Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar |
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| Authors: | Artur Benisek E Dachs Günther Redhammer Gerold Tippelt Georg Amthauer |
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| Institution: | Institut für Mineralogie, Universit?t Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria, AT
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| Abstract: | The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H2 in order to determine the activity of the annite component (a Ann) in iron biotites with variable degrees of the Tschermak's substitution (6]Fe + 4]Si = 6]Al + 4]Al). Based on the simplified relation a Ann = f H 2/foH2 (foH2 = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H2O. This assemblage was exposed to various f H 2 conditions (f H 2 < foH2) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f H 2 in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f H 2. By using opposite biotite starting compositions, the equilibrium composition as a function of f H2 was bracketed. For type II, f H 2 in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H2O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H2O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The 4]Al and 6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance 6]Al, and all Fe-Al biotites contain 8–10% Fe3+. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe3+-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar–1 and of 15.06 ± 0.02 Jbar–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter WAnnSid is sufficient to represent the data within error. This was constrained as: W AnnSid = –29 ± 4 kJmol–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed. |
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