Modeling the Al distribution in the amphibole structure has important implications for the geobarometry of igneous and metamorphic processes [1,2,3]. Most thermodynamic analyses assign VIAl only at M(2) andIVAl only at T(1)[4]; however, crystal-structure work has shown that this model is inadequate, at least for calcic amphiboles where VIAl disorders between the M(2) and M(3) sites[5] and IVAl disorders between the T(1) and T(2) sites[6] (at high mg# or high T, respectively). In this work, we report an FTIR study in the OH-stretching region of three amphibole compositions with different Si/Al ratios at the T sites (richterite, fluoro-edenite and pargasite). In the amphibole structure, the H atom forms a hydrogen bond to the closest O(7) atom [7]. When the T(1)-O(7)-T(1) linkages are of the type Si-O(7)-Al, the deficit in bond valence at the O(7) anion must be alleviated via a stronger O(3)-H…O(7) hydrogen bonding, and this feature affects significantly the O-H stretching frequency. Hence careful analysis of the OH-spectra may detect SRO of cations at the T(1) sites in amphiboles. In synthetic richterite, all the tetrahedra are occupied by Si, and all the T(1)-O(7)-T(1) linkages are of the Si-O(7)-Si type; accordingly, the OH-stretching spectrum consists of a single band at 3730 cm-1, which is assigned to the MgMgMg-OH-ANa:SiSi local configuration (neglecting minor deviations toward tremolite). In pargasite all the T(2) sites are occupied by Si and half of the T(1) sites are occupied by Al, hence all the T(1)-O(7)-T(1) linkages in the structure must be of the Si-O(7)-Al type in order to avoid insufficient bond-strength contribution to the O(7) oxygen atom, and thus all H atoms are involved in an hydrogen bond with the closest O(7) atom. Accordingly, a single band is observed at 3709 cm-1, and is assigned to the local MgMgMg-OH-ANa:SiAl configuration. In edenite, the composition of the T sites is Si7Al1; hence, half of the T(1)-O(7)-T(1) linkages must be of the Si-O(7)-Si type, and half must be of the Si-O(7)-Al type. Hence, there are two possible patterns of order between T(1)Si and T(1)Al, and these must have different spectral expressions in the infrared. The first (“non-clustered”, or “fully ordered”) pattern has Si-O(7)-Si linkages regularly alternating with Si-O(7)-Al linkages. The second pattern has clusters of Si-O(7)-Al linkages alternating with clusters of Si-O(7)-Si linkages (the total number of the different linkages in the two cluster being equal). We can label this second pattern as “clustered pattern”. In the first model, all OH-stretching bands must be of “pargasite”-type and should be observed at ~ 3709 cm-1 [or at lower wavenumber if cations different from Mg occur at the M(1,3) octahedra]. In the second model, we must observe two bands with almost the same intensity at 3730 (richterite-type) and 3709 cm-1 (pargasite-type) (or at lower wavenumbers in the presence of different octahedral cations). The OH-stretching spectra of edenites systematically show only absorptions at frequency < 3710 cm-1, thus confirming complete short-range order between T(1)]Si and T(1)Al throughout the double-chain. [1] Spear, F.S. (1981) Am. J. Sc., 281, 697-734. [2] Hammarstrom, J.M. and Zen, E-an (1986) Am. Min., 71, 1297-1313. [3] Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson, V.B. (1987) Am. Min., 72, 231-239. [4] Graham, C.M. and Navrotsky, A. (1986) Contrib. Mineral. Petrol., 93, 18-32. [5] Oberti, R., Hawthorne, F.C., Ungaretti, L., and Cannillo, E. (1995a) Can. Min., 33, 867-878. [6] Oberti, R., Ungaretti, L., Cannillo, E., Hawthorne, F.C., and Memmi, I. (1995b) Eur. J. Min., 7, 1049-1063. [7] Della Ventura, G., Hawthorne, F.C., Robert, J.-L., Delbove, F., Welch, M.D., Raudsepp, M. (1999) Eur. J. Min., 11, 79-94.

DELLA VENTURA, G., Bellatreccia, F., Oberti, R., Cámara, F. (2007). Short-range order in amphiboles: Si and Al at T(1) in edenite, 17-18.

Short-range order in amphiboles: Si and Al at T(1) in edenite

DELLA VENTURA, Giancarlo;BELLATRECCIA, FABIO;
2007-01-01

Abstract

Modeling the Al distribution in the amphibole structure has important implications for the geobarometry of igneous and metamorphic processes [1,2,3]. Most thermodynamic analyses assign VIAl only at M(2) andIVAl only at T(1)[4]; however, crystal-structure work has shown that this model is inadequate, at least for calcic amphiboles where VIAl disorders between the M(2) and M(3) sites[5] and IVAl disorders between the T(1) and T(2) sites[6] (at high mg# or high T, respectively). In this work, we report an FTIR study in the OH-stretching region of three amphibole compositions with different Si/Al ratios at the T sites (richterite, fluoro-edenite and pargasite). In the amphibole structure, the H atom forms a hydrogen bond to the closest O(7) atom [7]. When the T(1)-O(7)-T(1) linkages are of the type Si-O(7)-Al, the deficit in bond valence at the O(7) anion must be alleviated via a stronger O(3)-H…O(7) hydrogen bonding, and this feature affects significantly the O-H stretching frequency. Hence careful analysis of the OH-spectra may detect SRO of cations at the T(1) sites in amphiboles. In synthetic richterite, all the tetrahedra are occupied by Si, and all the T(1)-O(7)-T(1) linkages are of the Si-O(7)-Si type; accordingly, the OH-stretching spectrum consists of a single band at 3730 cm-1, which is assigned to the MgMgMg-OH-ANa:SiSi local configuration (neglecting minor deviations toward tremolite). In pargasite all the T(2) sites are occupied by Si and half of the T(1) sites are occupied by Al, hence all the T(1)-O(7)-T(1) linkages in the structure must be of the Si-O(7)-Al type in order to avoid insufficient bond-strength contribution to the O(7) oxygen atom, and thus all H atoms are involved in an hydrogen bond with the closest O(7) atom. Accordingly, a single band is observed at 3709 cm-1, and is assigned to the local MgMgMg-OH-ANa:SiAl configuration. In edenite, the composition of the T sites is Si7Al1; hence, half of the T(1)-O(7)-T(1) linkages must be of the Si-O(7)-Si type, and half must be of the Si-O(7)-Al type. Hence, there are two possible patterns of order between T(1)Si and T(1)Al, and these must have different spectral expressions in the infrared. The first (“non-clustered”, or “fully ordered”) pattern has Si-O(7)-Si linkages regularly alternating with Si-O(7)-Al linkages. The second pattern has clusters of Si-O(7)-Al linkages alternating with clusters of Si-O(7)-Si linkages (the total number of the different linkages in the two cluster being equal). We can label this second pattern as “clustered pattern”. In the first model, all OH-stretching bands must be of “pargasite”-type and should be observed at ~ 3709 cm-1 [or at lower wavenumber if cations different from Mg occur at the M(1,3) octahedra]. In the second model, we must observe two bands with almost the same intensity at 3730 (richterite-type) and 3709 cm-1 (pargasite-type) (or at lower wavenumbers in the presence of different octahedral cations). The OH-stretching spectra of edenites systematically show only absorptions at frequency < 3710 cm-1, thus confirming complete short-range order between T(1)]Si and T(1)Al throughout the double-chain. [1] Spear, F.S. (1981) Am. J. Sc., 281, 697-734. [2] Hammarstrom, J.M. and Zen, E-an (1986) Am. Min., 71, 1297-1313. [3] Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson, V.B. (1987) Am. Min., 72, 231-239. [4] Graham, C.M. and Navrotsky, A. (1986) Contrib. Mineral. Petrol., 93, 18-32. [5] Oberti, R., Hawthorne, F.C., Ungaretti, L., and Cannillo, E. (1995a) Can. Min., 33, 867-878. [6] Oberti, R., Ungaretti, L., Cannillo, E., Hawthorne, F.C., and Memmi, I. (1995b) Eur. J. Min., 7, 1049-1063. [7] Della Ventura, G., Hawthorne, F.C., Robert, J.-L., Delbove, F., Welch, M.D., Raudsepp, M. (1999) Eur. J. Min., 11, 79-94.
2007
DELLA VENTURA, G., Bellatreccia, F., Oberti, R., Cámara, F. (2007). Short-range order in amphiboles: Si and Al at T(1) in edenite, 17-18.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11590/175734
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