Database of Amphibole Structures

A. Umran Dogan (adogan@uiowa.edu)¹, Meral Dogan² & Serhat Ozbay¹

¹ Department of Geological Engineering, & Center for Applied Mineralogy, Ankara University, Ankara, Turkey

² Department of Geological Engineering, Hacettepe University, Ankara, Turkey

In this study, structures of amphiboles from various places of the world have been re-calculated. The main aim of the work is to create a database for amphibole structures and to correct miscalculations in the literature. Cr₂O₃, NiO, ZnO, Li₂O previously excluded from the structural formula have been incorporated in the structure. It is assumed that the tetrahedral sites which are not filled by Si are occupied by Al, and the remaining Al atoms are in the octahedral coordination. Octahedral sites included B+C and A sites. Also, the octahedral B+C site which is not filled totally by the other cations is occupied by Na, and the remaining Na atoms are in the octahedral A site. The most importantly, the average formula of thirty end-members of amphibole group minerals have been calculated. The calculation procedure for structure of amphiboles is described and is composed of nine columns. Column 1 lists the formula of oxides concerned. Column 2 lists the composition of the mineral expressed as weight percentages of the constituent oxides. Column 3 shows molecular weight of oxides concerned. Column 4 is derived by dividing weight percent of each oxide to molecular weight of the oxides. Column 5 is derived from column 4 by multiplying by the numbers of oxygen atoms associated with each of the element concerned and at the end of column 5, the total (S) is calculated. Since amphibole formula is calculated based on 24 (or 23) oxygen atoms, this is performed by multiplying each oxide by 24/S or (23/S). Column 7 lists the elements used in structural calculations of amphiboles. Column 8 gives the number of cations associated with oxygens in column 6. Therefore, Column 8 includes adjusted number of ions in the formula. Finally, Column 9 consist of the number of ions in the formula.

Database of Pyroxene Structures

Meral Dogan (adogan@uiowa.edu)¹, A. Umran Dogan² & Zafer Dogruel²

¹ Department of Geological Engineering, Hacettepe University, Ankara, Turkey

² Department of Geological Engineering, & Center for Applied Mineralogy, Ankara, Turkey

In this study, structures of pyroxenes from various places of the world have been re-calculated. The main aim of the work is to create a database for pyroxene structures and to correct miscalculations in the literature. P₂O₅, V₂O₅, Cr₂O₃, Li₂O, NiO, ZnO previously excluded from the structural formula, have been incorporated in the structure. It is assumed that the tetrahedral sites which are not filled by Si are occupied by Al, and the remaining Al atoms are in the octahedral site. The most importantly, the average end-members of pyroxene group minerals have been calculated. The pyroxene structures are calculated based upon 6 oxygens. The calculation procedure is described and is composed of nine columns. Column 1 lists the formula of oxides concerned. Column 2 lists the composition of the mineral expressed as weight percentages of the constituent oxides. Column 3 shows molecular weight of oxides concerned. Column 4 is derived by dividing weight percent of each oxide to molecular weight of the oxides. Column 5 is derived from column 4 by multiplying by the numbers of oxygen atoms associated with each of the element concerned and at the end of column 5, the total (S) is calculated. Since pyroxene formula is calculated based on 6 oxygen atoms, this is performed by multiplying each oxide by 6/S Column 7, lists the elements used in structural calculations of pyroxenes. Column 8 gives the number of cations associated with oxygens in column 6. Column 8 includes adjusted number of ions in the formula assuming the total number of ions in the tetrahedral sites is 2. Finally, column 9 consist of the number of ions in the formula.

Experiments and Thermodynamic-Kinetic Modelling of a Combined Diffusion + Interface-Controlled Reaction and Implications for Thermobarometry

Ralf Dohmen (dohmen@min.uni-koeln.de)¹, Sumit Chakraborty (sumit.chakraborty @ruhr-uni-bochum.de)², Herbert Palme¹ & Werner Rammensee¹

¹ Institut fuer Mineralogie, Universitaet zu Koeln, Zuelpicher Strasse 49B, 50674 Koeln, Germany

² Institut fuer Geologie, Mineralogie und Geophysik, Ruhr-Universitaet Bochum, 44780 Bochum, Germany

Conventional petrological interpretation of mineral reactions is based on the assumptions that (i) these reactions are either interface- or diffusion- controlled and (ii) equilibrium distribution of elements at crystal surfaces is attained instantaneously. We have used in situ observations and numerical modelling to demonstrate that there may be a need to revise both of these postulates.

Experiments in a Knudsen-cell mass spectrometer have shown that major element compositions of silicates and oxides (e.g. olivine, pyroxene, spinels) can be changed by reaction with a vapor phase [Dohmen et al., 1998]. Using the olivine + Fe-metal system as an example, experiments of different durations were carried out at 1400°C. The reaction progress is monitored by measuring vapor pressures of individual species (e.g. Mg, SiO, Fe etc.) during experiments, as well as by measurement of compositional profiles in crystals after the anneals. These results were reproduced in a numerical simulation which accounted for compositionally dependent diffusion in olivine, surface controlled evaporation/condensation reactions and transport in/out of the system (partially open behavior).

A main finding of the study is that the composition (Fe/Mg ratio) at the surface of olivine varies with time at constant temperature - i.e. the final surface composition was not achieved instantaneously compared to the diffusion time scale, even during exchange with a vapor phase at high temperatures. In other words, element partitioning can be kinetically inhibited due to the coupling between interfacial processes and diffusion rates and one of these do not necessarily dominate. This is contrary to the classic assumptions in applications such as geothermometry, geospeedometry [Lasaga, 1983], trace element modelling in magmatic systems etc. The results of this study are relevant for any analogous reaction system, e.g., exchange reactions that are mediated by an intergranular fluid phase.

Dohmen R, Chakraborty S, Palme HP & Rammensee W, Am Mineral, 83, 970-984, (1998).

Lasaga AC, Adv Phys Geochem, 3, 81-114, (1983).