Kinetics of Ordering Processes in Epidote Solid Solution

Karl Thomas Fehr (fehr@petro1.min.uni-muenchen.de)

Institute of Mineralogy, Theresienstr. 41, 80333 Munich, Germany

The epidote structure contains 3 octahedral sites, two of them can be occupied by Al and Fe³⁺ (M1, M3). The occupancies lead to an intracrystalline disorder state and a non-convergent ordering process occurs which can be described by the reaction

Fe³⁺,M3 + Al,M1 = Fe³⁺,M1 + Al, M3.

As shown by Mössbauer spectroscopy this intracrystalline exchange reaction occurs in Fe-rich epidotes (Fe pfu > 0.7) only (Fehr & Heuss-Aßbichler, 1997). The kinetic constants for the disordering and ordering reactions were studied at 500 <T<650°C at 0.3 GPa/HM buffer. Equilibrium is achieved rapidly within a week. The occurrence of shared edges between M1 and M3 sites favors the site exchange of Fe³⁺. The data clearly supports the lower Kd values of Fehr & Heuss-Aßbichler (1997) and no hint was observed for occupation of Fe³⁺ on M2 site. In all epidotes with Fe pfu < 0.7 all Fe³⁺ is located on M3 exclusively. In the compositional range 0.5 < Fe pfu < 0.7 the Mössbauer spectra consist of two M3 doublets implying the coexistence of two distinct epidote phases with different environments of their M3 sites. This solvus is the result of convergent ordering processes of Al and Fe³⁺ on M3 site. The kinetic constants were studied at 500<T<650°C at 0.3 GPa/HM buffer. The sizes of the exsolved epidote phases are in the nanoscale as shown by HRTEM and in microscale the samples are homogeneous according x-ray and microprobe data. The ordering process in Fe-rich epidotes takes place very much slower than the disordering process due to the formation of new domains with different local order. During disorder reaction progress the primary high amount of domains with different local order decreases and consequently the activation energy for the disorder process is smaller compared to the ordering reaction.

Fehr KT & Heuss-Assbichler S, N. Jb. Miner. Abh., 172, 43 - 67, (1997).

Phlogopite Exsolutions within Quasi-Periodic Sequences of Muscovite: A First Evidence for a Higher Temperature Re-Equilibration Studied by HRTEM and AEM Techniques

Cristiano Ferraris (cferraris@freedom4u.net)¹ & Bernard Grobéty (bernard.grobety@unifr.ch)²

¹ Department of Earth Sciences, University of Siena, Via Laterina 8, 53100 Siena, Italy

² Institute of Mineralogy and Petrography, University of Fribourg, CH-1700 Fribourg, Switzerland

Nano to micro-scale exsolutions of ferro-aluminian phlogopite (Phl) within pluri centimeter crystals of muscovite (Ms-matrix) from a pegmatite (Gorduno - Lepontine Domain, Central-Western Alps, Switzerland) were discovered using High Resolution Transmission (HRTEM) and Analytical Electron Microscopy (AEM). The Ms-matrix is a 2 M1 polytype with occasional disordered sequences. The Phl exsolution lamellae, covering a large range of sizes, are parallel to the basal plane of the host. The interfaces parallel to (001) are straight, whereas the interfaces perpendicular to the basal plane are irregular. The lamellae are concentrated on levels of the Ms-matrix with disordered stacking. The Ms-matrix has close to end-member composition with 0.15 p.f.u. tri-octahedral cations. The Phl-lamellae show considerable di-octahedral and eastonite substitution and are zoned with rim slightly richer in di-octahedral component. The Ms-matrix itself is homogeneously exsolved into two sets of nanometer-sized lamellae at an angle of 40° to the basal plane. The Phl-lamellae and the fine, homogeneously nucleated lamellae in the Ms-matrix are interpreted as the result of exsolution of a primary pegmatitic muscovite, richer in tri-octahedral component. Micas along the di- tri-octahedral joint exsolve with increasing temperature and the composition of the exsolved matrix point to a equilibration temperature of 600°C. The recalculated primary composition point to a crystallization temperature around 500°C. As heat source of the post-magmatic increase of temperature acted probably the nearby, younger Novate intrusion body.

An Experimentally Determined Petrogenetic Grid for Metapelites at Medium-high Pressure

Fabio Ferri (fabio@expe.terra.unimi.it), Arrigo Gregnanin (greg@r10.terra.unimi.it) & Stefano Poli (stefano@biko.terra.unimi.it)

Dip. Scienze della Terra, Università di Milano, Via Botticelli 23 I-20133 Milano, Italy

Despite the advent of internally consistent thermodynamic databases, currently available petrogenetic grids for metapelites are contradictory. Phase relationships in metapelites are investigated on four synthetic compositions in the model system CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O. Experiments were carried out in a piston cylinder apparatus at pressures ranging from 1.2 to 2.6 GPa and temperatures from 550°C to 650°C, at fluid saturated conditions and fO₂ buffered by graphite. Experiments were characterized by XRD, BSE images and EMPA. All assemblages contain quartz and muscovite-phengite solid solution; a Ca-phase (zoisite or lawsonite) is present in all but one experiments. Garnet-chlorite-biotite assemblage is stable at 1.60 GPa 550°C; talc-chloritoid pair is stable with biotite at 2.35 GPa 600°C or with garnet at 2.50 GPa 600°C and 2.65 GPa 625°C. Profiting of previous experimental data presented by Benciolini et al. (1996), garnet-chlorite pair is shown to be stable at any pressure below ca. 550°C, chloritoid-biotite assemblages occur in a narrow temperature range up to ca. 2.3 GPa, and staurolite-biotite pair is stable in the high temperature portion up to 1.7 GPa. The univariant reactions biotite + chlorite = talc + garnet and chloritoid + biotite = talc + garnet are responsible for the widespread occurrence of talc-schists at eclogite facies conditions. White mica at 2.5 GPa 600°C has a Si content of 3.4-3.5 a.p.f.u. (11 O) and a Mg/(Mg + Fe) = 0.5; chloritoid has a Mg/(Mg + Fe) = 0.3 and garnet has pyrope and grossular fractions of 0.1 and 0.3, respectively. The Al content in biotite progressively decrease with pressure down to 1.05 a.p.f.u. (11 O) at 2.3 GPa 600°C. Schreinemakers' rules are used to unravel experimental data assuming that Mg/(Mg+Fe) increases in the order: garnet<staurolite<chloritoid<biotite<chlorite<talc and chlorite on the Mg-rich (or Al-rich) side of the join talc-chloritoid.

Benciolini N, Poli S & Valle M, Terra abs, 8, 7, (1996).