120(1)
(OW–H2)\ (0001)
7(2)
(b) Canada Si–O2
1.607(3)
O1–Si–O2
107.4(1)
Si–O1
1.608(3)
O1–Si–O4
108.1(1)
Si–O4
1.615(3)
O1–Si–O3
110.4(1)
Si–O3
1.619(3)
O2–Si–O4
111.8(1)
\Si–O[
1.612
O2–Si–O3
111.9(1)
O4–Si–O3
107.1(1)
Al–O2
1.721(4)
Al–O1
1.729(4)
O2–Al–O1
106.0(1)
Al–O4
1.737(3)
O2–Al–O3
113.8(2)
Al–O3
1.746(3)
O2–Al–O4
114.6(2)
\Al–O[
1.733
O1–Al–O3
107.2(2)
O1–Al–O4
109.4(2)
C1–OC1 (x3)
1.283(7)
O3–Al–O4
105.6(2)
123
202
Phys Chem Minerals (2009) 36:193–206
Table 4 continued C2–OC2 (x3)
1.288(5)
Na1–OW
2.358(9)
H1–OW–H2
104(2)
Na1–Ow–Na1
154.9(5)
Na1–O2 (x3)
2.432(4)
Na1–O1 (x3)
2.861(5)
OW–H1
0.945(17)
Na1–OW0
2.889(9)
OWO2
3.153(9)
H1O2
2.53(3)
Na2–OC2
2.408(9)
OW–H1O2
124(2)
Na2–OC1
2.422(9)
OWO4
3.426(9)
Na2–OC10 Na2–O4
2.425(9) 2.425(4)
H1O4 OW–H1O4
2.62(2) 143(2)
(OW–H1)\ (0001)
8(2)
Na2–OC2’
2.442(7)
Na2–O3
2.443(5)
Na2–OC200
2.457(7)
OW–H2
0.957(16)
Na2–OC100
2.502(13)
OWO3
3.38(1)
Na2–O1
2.521(4)
H2O3
2.47(2)
Na2–O40
2.891(5)
OW–H2O3
158(1)
OWO2
3.228(9)
H2O2
2.69(3)
OW–H2O2
116(1)
(OW–H2)\ (0001)
8(2)
Fig. 6 Polarized-light spectra of cancrinite C38834 from Canada, 10 lm thick (hk0) section
Fig. 7 Polarized-light spectrum of cancrinite from Cameroun, 30 lm thick (hk0) section
Fig. 5 Unpolarized spectrum of cancrinite C38834 from Canada, 10 lm thick (hk0) section
H2 sites (Table 4a) or, more likely, by the lower resolution of the diffraction data of this cancrinite (Table 1). The H2O molecule does not lie on a plane k(0001), being z(OW) = z(H1) = z(H2) (Table 3a, b). Any attempt to fix OW, H1, and H2 on a plane perpendicular to [0001] gave unstable refinements. Likely, the configuration of the noncoplanar OW, H1, and H2 is energetically more favorable for the interaction with the Na1-site and for the H-bonding to the oxygens belonging to the tetrahedral framework. Four possible hydrogen bonds, two for the H1 (i.e., OW–H1O2 and OW–H1O4) and two for H2 (i.e.,
123
OW–H2O3 and OW–H2O2), have been reported in Table 4a, b and are shown in Fig. 8a. The Na1-site forms a distorted coordination polyhedron, with CN = 8 and ˚ (Table 4a, b). Two of maximum bond distance *2.89 A the vertexes are represented by water molecules (Table 4a, b), above and below the equatorial plane where the further bonded oxygens (i.e., O1 and O2, Fig. 8b; Table 4a, b), belonging to the tetrahedral framework, lie.
Interpretation of the FTIR data The H2O/OH absorptions One point of uncertainty in the cancrinite structure is the exact location and bonding of the water molecules. Attempts to solve this problem with proton magnetic resonance (PMR) (Sokolov et al. 1977) or infrared
Phys Chem Minerals (2009) 36:193–206
Fig. 8 Configuration of the hydrogen bonds in cancrinite. The data from the Cameroun sample are shown (Table 3a, b); the H2O molecular plane is stippled; black tetrahedra are Al and white tetrahedra are Si
spectroscopy (Galitskii et al. 1978; Ballirano and Maras 2004) gave conflicting results. In particular both the PMR study of Sokolov et al. (1977) and the FTIR work of Galitskii et al. (1978) indicate the presence of at least two different types of water molecule in the cancrinite structure. However, the combination of high-quality neutron diffraction data and polarized single-crystal FTIR data available here may shed some light on this unresolved problem. The neutron structure refinements, which constitute the obvious starting point for any discussion on this issue, show that cancrinite has only one type of H2O molecule which is localized within the undecahedral cages (Figs. 3, 4). The data exclude the presence of additional water molecules within the broad channels, as suggested by Galitskii et al. (1978). The water molecule is almost symmetric, with very similar (within experimental uncertainty, Table 4a, b; Fig. 4) Ow–H1 and OW–H2 bond distances; in addition, the
203
molecule is slightly tilted with respect the (0001) plane (Fig. 4), the titling angle being *8 (Table 4a, b). ˚ The OwO donor–acceptor distances are all [2.7 A (Table 4a, b), hence, the hydrogen bonds in cancrinite are to be classified as weak (e.g., Novak 1974; Beran and Libowitzky 1999); in agreement with this, the OwO contacts are far from being linear, with the Ow-HO angles ranging between *116 and *160 (Table 4a, b). The configuration of the hydrogen bonds is displayed in Fig. 8 where it is evident that both H1 and H2 hydrogen atoms are involved in very weak and bifurcated hydrogen bond with the O4 ? O2 and O3 ? O2 oxygens, respectively. On this basis, both the OH-stretching bands present in the spectrum of cancrinite (Fig. 6) must be assigned to the same H2O molecule, in agreement with the presence of a single bending mode at 1,630 cm-1 (Fig. 6). Therefore, the 3,602 cm-1 band is assigned to the anti-symmetric stretching mode (m3), while the 3,531 cm-1 band is assigned to the symmetric stretching mode (m1) of this water molecule. The m3–m1 separation is *70 cm-1; this value is in close agreement with the m3–m1 frequency separation calculated from the FS (frequency-splitting) correlation of Schiffer et al. (1976). The extensive spectroscopic work done on microporous materials like beryl or cordierite (see Wood and Nassau 1967; Charoy et al. 1996; Kolesov and Geiger 2000, among others) shows that, in the case of free water molecules (point symmetry C2v), the polarization of the m3 mode is inversely related to that of both m1 and m2; in particular, the polarization of the m3 mode gives the direction of the HH vector, whereas the polarization of the m1 and m2 modes give the direction of the molecular plane. Differently from beryl, in cancrinite both the HH vector and the molecular plane are oriented perpendicular to the c axis (Fig. 4), thus all the internal vibrational modes of the water molecule are polarized for E \ c (Fig. 6). Due to the tilting of the molecular plane with respect to (0001), the m1 mode actually has a component parallel to c, and in agreement with this, it shows a residual intensity for E k c (Fig. 6). The relationship between the O–H stretching frequencies and the geometry of the O–HO bond-system for inorganic compounds has been the subject of several papers (see Nakamoto et al. 1955; Huggins and Pimentel 1956; Novak 1974; Mikenda 1986 among others). Refined correlations for minerals have been recently presented by Libowitzky (1999), based on the analysis of a large data set of well-characterized and selected structural and spectroscopic data. An important issue discussed by Libowitzky (1999) is that, although the O–H stretching frequency in the IR spectrum of a mineral is clearly related to the OwO and HO distances, systematic deviations from the general trends are observed; these deviations are enhanced in the
123
204
case of strong H bonds or in the case of non-linear OwO or bifurcated HO bonds. In particular, bent H bonds yield constantly too high wavenumbers in the d(O O)–m plot, while yielding frequency values close to the theoretical ones in the d(HO)–m plot. A different situation is observed in the case of bifurcated H bonds: in such a case, frequency values systematically below the trend are observed, due to the doubled attractive force of the two proton acceptors. Accordingly, for the examined cancrinites, the observed frequencies are systematically lower than those calculated using either the (Ow–O)-m relationship for silicates only [m(cm-1) = 3,622–239 9 109 9 exp(-d(OO)/0.1346] or the (HO)-m relationship [m(cm-1) = 3,680–411 9 103 9 exp(-d(OO)/0.2683; both from Libowitzky 1999]. The minor absorptions at 4,108 and 3,848 cm-1 (Fig. 6) now deserve additional comments. The former is strongly polarized for E k c, and its frequency is compatible with the combination of the stretching plus a bending mode of an OH group, or to the combination of the stretching mode of an OH group with the stretching mode of a metal-oxygen (es. Na–O) bond (Ihinger et al. 1994). Zeolites commonly contain OH groups connected with Si–Al linkages (Brønsted sites). For weak hydrogen bridges, these OH groups are characterized by O–H stretching vibrations (mOH) located around 3,615 cm-1, and in-plane (dOH) or out-of-plane (cOH) modes located, respectively, around 1,000 and 500 cm-1 (Zecchina et al. 2002). For example, in some synthetic zeolites (ZSM-5 or H-mordenite), mOH *3,615–3,605 cm-1, dOH *1,090–1,060 cm-1 and cOH *420–320 cm-1 (Zecchina et al. 1996). As discussed above, the neutron diffraction refinements do not show any hydroxyl group in the examined samples. A closer inspection of the FTIR spectra, see for example the enlargement of the polarized spectrum of cancrinite from Cameroun (Fig. 9), however show the presence of a relatively sharp absorption at 3,590 cm-1 which shows-up in the E k c spectrum; this band is completely overlapped by the broader band at 3,602 cm-1 for E \ c (Fig. 9). The 3,590 cm-1 component might be assigned to O–H groups locally present in the structure, with a concentration insufficient to be localized by neutron diffraction. In such a case, the absorption at 4,108 cm-1 could be assigned to the combination of a mOH at 3,590 cm-1 with an out-of-plane cOH bending around 518 cm-1. Assignment of the band at 3,848 cm-1 is not straightforward and possibly involves a combination between the internal (m3 and m1) and the external (translational T and librational R) modes of the water molecule, which are typically located at low frequency. An example is provided by beryl, where the band at 3,860 cm-1 observed in the IR spectrum is assigned to a combination between the m3 at 3,694 cm-1 and the Rx libration (rocking on the molecular
123
Phys Chem Minerals (2009) 36:193–206
Fig. 9 Polarized-light spectrum of cancrinite from Cameroun, 30 lm thick (hk0) section. Solid line: E k c, broken line E \ c
plane) at 170 cm-1 (Wood and Nassau 1967). This interpretation is supported by the polarization of the 3,848 cm-1 band for E \ c (Fig. 6) which is consistent with the combinations of the symmetries of the m3 (B2) and Rx (A1) modes. The absorptions of the carbonate group As shown in Fig. 7, the carbonate absorption in the 1,300– 1,500 cm-1 region is complex and consists of several overlapping bands; at least five prominent components centered at 1,512; 1,480; 1,434; 1,397; and 1,381 cm-1 are resolved, plus several additional shoulders. These bands are all strongly polarized for E \ c and thus must all be assigned to C–O bonds which, according to the structure refinements, are all perpendicular the c crystallographic axis. The site symmetry of the free CO3 group in calcite is D3, hence a single, doubly degenerate (E) m3 broad band is observed (e.g., Nakamoto 1963; White 1974); this is not the case for the CO3 group in cancrinite (Fig. 7), where the disorder of the extraframework Na and Ca cations, bonded to the OC1 and OC2 oxygens, locally reduces the site symmetry of the CO3 group to Cs, like in aragonite. In such a case, the degeneracy is raised and two groups of splitted bands are observed (Fig. 7). The spectra of Fig. 7 show two additional sharp bands at 859 and 768 cm-1, respectively, which are totally polarized for E k c and thus must again be assigned to the vibration of the carbonate group in the studied sample. The 859 cm-1 component can be easily assigned to the m2, symmetric bending mode which is found at 866 cm-1 in aragonite and 879 cm-1 in calcite (Nakamoto 1963). The frequency of the 768 cm-1 band is relatively close to that of the m4, antisymmetric bending
Phys Chem Minerals (2009) 36:193–206
mode of the carbonate group (around 705 cm-1 in both calcite and aragonite: Nakamoto 1963). The assignment to this band to a carbonate group is also supported by the fact that the powder spectra of several cancrinite-group minerals with different CO3/SO4 ratio show that the intensity of this band is roughly proportional to the CO3 content of the sample (unpublished data). However, the polarization of the m4 mode is expected to be \ c in carbonates, in contrast to what is observed in Fig. 7. For this reason, we tentatively assign both the 859 and the 768 cm-1 bands to a splitted m2 bending, where each m2 component is associated with each of the two groups of splitted m3 bands. Acknowledgments F. Ca´mara (Pavia) kindly oriented the crystals studied by FTIR using X-ray diffraction; Dr. M. Picard, Canadian Museum of Nature (Ottawa) provided the studied specimen from Canada. E. Libowitzky (Wien) gave helpful suggestions for the interpretation of the FTIR data. Thanks to W. Depmeier and to an anonymous referee for positive criticism. Financial support was provided by MIUR Project 2006040119_004 to GGD and MIUR Project 2005043715_005 to GDV. Part of this research was done at the Laboratoire de Mineralogie, Museum National d’Histoire Naturelle, Paris, thanks to the support from the SYNTHESYS Project (http://www.synthesys.info/), financed by European Community Research Infrastructure Action under the FP6 ‘‘Structuring the European Research Area’’ Programme.
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