Chromium Accumulation on Clays: Volkonskoite as an Example

Yulya Simakova (cryst@geo.komi.ru)

54, Pervomaiskaya st., Syktyvkar, Institution of Geology, Russia

Volkonskoite is unique mineral from smectite group containing dominant chromium in the octahedral positions. This mineral is formed on different organic remains (sometimes on whole tree trunks) buried in productive sandstones from West Ural (Perm and Kirov region). Chemical analysis shows great variations in chromium content in volkonskoites (Cr ~ 14-30%). Our investigations indicate that chromium accumulation begins firstly in clay matter that has been formed on cell membranes of silicified wood, boundaries of quartz grains and other parts, where an organic matter can remains and micro-organisms can spread. Fixation of chromium in clayey matter can be promoted by the presence of finely disseminated inclusions of iron hydroxides that often marked the inner structure of silicified wood. Modelling experiments were carried out for the determination of comparative chromium sorption and desorption on smectites at the combined or separate presence of humic acids, FeOOH, Fe²⁺ to determine the basic reason of chromium accumulation in volkonskoites. The competing smectites were the whole samples of Triassic montmorillonite (Vychegda) and Permian clay from volkonskoite bearing rocks (Perm region). This experiments shows that chromium sorption is especially high on the montmorillonite clay at the presence of Fe²⁺ and hoetite and desorption of sorbed chromium is relatively low. Desorption of chromium from volkonskoite under oxic and unoxic conditions is too small. The influence of humic substances and ferrous iron increased the chromium sorption on clay minerals.

Vogel, Tamman, Fulcher, Kauzmann, Adam, Gibbs: A new Interpretation for Newly Measured Viscosities for Silicate Melts

Anne Sipp (richet@ipgp.jussieu.fr), Yan Bottinga (yb@ipgp.jussieu.fr) & Pascal Richet

Géomatériaux, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 05, France

Equilibrium values of the viscosity have been measured in and above the glass transition range for stoichiometric stablilized melts of albite, anorthite and diopside compositions. Along with high-temperature data obtained up to 2400 K, these measurements provide the basis for an expression of the temperature dependence of the configurational entropy. This expression harmonizes the duality of the theoretical Adam-Gibbs model and the empirical Vogel-Tamman-Fulcher equation. Moreover, it suggests a rational interpretation of the Kauzmann paradox. The proposed configurational entropy expression reproduces quite well residual configurational entropies calorimetrically determined for albitic, anorthitic and diopsidic melts and the calorimetrically determined specific heat constants.

Ti³⁺ Incorporation in Synthetic Diopside Investigated by Optical Absorption Spectroscopy and Microprobe Methods

Henrik Skogby (henrik.skogby@nrm.se)¹, Ulf Hålenius & Per Kristiansson²

¹ Swedish Museum of Natural History, Department of Mineralogy, Box 50007, SE-10405 Stockholm, Sweden

² Lund Institute of Technology, Department of Nuclear Physics, Box 118, SE-22100 Lund, Sweden

Titanium normally occurs in the tetravalent state in terrestrial minerals, whereas the trivalent state is known from the more reduced environments representative for lunar rocks and meteorites. Pyroxenes from such occurrences containing Ti³⁺ have been studied by optical spectroscopy (e. g. Mao and Bell, 1974), but the lack of good-quality samples has made the interpretation of the spectral features difficult. In order to provide useful material for further optical spectroscopy studies, and to investigate the substitution mechanisms connected with Ti³⁺ and Ti⁴⁺ incorporation, a series of nine diopside single crystal samples with different Ti contents were synthesised in this work. The samples were synthesised by a flux-growth method under low oxygen fugacities using a H₂-CO₂ gas-flow furnace, and ranged in size up to 0.3 x 0.5 x 2 mm. Electron microprobe and nuclear reaction analyses show that Ti_tot varies from 0.08 to 0.33 apfu and correlates with Na and B stemming from the flux. These elements provide the charge balance necessary for Ti³⁺ and Ti⁴⁺ incorporation at the octahedral site. Calculations of structural formulae further indicate that a substantial part of Ti⁴⁺ occur at the tetrahedral position, which is also confirmed by XRD studies (Nazzareni et al., 1999). Polarised optical spectra obtained on oriented sample sections reveal three major absorption features; one broad band centred around 24 000 cm^-1 being strongly polarised in the crystallographic b-direction and two more narrow bands centred at 18 500 cm^-1 and 15 700 cm^-1 being polarised mainly in the crystallographic c-direction. The two bands at the lower wavenumbers are correlated with the Ti³⁺ content and are assigned to spin-allowed electronic d-d transitions in Ti³⁺ at the M1 site. The broad band at 24 000 cm^-1 is well correlated with the product of the Ti³⁺ and Ti⁴⁺ concentrations and is interpreted as being caused by a Ti³⁺-Ti⁴⁺ charge-transfer transition. The pleochroic behaviour of this band indicates that the charge-transfer transition occur between Ti³⁺ at the M1 site and Ti⁴⁺ at the tetrahedral site.

Mao HK & Bell PM, Carnegie Inst. Yearbook, 73, 488-492, (1974).

Nazzareni S, Molin GM & Dal Negro A, Plinius, 22, 286, (1999).