Structure of Hydrous Rhyolite and Basalt Glasses: Raman Spectroscopic Study

Nikolay Zotov (nikolay.zotov@uni-bayreuth.de), John Sowerby & Hans Keppler

Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

Different models are proposed for the incorporation of water in aluminosilicate glasses and melts, some of which involve breaking of T-O-T bonds (Mysen et al., 1986) while others imply no depolymerization of the structure by water (Kohn et al., 1992).

We present new Raman data for hydrous rhyolite and basalt glasses with varying water contents. Fe-free hydrous rhyolite glasses were synthesized at 1100°C and 1-2 kbar pressure in externally heated TZM autoclaves. Fe-free hydrous basalt glasses were synthesized at 1400°C and 10 kbar in a piston cylinder apparatus. The spectra are interpreted using vibrational density of states and Raman spectra calculations (Zotov et al., 1999).

Systematic changes of all T-O Raman bands (T=Si,Al) in the range 200-1200 cm^-1 with increasing total water content (TWC) are observed. They indicate that: (1) the T-O-T bond angle distributions become more narrow with increasing TWC; (2) water depolymerizes the structure of rhyolite and basalt glasses and (3) leads to changes in the Q-species distribution. The later effect is more pronounced in the basalt glasses. A new peak at about 650 cm^-1 with low depolarization ratio is observed in the basalt glasses, the behaviour of which also implies changes in the structure with increasing TWC.

The O-H Raman bands in the range 2700-3650 cm^-1 indicate that both OH groups and H₂O molecules participate in hydrogen bonding regardless of the OH - H₂O distribution and suggest that the increase of depolymerization leads to an increase of the strongly hydrogen bonded OH groups.

Kohn SC, Dupree R & Mortuza MG, Chem. Geol., 96, 399-409

Mysen B & Virgo D, Chem. Geol, 57, 333-358

Zotov N, Ebbsjö I, Timpel D & Keppler H, Phys. Rev. B, 60, 6383-6397

Loparite, Lewisite and Bismutocolumbite: Crystal Structures and Topology

Natalia Zubkova (nzubkova@mail.ru) & Dmitry Pushcharovsky (dmitp@geol.msu.ru)

Moscow 125212, Leningradskoye sh., 50, 77, Russia

This work summarises the results of the XRD studies of three natural oxides (loparite, lewisite and bismutocolumbite) and examines their topological relationships, polyhedral stacking variations and cationic ordering. Loparite, (Ti,Nb)(Na,Ce,Ca,Sr)O₃, (sp.gr Pn3m) according to its chemical composition and XRD powder diagram is considered as a member of perovskite group (general formula ABO₃). Lattice dimensions of loparite from Lovozero alkaline massive (Kola Peninsula) are doubled in comparison with cubic perovskite because of the ordering of both A- and B-cationic sites. Structurally loparite with general formula A'A''₃B'₂B''₂O₁₂ is related to HP silicate Ca₂TiSiO₆. Loparite as well as lewisite, (Ca,Sb³⁺,Fe³⁺,Al,Na,Mn,˜)₂(Sb⁵⁺,Ti)₂O₆(OH) (pyrochlore-like structure, general formula A₂B₂O₇, sp.gr. Fd3m) comprise octahedral framework. In modern structural classification offered by professor Lima-de-Faria both minerals belong to atomic structures with close packing of large ions. We investigated the sample of lewisite from Tripui (Brazil). The comparison of two models with static distribution of Sb³⁺ (splitting of A-site) and dynamic one (anharmonic thermal displacements of A-cations) allowed to prefer the latter. The structure is based on a defect simple cubic packing of anions with A- and B-atoms occupying 1/2 of cubic voids. A-cations have cubic coordination and B-atoms - 6-fold coordination due to the vacancies in anionic packing. The structure of bismutocolumbite, (Bi_0.99Sb_0.01)(Nb_0.79Ta_0.21)O₄, from Eastern Siberia heritages the features of both perovskite and pyrochlore structural types. The structure is closely related to stibiotantalite, SbTaO₄, (general formula ABO₄, sp.gr. Pnna) and consists of the sheets of corner linked (Nb,Ta)-octahedra (similarly to perovskite) separated by highly asymmetric due to the lone-pair electrons Bi-polyhedra (4+2). However the sheets are corrugated and their configuration is similar to octahedral arrangement (111) in pyrochlore. The structure of bismutocolumbite with close packing index 54 can not be considered as an atomic one in contrast to loparite and lewisite.

Estimation of Oligoclase, Biotite and Orthoclase Dissolution Rates During the Granite Hydrothermal Alteration

Pierpaolo Zuddas (zuddas@ipgp.jussieu.fr) & Francois Seimbille

Laboratoire de Geochimie des Eaux (CNRS 7047). Institut de Physique du Globe et Universite Paris 7, France

We propose a new experimental evaluation of the simultaneous dissolution rate of orthoclase, biotite and plagioclase (oligoclase) during the interaction between a powered granite and a fluid artificially enriched in ⁸⁴Sr and ³⁹K. Experiments were carried out at 180°C and 10 bars for 1 year using batch reactors and fluids saturated with kaolinite, low-temperature albite, prehnite, calcite adularia and quartz. During interaction the chemical composition of the fluids remains quite constant while enriched in strontium and potassium isotopes coming from the dissolution of the granite minerals. We found that the evolution of strontium and potassium isotope ratios in solution allows to identify a single rock end-member indicating that the dissolving mineral assemblage remains constant during the time of interaction. Associating the mass conservation low to the mixing isotopic equations for both K and Sr we found that the mass of dissolved oligoclase is 3-4 times higher the mass of dissolved biotite and 10 times higher the mass of dissolved orthoclase. Because of the influence of the neogenic phases on the molar dissolution budget, the rate of potassium release was estimated assuming initially that the (⁴¹K/³⁹K) ratio is in continuos equilibrium with neogenic phases and additionally that such equilibrium is attained at a given instant only. The proportion of dissolved minerals associated to the overall rate of potassium dissolution normalised to the mineral surface area allows to estimate that, in our experimental conditions, oligoclase dissolves at @ 10^-12 mol.m^-2.s^-1 while biotite and orthoclase dissolves at @ 10^-13 mol.m^-2.s^-1.