The Correctness of Thermodynamic Properties Components of Magmatics Melts

Vladimir Karzhavin (root@geo.kolasc.net.ru)

Geological Institute, Kola Science centre RAS, Apatity 184200, Russia

The information about thermodynamic properties of substances is given in some papers and reference media as intercoordinated data on temperature-dependence of thermal capacity, entropy, enthalpy of formation and Gibbs energy. There are, however, great discrepancies in the data on the number of substances. They concern thermodynamic properties of compounds in a metastable state under standard conditions, which are offered, or used, for the study of crystallization of complex magmatic systems or for a construction of phase diagrams. To substantiate the applicability of the discussed thermodynamic properties of chemical compounds, we have considered sodium tetraborate as a representative example. The given chemical compound is one of most thoroughly investigated experimentally (calorimetry) in a wide temperature interval, and the thermodynamic properties of this compound in liquid, glassy and crystalline states have been determined. The carried out research has allowed us to establish a definite relationship between thermodynamic functions of phases in a wide temperature interval. So, the difference in the entropy, enthalpy of formation and Gibbs energy between glassy and crystalline phases of a substance is described by the following equations:

G^gl-G^crys=H^gl-H^crys-(S^gl-S^crys)T

or

G^*T = H^*T - TS^*T

The melts are thermodynamically nonequilibrum and metastable. Depending on conditions of phase transformations (crystallization, vitrification), metastable phases are formed along with stable. The analysis and registration of their properties when constructing an objective picture of phase transformations in multisystems contributes to the study of crystallization paths of a liquid phase both in equilibrium, and in nonequilibrum conditions.

FTIR and TEM Investigation of Mantle Olivines: Intrinsic and Extrinsic Mode of Hydrogen Occurrence

Nataliya Khisina (urusov@geol.msu.ru)^1,2, Richard Wirth (wirth@gfz-potsdam.de)², Michel Andrut³ & Andrew Ukhanov¹

¹ Institute of Geochemistry and Analytical Chemistry of Russian Academy of Science, Kosygin st. 19, 117975 Moscow Russia

² GeoForschungsZentrum Potsdam, Telegrafenberg D- 14473 Potsdam, Germany

³ Institute for Mineralogy and Crystallography, University of Vienna - Geozentrum, Althanstr. 14, A- 1090 Austria

The mode of hydrogen incorporation in olivine is a debated topic which is related to the problem of water storage in the mantle. The OH-absorption bands observed in IR spectra of olivine (Libowitzky and Beran, 1995) can be referred to either hydrogen incorporated in the olivine structure or to extrinsic hydrogen as hydrous minerals included in olivine. It is suggested (Khisina et al, 2000) that inclusions of so called DHMS (dense hydrous magnesium silicates) which are stable at high pressure-high temperature conditions are present in olivine. Olivines from two mantle nodules in kimberlites were investigated by TEM and FTIR in order to detect the mode of hydrogen occurrence. Both small (up to several tens nm in size) and "large" (up to several hundreds nm in size) inclusions are observed in the interior of the olivine grains. EELS measurements show that inclusions of both types contain OH^- (or water).

AEM measurements indicate that the Mg/Si ratio decreases in inclusions compared to the host olivine. HRTEM images and SAED patterns from small inclusions exhibit a superperiodicity along certain directions of olivine structure indicating a cation-deficient olivine structure with ordered OH- -bearing point defects. Talc+serpentine intergrowth together with the sequence of 10Å-Phase - kerolite - talc transformations are identified in large inclusions. 10Å-Phase is a DHMS synthesized at high pressures (Bauer & Sclar, 1981) and observed in a natural sample by Wirth & Khisina (1998). Thus, two types of hydrogen incorporation into olivine are determined: intrinsic hydrogen as ordered OH- -bearing point defects in "hydrous olivine" (small inclusions) and extrinsic hydrogen included in hydrous minerals ("large" inclusions). The data show that some of the OH absorption bands observed in IR spectra of olivines should be unambly referred to serpentine, talc and 10Å-Phase (Furmagalli et al. 1999).

Bauer JF & Sclar CB, Amer. Mineral, 66, 576-585, (1981).

Fumagalli P, Poli S, Don Snyder, Stixrude L, Wirth R, AGU Fall Meeting Abs, (1999).

Khisina NR, Wirth R, Langer K, Andrut M, Ukhanov A, Miner. Mag, in press, (2000).

Libowitzky E, Beran A, Phys. Chem. Minerals, 22, 387-392, (1995).

Wirth R, Khisina NR, AGU Fall Meeting supplement, 79, 10, (1998).

Structural Allocations and Distortion of Iron Ions' Polyhedra in Low Cordierite: Single Crystal Spectroscopic Study at 80 - 700 K

Vladimir M. Khomenko (vladkhom@hotmail.com)¹, Klaus Langer¹ & Charles A. Geiger²

¹ Institut für Angewandte Geowissenschaften I, TU Berlin BH-1, Ernst Reuter Platz 1, D-10587 Berlin, Germany

² Institut für Geowissenschaften der Universität Kiel, Olshauenstr. 40, D-24098 Kiel, Germany

Single crystal electronic absorption spectra of 21 natural cordierites were measured between 35000-1000 cm^-1 using microscope-spectrometric techniques. The temperature behavior of the integral intensities, widths and energies of the absorption bands were studied between 80-700K on five samples.

Both the electronic and Mössbauer spectra confirm the allocation of Fe²⁺ ions in two different structural sites: 90-99% of the total Fe²⁺ occupies the octahedral position, whereas a minor amount resides in a non-octahedral sites with a maximal fraction, about 10%, in the magnesian cordierites. Fe-contents of this second type of structurally bound iron, recalculated from Mössbauer data, correlates linearly with the integral intensity of an absorption band around 10500 cm^-1 in the ß- and (gamma)-polarized spectra. Compositional and temperature dependencies of an (alpha)-polarized band doublet with maxima at about 10000 and 8200 cm^-1 coincide with those predicted for spin allowed transitions ⁵T_2g-⁵E_g derived from octahedral Fe²⁺ ions. The integral intensity of the doublet-band increases with temperature and correlates linearly with the iron content. The ⁵E_g-level splitting increases with increasing Fe-content and on increasing temperature indicating raising distortion of the Fe²⁺-octahedra.

The polarization of a Fe²⁺Fe³⁺ charge transfer (CT) band at 17000 cm^-1 indicates that the interaction takes place along the T11-M direction, i.e. between the predominant octahedral Fe²⁺ and Fe³⁺ in the T11 tetrahedron, both of which share edges. The intensities of the other weaker absorption bands caused by Fe²⁺ correlate neither with the doublet, nor with intensity of the CT band, thus confirming another site for corresponding Fe²⁺-factions. The unusually high molar extinction coefficient of the ß/(gamma)-polarized band at 10500 cm^-1, as well as its temperature independence, are both characteristic of absorption caused by Fe²⁺ ions in a non-centrosymmetric field. The assignments of the ß/(gamma)-polarized band at 10500 cm^-1 and the two weak bands at about 6000 and 4500 cm^-1 will be analyzed and the presence of corresponding minor tetrahedral Fe²⁺ factions will be discussed using a combination of chemical-structural and experimental results on temperature variations in cordierites electronic absorption spectra.