The Experimental Determination of the Chemical Activities of SiO₂ in MAS and CAS Melts

Joerg Fechner (.fechner@uni-koeln.de)¹, Herbert Palme (palme@min.uni-koeln.de)¹ & Sumit Chakraborty (sumit.chakraborty@rz.ruhr-uni-bochum.de)²

¹ Mineralogie und Geochemie, Zülpicher Str. 49b, 50674 Köln, Germany

² Institut für Mineralogie, Universitätsstr. 150, 44780 Bochum, Germany

The relationship of structural and thermodynamic properties of silicate melts is important for understanding the speciation of components of silicate melts. The parameter NBO/T (nonbridging oxygen per tetrahedron) (Mysen 1988), is frequently used as a structural parameter to indicate the degree of polymerisation of a silicate melt. The higher the degree of polymerization, the lower the activity coefficient of SiO₂ relative to a given standard state in a silicate melt is expected to be. We have experimentally investigated variations of SiO₂ activity coefficients in two series of experiments using melts in the CAS (CaO- Al₂O₃- SiO₂) and MAS (MgO- Al₂O₃- SiO₂) systems: (i) Constant NBO/T, variable SiO₂ and (ii) variable NBO/T, constant SiO₂. To facilitate equilibration and cover a wide range of compositions, it was necessary to run experiments at higher temperatures than those with conventional Pd-alloy techniques (e.g. see Fechner et al., this issue). As a consequence, we had to develop an analogous method based on Rh-metal, which is stable to higher temperatures. Silicate melts (CAS as well as MAS) with different compositions were equilibrated with a Rh ´sensor´-metal at 1650°C and an oxygen fugacity of IW-1.5, along with the pure SiO₂ standard + Rh-metal. NBO/T is calculated with the assumption that 2 Al (3+) and 1 M (2+) [M = Ca, Mg] form 2 tetrahedra with 4 bridging oxygens each (2 x Q₄). Additional Al or M are considered as network modifying cations.

We found that the activity coefficient of SiO₂ relative to a pure liquid component, 1 bar, T standard state is a strong function of SiO₂ content and relatively insensitive to variations in NBO/T. It thus appears that the activity coefficient of SiO₂, and consequently other silicate components (e.g. Mg₂SiO₄), for these systems depends on the mole fraction of SiO₂ rather than on NBO/T and models for calculation of activities of components in natural multicomponent melts should be consistent with this observation.

Mysen BO, Structure and Properties of Silicate Melts, Developments in Geochemistry, Elsevier, 4, (1988).

Experimental Study of the Effect of Fluorine and Phosphorus on Layering in Li-F Granites

Alexei Fedkin (alfed@iem.ac.ru)¹, Reimar Seltmann (rs@nhm.ac.uk)², George Zaraisky (zaraisky@iem.ac.ru)¹ & Nikolay Bezmen (bezmen@iem.ac.ru)¹

¹ IEM RAS Chernogolovka, Moscow District 142432, Russia

² Natural History Museum, Department of Mineralogy, Cromwell Road, London SW7 5BD, UK

The evidence for aluminosilicate melt depolymerization (Bezmen, 1992) and subsequent layered texture formation in granitic systems was obtained, and the effect of F-P rich fluid on superliquidus evolution was estimated. Melting of granitic compositions (natural granite powders) was conducted at T=800°C and P=2 kbar with high F (5.06 wt.%) and P₂O₅ (3.4 wt.%) concentrations in the silicate charge in presence of complex H-O-C fluid. These conditions provide development of layered solidification textures resulted either from the differentiation of a F-P-rich melt during a superliquidus evolution or controlled by nucleation and growth kinetics during non-equilibrium rapid cooling by self-organization. The three vertical zones of the run product are chemically and texturally different. The separation of SiO₂-rich melt during the superliquidus evolution resulted in crystallization of quartz in the middle part of the capsule after quenching, whereas the lower zone is impoverished in quartz. The upper zone is free of quartz and characterized by micrographic intergrowth textures and abundant dendrites at the upper-middle zone transition, which is obviously the result of quenching. The experiment also produced berlinite dissipated in the middle - lower zones. The gradual decrease of silica content in the resulting compositions up to almost SiO₂-free (0.31-0.83 wt.%) immiscibility phases is noted. It could be the result of high enrichment of the residual melt by volatile-rich fluid with high fluorine concentration after quartz crystallization. On the other hand, such relationships could form during subliquidus evolution or even quenching under non-equilibrium regime. Crystallization of quartz within the middle zone and formation of berlinite and F-Al-rich complexes give evidence for connection of both fluorine and phosphorus with alumina destroying- AlO₂ - tetrahedra (the feldspar-forming units) and making impossible feldspar formation (Xiong Xiao-Lin et al., 1999). Acidifying effect of fluorine and phosphorus and depolymerization of aluminosilicate structure can be the major factors in layering mechanism.

Bezmen NI, Experiment in Geosciences Mag., 1(2), 1-33, (1992).

Xiong Xiao-Lin, Zhao Zhen-Hua, Zhu Jin-Chu and Rao Bing, Geochemical Journal, 33, 199-214, (1999).

An Experimental Study of the Diasporite-Corundite Transformation (up to 30 kbar) as Occurring in Metakarstbauxites: An Extreme Example of Dehydration-controlled Recrystallisation and Al-mobility

Anne Feenstra (feenstra@gfz-potsdam.de) & Bernd Wunder (wunder@gfz-potsdam.de)

GeoForschungsZentrumm Potsdam, Experimental Petrology/ Geochemistry, Telegrafenberg, D-14473 Potsdam, Germany

Contrary to more common rock types, bauxitic rocks loose their water discontinuously during progressive metamorphism. This abrupt dehydration of bauxites occurs during thermal breakdown of their major phase diaspore ((alpha)-AlOOH) into corundum ((alpha)-Al₂O₃) + H₂O [a], releasing huge amounts of water during greenschist/blueschist-facies metamorphism. The phase transition can be studied in detail in the metamorphic core complex of Naxos, Greece, where metabauxite lenses are widespread in marbles. With increasing grade, diasporites are gradually transformed into corundites in a =2 km-wide transition zone, which corresponds to a metamorphic T-interval of 30-40°C. The non-univariant character of equilibrium [a] on Naxos is thought to be related chiefly to the internal buffering of P_H2O, owing to the large buffer capacity of the rock (containing 50-80% diaspore) and low metabauxite/marble permeabilities (Feenstra, 1985; Urai & Feenstra, 2000). A bauxite-marble system was experimentally studied in gas-pressure and solid-media equipment (up to 30 kbar), using natural fine-grained marble/dolomite and diasporite (grain size <5-10µm) from Naxos. A 6 mm-long (3 mmØ) diaspore-cylinder was fitted in a hollow =10 mm-long (5 mm-Ø) calcite/dolomite cylinder. This assemblage was placed in a tightly-fitting sealed gold tube and first recrystallized in the diaspore-field. Then the run temperature was slowly increased (heating rates: 10-30°C/h) and held at the final P,T-conditions in the corundum-field for several days. The experiments indicate that overstepping reaction [a] by =200°C results in a nearly complete transformation of diasporite into a medium-grained corundite, whereas minor overstepping results in partial dehydration and microcracking. The study also provides evidence for pronounced Al-mobility, particularly at higher pressures. Triggered by a 30-40°C vertical T-gradient, symmetric diasporite-carbonate assemblages were modified into asymmetric corundite-carbonate-(spinel) assemblages, with H₂O-(CO₂) principally trapped at the hotter top of the capsule. The dehydration experiments reflect petrological observations in the Naxos diaspore-corundum zone such as corundum-filled hydrofractures and increasing Al-mobility at the metabauxite-marble contact.

Feenstra A, Metamorphism of bauxites on Naxos, Greece. Geologica Ultraiectina,RU Utrecht, 39, 206pp, (1985).

Urai JL, Feenstra A, J. struct. Geol., in press, (2000).