The Mn-Al-Si-O system: an experimental study of phase relations applied to parageneses in manganese-rich ores and rocks
Abs-Wurmbach, Irmgard; Peters, Tjerk
published: Feb 11, 1999
manuscript accepted: Oct 15, 1998
manuscript received: Aug 14, 1998
ArtNo. ESP147051101011, Price: 29.00 €
Abstract As a prerequisite for an experimental study of the Mn-Al-Si-O system, the solubility of Al in braunite, partridgeite and hausmannite, as well as Mn3+ in galaxite and viridine, was investigated using hydrothermal cold-seal and piston-cylinder apparatus. Equilibrium relations were mainly determined at 600°C and 4 kbar, with fO2 controlled by the solid buffers Mn2O3/MnO2, Mn3O4/Mn2o3, Cu2O/CuO, Cu/Cu2O, Fe3O4/Fe2O3 and Ni/NiO. The fO2 for the reaction pyrolusite (pyr) + quartz = braunite (brn) + O2 and Al2SiO5 + pyr + qtz = viridine (vir) or kanonaite (kan) lies above the Mn2O3/MnO2 buffer. The reaction brn + qtz + vir = spessartine (sps) takes place at fO2 conditions between the Mn2O3/MnO2 and Mn3O4/Mn2O3 buffers, whereas the reaction brn + vir = sps + corundum (cor) + 02 lies just below the latter. Between the fO2 of the Cu2O/CuO and Cu/Cu2O buffers, the reactions brn + qtz = rhodonite (rdn) + O2, vir = sps + sillimanite (sil) + cor, brn + cor = hausmanite (haus) + sps + O2, brn + rdn = tephroite (teph ) + O2, brn = haus + teph + O2, haus + cor = galaxite (gal) + O2 and haus + sps = gal + teph + O2 take place with decreasing O2. At lower total pressure, higher temperature and appropriate fO2, braunite is not stable with tephroite, and will decompose to rhodonite and hausmannite. The resulting isothermal isobaric sections for the different buffers are compared with natural mineral assemblages in aluminous manganese-rich rocks. Most of the variability of mineral assemblages within a manganese deposit and the persistence of certain minerals like braunite, piemontite and viridine from very-low-grade to amphibolite-facies conditions can be explained by the experimentally derived equilibrium reactions. The most critical parameters for these reactions seem to be oxygen fugacity and, to a lesser extent, temperature and total pressure.