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Crystal-chemistry of magnesiocarpholite

¹ Laboratoire de Minéralogie Cristallographie de Paris, UMR 7590 du CNRS, case 115, Université Pierre et Marie Curie, 4 Place Jussieu, F-75252 Paris Cedex 05, France
² Dipartimento di Scienze della Terra, Università di Siena, Via Laterina 8, 1–53100 Siena, Italy

e-mail: memmi{at}unisi.it

This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)

	Abstract

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Magnesiocarpholite, MgAl₂Si₂O₆(OH)₄, is a high-pressure/low-temperature mineral occurring in the metamorphic rocks of southern Tuscany (Monte Leoni and Monte Argentario). Chemical, Mössbauer, FTIR and single-crystal XRD characterizations were carried out to understand the symmetry and crystal chemistry of the mineral, since forbidden diffractions violating the space group Ccca have been reported for ferrocarpholite and oblique extinction for carpholite. Tuscany magnesiocarpholite contains significant fluorine and minor potassium. Iron occurs mostly as ferrous iron, within one octahedral site. Crystal chemical formulae are: K_0.002(Mg_0.65Fe²⁺_0.32Fe³⁺_0.03)_=1.00 Al_1.97Si_2.00O_5.90(OH)_3.95F_0.15 and K_0.002(Mg_0.65Fe²⁺_0.34Fe³⁺_0.01)_=1.00 Al_1.98Si_2.00 O_5.92(OH)_3.96F_0.12, for Monte Argentario and Monte Leoni, respectively. Single-crystal X-ray diffraction refinements (R₁ = 0.020 and 0.024) largely confirm the apparent Ccca symmetry and indicate residual maxima corresponding to the K-site of non-stoichiometric potassium and fluorine-bearing carpholite. However, a few reflections violating the a glide plane were observed in the Monte Argentario specimen, which also shows anomalous values of the unit-cell parameters.

The Monte Argentario magnesiocarpholite also shows differences in the Raman and FTIR spectra, with respect to both Monte Leoni magnesiocarpholite and ferrocarpholite (four to five O-H stretching bands rather than three; different spectral features in the Si-O stretching region, pointing to more distorted bonding pattern in the Monte Argentario specimen). The contradictory results are explained in terms of different long-range (revealed by X-ray diffraction) and short-range order (revealed by FTIR and Raman spectroscopies), that involve local arrangements, like point defects.

Key-words: magnesiocarpholite, crystal structure, infrared spectroscopy, Raman spectroscopy, Mössbauer spectroscopy, order-disorder.

	Introduction

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Magnesiocarpholite, MgAl₂Si₂O₆(OH)₄, is a highpressure/low-temperature mineral characteristic of the blueschist metamorphic associations (Viswanathan & Seidel, 1979; Chopin & Schreyer, 1983). The mineral is isomorphically related to ferrocarpholite and carpholite s.s. (iron and manganese replacing magnesium, respectively). Potassium- and fluorine-bearing carpholites have been also reported (Chashka et al., 1973). All these minerals have common crystal structures (Ferraris et al., 1992; Viswanathan, 1981), with slight differences in the M1 site (the one hosting magnesium, iron and manganese), and the way by which fluorine replaces hydroxyl (Ghose et al., 1989).

The aim of this paper is two-fold:

1. to report the common occurrence of magnesiocarpholite in the metamorphic rocks of southern Tuscany, with possible implications on the local P/T metamorphic regimes.
   
2. to produce a comprehensive chemical, spectroscopic and structural characterization of the same magnesiocarpholite specimens. The approach is necessary because of the persisting uncertainty over the true symmetry. Ccca is the generally accepted space group, but contradictory evidence has been presented: forbidden X-ray diffractions in ferrocarpholite (Ferraris et al., 1992), oblique extinction in carpholite (Mottana & Schreyer, 1977).
   

	Occurrence

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Magnesiocarpholite comes from two outcrops of the Monticiano-Roccastrada Unit (Verrucano Formation, Northern Apennine, Italy), at Monte Leoni (ML) and Monte Argentario (MA). The Verrucano rocks (Triassic in age) mainly consist of metapelites and phyllites (Franceschelli et al., 1986) which, during the Apennine orogeny, experienced lowtemperature metamorphism. The outcrops extend from the pyrophyllite + quartz zone to the kyanite + quartz zone (Franceschelli et al., 1986). Recently, magnesiocarpholite from syn-folial quartz-calcite segregations, widespread in Verrucano rocks, has been reported (Theye et al., 1998; Giorgetti et al., 1998). The presence of magnesiocarpholite indicates a previously unrecognized high-pressure episode during the metamorphism of the Triassic metasedimentary sequence.

Magnesiocarpholite forms green, columnar or fibrous crystals, up to several centimetres in length. The individual fibres may be as small as a few micrometers wide. The crystals have prevalent parallel extinction (more evident in the MA samples). They may be highly fractured, and weathering products are seen as secondary phases within the fractures (Giorgetti et al., 2000). Magnesiocarpholite fibres included in large quartz crystals suggest the contemporaneous formation of these two minerals.

	Chemical composition

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
The carpholite structure allows a large variety of cationic and anionic substitutions (Ghose et al., 1989). Therefore, before any spectroscopic analysis, we have obtained a comprehensive view of the MA and ML chemistry, by extensive analysis of several different specimens, using a Philips XL30 Scanning Electron Microscope (SEM), equipped with an Energy Dispersive Spectrometer (EDS). MA and ML magnesiocarpholite share common compositional ranges; the magnesium mole fraction X_Mg = Mg/(Mg+Fe) spans from 0.63 to 0.71. The specimens are homogeneous, with very limited intracrystalline and intercrystalline variation.

Representative specimens from the two occurrences were selected for further analysis by the electron microprobe. The data (reported in Table 1) were obtained using a JEOL Superprobe JXA-8600 (University of Florence) operating at 20 kV acceleration voltage, with beam current of 15 nA, beam diameter of 5 µm and counting time of 10–30 seconds. Raw concentrations were recalculated according to Bence & Albee (1968). Iron was distributed between ferrous and ferric iron on the basis of the Mössbauer results (see below), which indicated ferric iron contents corresponding to 8.7 and 3.8 % of total iron (MA and ML, respectively).

=1.00

=1.00

	Mössbauer spectroscopy

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Mössbauer spectroscopy was carried out at room temperature (300 K), using 200 mg of powder embedded in an iron-free aluminium foil. The experiments were performed with the absorber oriented 54° to the k vector (magic angle technique) ruling out texture effects. Spectra shown in Fig. 1 were recorded on a 512 channels analyzer, in a constant acceleration mode, using a source of ⁵⁷Co diffused into a Rh matrix. Folding of the data was applied, reducing the number of channels to 256. Data were calibrated using -Fe. The ⁵⁷Fe hyperfine parameters (reported in Table 2) were derived using the MOSFIT program (Varret, 1981) and Lorentzian peak shape.

View larger version (11K): [in this window] [in a new window]   Fig. 1. 57Fe Mössbauer spectra of magnesiocarpholite from Monte Leoni (top) and from Monte Argentario (bottom).

1. The spectral parameters are constant, independent from the actual carpholite composition (in particular, going from magnesio- to ferrocarpholite). The unusually large value of the quadrupole splitting parameter (close to 3.20 mm/s), first emphasized by Seifert (1979) for ferrocarpholite, is also present in magnesiocarpholite. For comparison, iron octahedra in the TO serpentine lizardite display = 2.69 mm/s (Fuchs et al., 1998).
   
2. Ferrous iron largely dominates over ferric iron (that amounts to 3.8 % and 8.7 % of total iron only, in MA and ML, respectively).
   
3. The spectra indicate just one octahedral site for both ferrous and ferric iron.
   
4. No tetrahedral ferric iron occurs in magnesiocarpholite.
   

	FTIR and Raman results

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
The FTIR spectra were recorded using a Nicolet 560 ESP spectrometer. Samples were powdered and diluted in KBr at 0.5 and 1.5 % concentration, and pressed into disks. Polarized Raman spectra were obtained on carpholite single crystals in four different orientations (see Table 3) with a Labram (Jobin-Yvon) instrument, equipped with an Ar⁺ Laser ( = 514.5 nm); emission power was 300 mW with emission time of 10 s.

The O-H stretching vibrations characterize this region. So, any difference in the spectra in this region means different OH bonds. Main indications are:

1. MA and ML magnesiocarpholite have similar but not identical spectra. In particular, (Fig. 2) MA presents more FTIR bands than ML (e.g., four to five rather than three absorption bands). In turn, ML does not differ from ferrocarpholite (Ferraris et al., 1992).
   
2. The highest-energy vibrations (close to 3700–3650 cm^–1) are more evident in MA, and determine more complex spectra.
   
3. The strongest absorption bands occur at 3630–3621 cm^–1 and 3581–3583 cm^–1. The first one is attributed to the OH(1)-H(1) stretching vibration and the second one to OH(2)-H(2). In fact, H(1) does not form hydrogen bonds (see structural section) and H(2) forms weak bonds with O(3).
   
4. The previous band assignment is confirmed by polarized Raman spectroscopy (Fig. 3). Raman pleochroism is extremely evident, in terms of intensities and number of the bands, which dramatically change with the crystal orientation. Considering that the OH(1)-H(1) bond almost lies along c and the OH(2)-H(2) bond along a (see structural section), we expect that the first vibration will be enhanced for sections containing the c axis and polarizer parallel to z; on the other hand, the second one will be excited for sections containing the x axis, and polarizer parallel to it. The expectation is verified in the absorption scheme reported in Table 3.
   
5. The other bands in this spectral region are tentatively explained assuming different local octahedral environments, responsible for the FTIR and Raman shifts. For instance, OH(1) (not involved in any hydrogen bond with a basic frequency of 3630–3621 cm^–1) is shared among two M(1) and one Al(1) octahedral sites, with 7 as sum of the charges of the three corner-sharing octahedra; OH(2) (engaged in hydrogen bonding with O(3) with a frequency of 3590–3580 cm^–1) is shared among one M(1) and two Al(2) sites, giving 8 as sum of the octahedral charges. The two frequencies will be lowered in the case of cationic substitutions leading to higher sums (e.g., trivalent iron replacing for divalent one), and increased in the opposite case (e.g., lithium or vacancies in octahedral coordination). In conclusion, the weak 3700 cm^–1 band of ML might be due to lithium for magnesium replacement; the 3570 cm^–1 band of MA to Fe³⁺ for Fe²⁺ replacement.
   

View larger version (13K): [in this window] [in a new window]   Fig. 2. FTIR spectra spectra of magnesiocarpholite from Monte Argentario and Monte Leoni in the OH-stretching region.

View larger version (17K): [in this window] [in a new window]   Fig. 3. Polarized Raman spectra of magnesiocarpholite from Monte Argentario (top) and Monte Leoni (bottom) in the OH-stretching region; (1) = c axis horizontal and parallel to the polarization direction; (2) = c axis horizontal and normal to the polarization direction; (3) = c axis vertical and most developed crystal face parallel to the polarization direction; (4) = c axis vertical and most developed crystal face normal to the polarization direction.

This region of the FTIR spectrum corresponds to the stretching vibrations of the Si-O bonds (Fig. 4). Four crystallographically independent Si-O bonds (with bond distances of 1.600, 1.615, 1.631 and 1.637 Å in MA; 1.603, in MA; 1.603, 1.612, 1.630 and 1.642 Å in ML) occur in magnesiocarpholite. We therefore expect up to four different absorption frequencies, spanning over quite a range of energy.

View larger version (13K): [in this window] [in a new window]   Fig. 4. FTIR spectra of magnesiocarpholite from Monte Argentario and Monte Leoni in the 1200–900 cm–1 region corresponding to the stretching vibrations of the Si-O bonds.

The 650–400 cm^–1 region

The two FTIR spectra are characterized by common frequencies at 635 and 610 cm^–1. MA reveals also an additional 538 cm^–1 absorption.

	Structural data

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Preliminary qualitative investigation and selection of crystals were made by photographic techniques using a Weissenberg goniometer. The crystals were tiny needles elongated parallel to c; they almost invariably showed split diffraction spots; this feature, according to Viswanathan (1981) has been attributed to tectonic deformation. After several attempts, we finally succeeded in finding some fibres (0.2 x 0.05 x 0.05 mm in size) good enough for quantitative data collection.

Single-crystal data were collected by a Siemens P4 four-circle diffractometer, using graphite-monochromatized Mo-k radiation ( = 0.7107 Å), operated at 55 kV and 25 mA. The unit-cell data (Table 4) were determined and refined using the accurate positioning of 25 reflections, measured at high values. Whereas systematic extinctions univocally indicate Ccca as the space group of ML, a few weak forbidden reflections appear in the MA data collection (5 3 0, 7 1 0, 7 3 0, 9 5 0 and 9 13 0), apparently violating the a glide plane. After confirmation of the Ccca space group, 2428 and 1610 symmetry-related reflections in the 2–30° range were measured for ML and MA, respectively. No crystal decay or instrumental drift was observed during data collection. After correction for Lorentz, polarization and empirically determined absorption effects (-scan), the symmetry-related reflections were merged to produce two sets of 1037 unique reflections, with discrepancy factors among symmetry related reflections R(int) of 0.014 and 0.028 for ML and MA, respectively.

After refinement convergence, a difference Fourier synthesis showed the presence of a residual peak centred at 0, 3/4, 1/4 for both ML and MA. Although small (less than 1.5 e^–/ų), this peak is important because it corresponds to the K-site proposed by Ghose et al. (1989) to explain the crystal chemistry of nonstoichiometric, potassium- and fluorine-bearing carpholite. The K-site was introduced in the refinement with variable, partial occupancy and, eventually, the refinement converged at R₁ = 0.020 (866 reflections with F₀ > 4(F₀) and wR₂ = 0.062 (all data) in the case of ML magnesiocarpholite, and at R₁ = 0.024 and wR₂ = 0.094 in the case of MA magnesiocarpholite. At this stage, no peak larger than 0.5 e^–/ ų was present in the final difference Fourier synthesis. The final atomic positional and displacement parameters are given in Table 5. Selected bond distances are given in Table 6. Bond strength balance is given in Table 7.

	Estimated chemical compositions

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

	Bond geometries

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Apart from their greater precision, the present data (Table 6) match the previous data on magnesiocarpholite (Viswanathan, 1981) and ferrocarpholite (Ferraris et al., 1992). The following points, relevant to the present discussion, deserve further attention.

• — the M(1) site (hosting iron) definitely deviates from the ideal octahedral configuration, having two short, two intermediate and two long bonds. This asymmetric bond pattern explains the unusually high quadrupole splitting values in the Mössbauer spectra.
  
• — Although similar, the two Al sites have slightly different sizes; therefore, if any Fe³⁺ occurs within this site, Al(2) would be favoured in terms of ionic radii.
  
• — The average Si-O bond distance corresponds to the value expected for a pure silicon tetrahedron. However, the bonding geometry is quite distorted, as already pointed out while discussing the FTIR absorption bands.
  

	Bond strength balance

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Table 7 reports the bond strength balance, calculated following Brown & Altermatt (1985) for ML magnesiocarpholite (MA not reported because it does not differ significantly from ML). These data are in agreement with the previous interpretation of bond geometry and site population. In particular, all sums of positive charges leaving the cations do not deviate much from the expected value of 2 for M(1), 3 for Al(1) and Al(2), 4 for Si. At the same time, the positive charges reaching the anions approaches 2 for oxygens and 1 for hydroxyls. The major discrepancy involves OH(2), that sums up to 1.15 and 1.18 valence units for ML and MA, respectively. This excess charge is significant and evidence for the hydrogen bond connection existing between the donor atom OH(2) and the acceptor atom O(3), which shows the largest charge deficit (1.93 vs. 2.00 valence units). Conversely, there is no evidence for any hydrogen bond from OH(1).

	Conclusions

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
Iron is present in magnesiocarpholite essentially as ferrous iron, within one octahedral site. The reduced state of the mineral indicates that the Verrucano rocks underwent metamorphic evolution under mildly oxidizing conditions. Basically, ML and MA magnesiocarpholites both show important fluorine contents and small, but detectable, potassium contents. Other previous routine analytical determinations may have neglected search for these two elements, as potassium and fluorine perhaps are constantly present constituents. The average structural arrangement revealed by this study largely confirms both the apparent Ccca symmetry and the bonding pattern already known for the differently substituted carpholite-like minerals. One more structural evidence is the occurrence of residual maxima corresponding to the K-site of non stoichiometric potassium and fluorine-bearing carpholite (Ghose et al., 1989). Contradictory data still exist. At the level of X-ray diffraction, a few reflections violating the a glide plane in MA were observed. Furthermore, the larger the unit-cell volume (Table 4), the smaller was the refined iron content; this trend is just the opposite of what is expected for an ideal magnesium-iron solid solution series. Notwithstanding the common chemical composition, the two occurrences largely differ in spectroscopic features (four to five FTIR O-H stretching bands in MA, three in ML; more complex Raman pattern for MA than ML; different spectral pattern in the Si-O stretching region, pointing to more distorted bonding pattern of MA). Ferraris et al. (1992) investigated the real symmetry of magnesian carpholite, and suggested that order/disorder involving hydrogen atoms and cationic or "anionic" impurities might lead to real symmetry lower than the one revealed by X-rays. Magnesiocarpholite really shows contradictory results regarding the long-range ordering behaviour (i.e., the one revealed by X-ray diffraction) and the shortrange ordering behaviour (i.e., the one revealed by FTIR and Raman spectroscopies). Such controversial features may be explained on the basis of extended or point defects such as intergrowth of two different structures (e.g., pyroxene-amphibole in biopyriboles) and disordered distribution of faulted structures without clustering of defects. High-resolution transmission electron microscopy has been recently applied to the same ML specimens by Giorgetti et al. (2000), who characterized the alteration products, but did not find any clustered defects. We conclude that the short range ordering in MA and ML magnesiocarpholite originates from local arrangements like point defects.

	Acknowledgements

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
We thank Dr. M.L. Frezzotti for help in doing Raman analyses, PNRA for the Raman facilities and GNV for the microprobe facilities. Financial support was given from MURST to M. Mellini. The authors are indebted with Prof. G. Ferraris, S. Ghose and E. Libowitzky for careful revisions.

Received 16 May 2000
Modified version received 9 November 2000
Accepted 18 December 2000

	References

Top Abstract Introduction Occurrence Chemical composition Mossbauer spectroscopy FTIR and Raman results Structural data Estimated chemical compositions Bond geometries Bond strength balance Conclusions Acknowledgements References

 
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Brown, I.D. & Altermatt, D. (1985): Bond-valence parameters obtained from a systematic analysis of the inorganic structure database. Acta Cryst., B41, 244–247.

Chashka, A.I., Marchenko, E. Ya., Gurov, E.P., Khvostova, V.A., Peterson, R.M. (1973): Potassium- and fluorinecontaining carpholite. Zapiski Vsesoyuznovo Mineralogischeskovo Obschestva, 102, 82–86 (in Russian).

Chopin, C. & Schreyer, W. (1983): Magnesiocarpholite and magnesiochloritoid: two index minerals of pelitic blueschists and their preliminary phase relations in the model system MgO - Al₂O₃ - SiO₂ - H₂O. Amer. J. Sci., 283A, 72–96.

Ferraris, G., Ivaldi, G., Goffé, B. (1992): Structural study of a magnesian ferrocarpholite: Are carpholites monoclinic? N. Jb. Mineral. Mh., 1992, 337–347.

Franceschelli, M., Leoni, L., Memmi, I., Puxeddu, M. (1986): Regional distribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy. J. Metamorphic Geol., 4, 309–321.[CrossRef]

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Giorgetti, G., Goffé, B., Memmi, I., Nieto, F. (1998): Metamorphic evolution of Verrucano metasediments in northern Apennines: new petrological constraints. Eur. J. Mineral., 10, 1295–1308.[Abstract/Free Full Text][GeoRef]

Giorgetti, G., Memmi, I., Peacor, D.R. (2000): Contrast in processes and products of weathering of carpholite and associated phyllosilicates: a TEM study. Eur. J. Mineral., 12, 33–44.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

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Seifert, F. (1979): A note on the Mössbauer spectrum of ⁵⁷Fe in ferrocarpholite. Mineral. Mag., 43, 313–315.[CrossRef][ISI][GeoRef]

Theye, T., Reinhardt, J., Goffé, B., Jolivet, L., Brunet, C. (1998): Ferro- and magnesiocarpholite from the Monte Argentario (Italy): first evidence for high-pressure metamorphism of the metasedimentary Verrucano sequence, and significance for P-T path reconstruction. Eur. J. Mineral., 9, 859–873.[ISI]

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Weiser, F., Abs-Wurmbach, I., Seidel E. (1996): Crystal-chemical relations of iron in natural Mg-carpholites: a spectroscopic study. Phys. Chem. Minerals, 23, 237–238.

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by FUCHS, Y. Articles by MEMMI, I. Search for Related Content GeoRef GeoRef Citation