Journal of Conference Abstracts

High Pressure Study of CaSi₂O₅: Evolution of 5-fold Silicon with Pressure

N. L. Ross¹ (n.ross@ucl.ac.uk), R. J. Angel² (ross.angel@uni-bayreuth.de), F. Seifert² (fritz.seifert@uni-bayreuth.de) & T. F. Fliervoet² (timon.fliervoet@uni-bayreuth.de)

¹ Dept. Geological Sciences, University College London, London, U.K.

² Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany.

Silicon coordinated by five oxygen atoms plays a central rôle in a large number of dynamic processes that occur in silicates and silicate melts. The quenched high-pressure phase, CaSi₂O₅, is the first structure reported to contain stoichiometric SiO₅ groups in addition to SiO₆ octahedra and SiO₄ tetrahedra (Angel et al., 1996). CaSi₂O₅ has the same topology as that of titanite (CaTiSiO₅), with Si occupying both the tetrahedral and octahedral sites. The smaller size of Si results in significant distortion of the structure and reduction in symmetry to triclinic from the ideal C2/c symmetry of the titanite aristotype structure. The two symmetry-independent SiO₄ tetrahedra and two of the three SiO₆ octahedra display normal Si-O bond lengths and angles, but the third SiO₆ octahedron has five short bonds ranging from 1.66 - 1.82 Å, and one very long bond (2.82 Å). Its coordination is therefore better described as 5 + 1, similar to that found in some germanate high-pressure phases. Preliminary high-pressure single-crystal X-ray diffraction results indicate that the coordination of 5-fold Si changes to 6-fold coordination at approximately 4.4 GPa. Complementary high-pressure infrared spectra will also be presented tracing the evolution of the SiO₅ group with increasing pressure.

Coloured Lava Flows on the Earth: A Warning to Io Volcanologists

David A. Rothery¹ (D.A.Rothery@open.ac.uk), Tanya L. Babbs² (tlb1@leicester.ac.uk), Andrew J. L. Harris^1,3 (harris@kahana.pgd.hawaii.edu) & Martin J. Wooster¹ (M.J.Wooster@open.ac.uk)

¹ Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA.

² Department of Geology, Leicester University, Leicester, LE1 7RH.

³ Now at Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA.

In the early years after the Voyager encounters, the reddish or yellowish colour of Io's surface seemed to support a commonly held view that many, or even most, of Io's volcanic flows are composed of sulfur. Others favoured an interpretation that all, or most, of Io's flows are composed of silicates. Io's spectral properties have been interpreted as being consistent with basaltic rock covered by a thin (possibly only micrometers thick) coating of SO₂, its dissociation products polysulfur oxide (PSO) and S₂O, compounds involving sodium and sulfur such as Na₂S and NaHS, and other salts such as FeCl₂ . Debate is now focusing on whether Io as a whole needs to be volatile-rich in order to account for its apparently volatile-rich surface.

It seems inevitable that on Io, older flow surfaces must be mantled by fallout from eruption plumes or by related SO₂ frost. Post-Voyager changes to Io's surface albedo distribution revealed by the Hubble Space Telescope and on Galileo images would appear to demonstrate this. It is therefore unlikely that colours of older, mantled flows on Io can tell us anything directly about the flow composition. What is still in question is whether or not the bulk compositions of flows young enough to have not yet been mantled by fallout can be revealed by their spectral properties.

Pale coatings of amorphous silica, sulfur, and sulfur-bearing compounds can form on terrestrial basalt flows while they are still active and subsequently on timescales ranging from days to years after the eruption. We present examples from Kilauea (Hawaii), Etna (Sicily) and Cerro Negro (Nicaragua) and caution against undue reliance on spectral reflectance to determine the bulk properties of lava flows on Io, where sulfur and/or sulfur compounds are likely to dominate comparable (but probably more widespread) coatings.