Structural Response of Phengite 2M₁ to Temperature: An in situ Neutron Diffraction Study

Mainak Mookherjee (mm329@cam.ac.uk)¹, Simon A T Redfern (satr@cam.ac.uk)¹ & Alan Hewat (hewat@ill.fr)²

¹ Dept Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom

² Institut Laue langevin, 6,Rue Jules Horowitz, BP 156, 38042, Grenoble,Cedex 9, France

As part of a wider study into the relationship between M-site occupancies and the high-temperature structural stabilities of micas, a natural 2M₁ phengite from Greece, with chemical composition (K_0.95Na_0.05)(Al_0.76Fe_0.14Mg_0.1)₂(Si_3.25Al_0.75)O₁₀(OH)₂ has been investigated using in situ high-temperature neutron powder diffraction on the D2B diffractometer at ILL, Grenoble. Data were recorded with a high temperature resolution at nine temperatures from 293 to 973 K as well as on cooling at 773 and 573 K and the crystal structure was refined using the Rietveld method. In keeping with the results of Guggenheim et al. (1987), the average K-O bond length increased more rapidly than that of any other polyhedron on heating. The cell parameters show anisotropic thermal expansion (characteristic of sheet silicates), with coefficients (alpha)_a = 8.62(7), (alpha)_b = 9.85(7), (alpha)_c = 21.5(1), (alpha)_d001 = 1.378(6), and (alpha)_V = 40.5(5) x 10^-6 K^-1. This behaviour is compared to other published data for the temperature dependence of muscovite and phengite 2M₁ (Pavese et al., 1999) structures. The large neutron scattering length of hydrogen means that the hydrogen position may be refined accurately. We find that the O-H bond decreases in length from 0.1004(11) nm at 293 K to 0.0927(14) nm at 923K. The decreasing bond length of the hydroxyl group is likely due to incipient dehydrogenation of those hydrogens bonded further from O3. The reduction in the OH bond length is accompanied by a decrease in the M-O3 bond length, which can be understood in terms of attraction of the M cation to the underbonded O3 on dehydroxylation. We find no evidence for changes in tetrahedral order on heating, and suggest that the main structural accommodations to temperature observed in this and other studies of 2M₁ mica under these conditions are associated with the nature of the bonds to hydrogen at the shared edge (O3-O3') of the M2 sites.

Guggenhiem S, Chang Y-H & Koster van Groos AF., Amer. Mineral., 72, 537-550, (1987).

Pavese A, Ferraris G, Pischedda V & Ibberson R, Eur. J. Mineral, 11, 309-320, (1999).

Solubility and Speciation of CO₂ in Synthetic Phonolitic Melt

Yann Morizet (yann.morizet@bris.ac.uk), Simon Kohn (simon.kohn@bris.ac.uk) & Richard Brooker (r.a.brooker@bris.ac.uk)

Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK

Carbon dioxide is the second most important volatile in magmas after water. It plays an important role during melting, eruptive processes and degassing. The behaviour of CO₂ in melts is much less well understood then that of water. CO₂ solubility was measured in a phonolite composition by performing experiments at 10-25 kbars and 1300-1550°C in a piston cylinder apparatus. The starting composition was a synthetic Fe-free phonolite composition prepared from an oxide mixture with Na₂CO₃ as a source of CO₂. During the experiments, an excess CO₂ fluid is present in all cases. Glasses were recovered from the experiments and analysed for CO₂ using a LECO C/S 300 analyser. The results suggest that the solubility at this range of pressure and temperature are 0.8-2.4wt%. It appears that there is a negative correlation between temperature and CO₂ solubility as reported previously for albite (Stolper et al., 1987), diopside (Rai et al., 1983), ol-leucitite (Thibault et al., 1994), and other compositions (Brooker et al., in press). The temperature dependence is larger at higher pressure. As for all compositions studied previously there is positive correlation between pressure and CO₂ solubility. The data for all pressure and temperature can be modelled as a function of CO₂ fugacity. Dissolved CO₂ can occur as two species in silicate glasses: molecular CO₂ or carbonates (CO₃^2-). FTIR and NMR analyses are currently being used to study the behaviour of the C species. Data suggest that CO₃^2- is by far the most abundant species with minor molecular CO₂. No evidence for CO or other reduced species was found. In few cases, water diffusion into the capsule may be significant and has an important effect on speciation. However, a separate series of experiments shows no effect of water on CO₂ solubility at least up to 4wt% H₂O in the melt.

Stolper E, Fine G, Johnson T, Newman S, Amer. Mineral., 72, 1071-1085, (1987).

Rai CS, Sharma SK, Muenow DW, Matson DW, Byers CD, Geochim. Cosmochim. Acta, 47, 953-958, (1983).

Thibault Y, Holloway JR, Contrib. Mineral. Petrol, 116, 216-224, (1994).

Brooker RA, Kohn SC, Holloway JR, McMillan PF, Chem. Geol, in press