Ilvaite-manganilvaite series minerals in jasper and iron-manganese ore from the Northern Chichibu belt, central Shikoku, Japan

Ilvaite, a hydrous calcium mixed-valent iron silicate, is found in red hematitic chert (jasper) lenses in altered basalt and an iron-manganese ore from the Sumaizuku unit (Middle Jurassic accretionary complex) of the Northern Chichibu belt, SW Japan. The jasper lenses are composed of ilvaite, andradite, stilpnomelane, riebeckite, hematite and quartz. Ilvaite in the jasper lenses shows an unusual spherical morphology (similar to 100 mu m in diameter) and each ilvaite spherule exhibits a foam-like microstructure with recrystallized quartz grains. Ilvaite also occurs as discrete euhedral crystals (up to 500 mu m in length) in quartz segregation veins or monomineralic ilvaite veins. Ilvaite in the jasper lenses is close to the endmember composition, but the material in the ironmanganese ore is rich in Mn-[6](2+) (up to 0.84 atoms per formula unit) corresponding to manganilvaite. Thermodynamic modeling shows that the ilvaite-andradite-hematite-quartz association (ilvaite + O-2 = hematite + Wandradite + quartz) is stable under the peak P-T conditions of the Sumaizuku unit (similar to 0.35 GPa, 230-250 degrees C) and oxygen fugacity slightly above the hematite-magnetite buffer (log f(O2) = similar to -38). Ilvaite spherules are intimately associated with colloform-textured andradite, suggesting that they probably originated from a Ca-Fe-Si colloidal precursor deposited along hydrothermal fluid conduits within the Early Permian oceanic crust, and recrystallized during subduction-related low-grade metamorphism.