Synthetic Mn3+-kyanite and viridine, (Al2-xMn
x
3+
) SiO5, in the system Al2O3-MnO-MnO2-SiO2 |
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| Authors: | I Abs-Wurmbach Prof Dr K Langer |
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| Institution: | 1. Institut für Mineralogie, Ruhr-Universit?t Bochum, Federal Bepublic of Germany
2. Mineralogisch-Petrologisches Institut der Universit?t Bonn Poppelsdorfer Schlo?, D-5300, Bonn, Federal Bepublic of Germany
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| Abstract: | Experiments on the join Al2SiO5-“Mn2SiO5” of the system Al2O3-SiO2-MnO-MnO2 in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn2O3, and high purity SiO2 as starting materials and using /O2-buffer techniques to preserve the Mn3+ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al1.88Mn 0.12 3+ )SiO5 and, thus, equals approximately that on the join Al2SiO5-“Fe2SiO5” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn3+ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a o, b o, c o, and V o to increase by 0.015, 0.009, 0.014 Å, and 1.6 Å3, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn3+-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusitess (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al1.86Mn 0.14 3+ )SiO5-(Al1.56Mn 0,44 3+ )SiO5. Thus, Al→Mn3+ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn3+ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a o, b o, c o, and V o to increase by 0.118, 0.029, 0.047 Å and 9.4 Å3, resp. At “Mn2SiO5”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn2SiO5”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn2+Mn3+O8/Si04]), pyrolusite and SiO2 were found to coexist with the Mn3+-saturated ky ss or and ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn3+). Pure synthetic braunites had the lattice constants a o 9.425, c o, 18.700 Å, V o 1661.1 Å3 (ideal compn.) and a o 9.374, c o 18.593 Å3, V o 1633.6 Å3 (1 Al for 1 Mn3+). Stable coexistence of the Mn2+-bearing phase braunite with the Mn4+-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO2-SiO2. |
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