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Structural Investigations of the Two Polymorphs of Synthetic Fe-Cordierite and Raman Spectroscopy of Hexagonal Fe-Cordierite

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Structural Investigations of the Two Polymorphs of Synthetic Fe-Cordierite and Raman Spectroscopy of Hexagonal Fe-Cordierite Miner Petrol DOI 10.1007/s00710-013-0313-3 ORIGINAL PAPER Structural investigations of the two polymorphs of synthetic Fe-cordierite and Raman spectroscopy of hexagonal Fe-cordierite Udo Haefeker & Reinhard Kaindl & Peter Tropper & Hannes Krüger & Volker Kahlenberg & Maria Orlova Received: 11 June 2013 /Accepted: 25 September 2013 # The Author(s) 2013. This article is published with open access at Springerlink.com Abstract The crystal structures of synthetic hexagonal and Introduction orthorhombic Fe-cordierite polymorphs with the space groups P6/mcc and Cccm were refined from single-crystal X-ray Cordierite with the simplified formula (Mg, Fe)2Al4Si5O18*n(H2O, diffraction data to R1, hex=3.14 % and R1, ortho=4.48 %. The CO2) is a framework silicate whose structure consists of 2+ substitution of the larger Fe for Mg leads to multiple struc- corner-sharing T2 tetrahedra forming six-membered rings tural changes and an increase of the unit cell volumes, with a, c which are crosslinked by additional T1 tetrahedra. Crystal (hex)=9.8801(16) Å, 9.2852(5) Å and a, b, c (ortho)= structure and Al/Si-ordering of Mg-Fe-cordierites were inves- 17.2306(2) Å, 9.8239(1) Å, 9.2892(1) Å in the end-members. tigated by various authors (e.g., Gibbs 1966; Langer and Furthermore Fe incorporation results in an increase of the Schreyer 1969; Meagher and Gibbs 1977; Cohen et al. volumes of the octahedra, although the diameters of the octa- 1977; Hochella et al. 1979; Putnis 1980; Wallace and Wenk hedra in direction of the c-axis decrease in both polymorphs. 1980; Armbruster and Bloss 1981; Putnis and Bish 1983; X-ray powder diffraction analysis indicates a high degree of Al/ Gunter et al. 1984; Mirwald and Kirchner 1984;Armbruster Si ordering in the orthorhombic polymorph, the Miyashiro 1985a, b;Putnisetal.1985;Putnisetal.1987;Malcherek distortion index is ~0.24. Estimations of site occupancies based et al. 2001; Bertoldi et al. 2004 and references therein; on the determined tetrahedral volumes result in the following Balassone et al. 2004;Miletichetal.2010). The tetrahedral values for hexagonal Fe-cordierite: ~73 % Al for T1 and ~28 % positions are occupied by Si and Al. Cordierite occurs in two Al for T2. For the first time Raman spectroscopy was polymorphs which are distinguished by the Al/Si ordering. performed on the hexagonal Fe-cordierite polymorph. In the Most of the natural cordierites adopt the structure of the hexagonal Fe-cordierite polymorph most Raman peaks are orthorhombic low-temperature polymorph with space group shifted towards lower wavenumbers when compared with the symmetry Cccm and a fully ordered Al/Si distribution where Mg-end-member. Al occupies the T11andT26positionsandSioccupiesthe T16, T21andT23 tetrahedral sites (e.g., Cohen et al. 1977; Meagher and Gibbs 1977; Hochella et al. 1979). The hexag- Editorial handling: A. Beran onal high-temperature polymorph indialite (Miyashiro 1957) Electronic supplementary material The online version of this article with space group symmetry P6/mcc shows a disordered Al/Si (doi:10.1007/s00710-013-0313-3) contains supplementary material, distribution and is isostructural with beryl (e.g., Meagher and which is available to authorized users. Gibbs 1977 and references therein). The T tetrahedra are : : : : 1 U. Haefeker (*) P. Tropper H. Krüger V. Kahlenberg preferentially occupied by Al and the T2 tetrahedra by Si M. Orlova (e.g., Meagher and Gibbs 1977;Putnisetal.1985; Putnis Institute of Mineralogy and Petrography, University Innsbruck, and Angel 1985; Winkler et al. 1991). Selected bond distances Innrain 52, 6020 Innsbruck, Austria e-mail: [email protected] and angles of the Mg-cordierite polymorphs are given in Table 1. The information provided for hexagonal Mg- R. Kaindl cordierite is based on crystallographic data of Winkler et al. – MATERIALS Institute for Surface Technologies and Photonics, (1991). The octahedral M-sites are mainly occupied by Mg Functional Surfaces, JOANNEUM RESEARCH 2+ Forschungsgesellschaft mbH, Leobner Straße 94, 8712 Niklasdorf, and Fe and Mg-Fe mixing in orthorhombic cordierite is Austria assumed to be ideal (e.g., Boberski and Schreyer 1990). 90 U. Haefeker et al. Table 1 Structural data of both * ** polymorphs of Mg- and Cordierite Mg-hexagonal Mg-orthorhombic Fe-hexagonal Fe-orthorhombic Fe-cordierite Unit Cell (Å) a=9.7683 a=17.0552(1) a=9.8801(6) a=17.2306(2) b=9.7240(1) b=9.8239(1) c=9.3408 c=9.3480(1) c=9.2852(5) c=9.2892(1) Cell Volume (Å3) 772 1550.323(7) 784.95(8) 1572.40(3) <M-O>(Å) 2.102 2.123 2.153(2) 2.157(6) Octahedral Vol. (Å3) 11.739 12.011 12.400 12.411 λ, σ2 octahedra 1.037, 115.87 1.041, 129.66 1.048, 147.42 1.051, 155.41 <T1-O>, <T11-O>(Å) 1.721 1.754(2) 1.715(2) 1.746(7) <T16-O>(Å) 1.624(2) 1.632(6) <T2-O>, <T21-O>(Å) 1.648 1.609(3) 1.652(2) 1.601(8) <T23-O>(Å) 1.603(3) 1.634(9) <T26-O>(Å) 1.739(3) 1.736(8) 3 Vo l. T 1,Vol.T11(Å) 2.460 2.594 2.481 2.590 3 Vol. T16(Å)2.1222.179 3 Vo l. T 2,Vol.T21(Å) 2.293 2.132 2.308 2.100 3 Vol. T23(Å) 2.110 2.234 3 Vol. T26(Å)2.6882.671 2 λ, σ :T1,T11 1.042, 156.55 1.045, 165.20 1.028, 109.40 1.036, 137.14 2 λ, σ :T16 1.023, 90.35 1.016, 64.81 2 λ, σ :T2,T21 1.001, 3.85 1.002, 7.92 1.001, 4.84 1.001, 5.95 2 Data based on *Winkler et al. λ, σ :T23 1.001, 6.030 1.002, 6.52 (Winkler et al. 1991)and 2 λ, σ :T26 1.002, 10.75 1.003, 14.16 **Haefeker et al. (2012) to 99 % of the total Fe2+ occupies the octahedral site Schreyer 1969;McMillanetal.1984; Güttler et al. 1989; (Khomenko et al. 2001). The amount of Fe3+ in natural cordi- Poon et al. 1990), volatile content (Armbruster and Bloss erites is insignificant (Geiger et al. 2000). The stacked tetrahe- 1982; Vry et al. 1990;Kalt2000; Kolesov and Geiger 2000; dral rings form channels parallel to the crystallographic c-axis Yakubovich et al. 2004;Bul’bak and Shvedenkov 2005; which can incorporate molecules such as CO2 and H2O, and Khomenko and Langer 2005;Kaindletal.2006;Kolesov also large cations such as Na+ or K+. Two channel sites (Ch0, 2006; Nasdala et al. 2006; Rigby et al. 2008;Weikusatetal. Ch¼)canbedistinguishedinthestructure,accordingtothe 2010), structural heterogeneity as well as energetic and struc- general formula: (M)2(T11)2(T26)2(T23)2(T21)2(T16)O18(Ch0, tural changes under high pressure (Geiger and Grams 2003; Ch1/4). Most natural cordierites show XFe (Fe/Fe + Mg + Likhacheva et al. 2011). Group theoretic analysis yields 36 Mn) <0.9. Under certain experimental conditions it was possi- Raman active vibrations for hexagonal Mg-cordierite and 87 ble to synthesize pure Fe-cordierites (e.g., Gunter et al. 1984; for the orthorhombic end-member (e.g. McMillan et al. 1984; Boberski and Schreyer 1990;Kaindletal.2011). The incorpo- Poon et al. 1990; Kaindl et al. 2011). Local structural hetero- ration of Fe2+ into the octahedral positions of orthorhombic geneities and thermodynamic mixing properties in natural Fe- cordierite then leads to multiple structural changes when com- Mg-cordierites have also been studied with IR spectroscopy pared with Mg-cordierite. The average M-O distance increases (Geiger and Grams 2003). The synthetic Fe-end-member has which leads to an elongation of the crystallographic a and b- been investigated with X-ray powder diffraction and IR spec- axis. The average T11-O distances decrease slightly resulting in troscopic methods (Gunter et al. 1984;BoberskiandSchreyer a contraction of the c-axis (e.g., Wallace and Wenk 1980; 1990) but not using Raman spectroscopy so far. Quantum- Armbruster 1985b; Boberski and Schreyer 1990; Malcherek mechanical calculations of Raman spectra of ordered Mg- and et al. 2001; Geiger and Grams 2003). All polyhedra in both Fe-cordierites yielded a detailed peak assignment and a theo- cordierite polymorphs are distorted. The polyhedral distor- retical description of the effects of the Mg-Fe exchange and Al/ tion can be quantified with the quadratic elongation λ and Si ordering (Kaindl et al. 2011). The effect of Fe incorporation the angle variance σ2 (Robinson et al. 1971)andthese on six selected Raman bands of synthetic hydrous orthorhom- parameters are also given in Table 1. Several authors investi- bic cordierite has been investigated by Haefeker et al. (2012). gated cordierite with spectroscopic methods focussing on The aim of this work is to provide detailed structural data for topics like polymorphism and Al/Si ordering (Langer and both polymorphs of synthetic Fe-cordierite and for the first Structural and spectroscopic investigations of Fe-cordierite time Raman spectroscopic data for the hexagonal polymorph. The spectral resolution of 1.4 cm−1 (green) and 0.9 cm−1 (red) A thorough literature search showed that appropriate crystal- was determined by measuring the Rayleigh line. To obtain lographic data of both polymorphs and Raman spectra of high accuracy, the system was calibrated with emission lines synthetic hexagonal Fe-cordierite are not available in the liter- of a Ne spectral calibration lamp and the resulting deviation ature yet. The investigations described in this contribution was ±0.24 cm−1. The spectral software suite LabSPEC 5 include comprehensive structural refinements based on X-ray (Horiba 2005) was used to fit the Raman spectra and the single-crystal and powder diffraction, as well as descriptions of line-segments baseline correction and convoluted Gauss- single bands and peaks of hexagonal cordierite Raman spectra.
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